Instrument Science Report WFPC2 2002-02
Updated WFPC2 Flatfield
Reference Files for 1995 - 2001
Anton M. Koekemoer, John Biretta and Jennifer Mack
March 1, 2002
ABSTRACT
We present updated flatfields for WFPC2 using on-orbit data from Earthflat exposures,
covering dates ranging from September 1995 - May 2001. We divide the flatfields into
epochs depending on the appearance of major new features, thereby substantially diminishing the effect of such features on the data by calibrating their variation with time. We
also quantify their wavelength dependence by examining Earthflats from a wide range of
filters, and measure the time dependence of large-scale flatfield features that may be
caused by long-term changes in the camera geometry. In addition, we are able to measure
pixel-to-pixel structure to levels below ~0.3% for the PC and ~0.2% for the WF chips.
Here we describe the procedures for creating the new flats, which are now available in the
standard CDBS processing pipeline as updated WFPC2 calibration reference files.
Introduction
While the calibration of WFPC2 has been remarkably stable during its 8 years onorbit, small changes have been slowly accumulating and eroding previously attained calibration accuracies. Among these changes are long-term evolution of the flatfields - in
particular, new dust spots can appear on the optics, and changes in the camera geometry
can manifest themselves as significant large-scale changes in the flatfields. To date, the
primary set of calibration reference flatfield files for WFPC2 have been based on a combination of ground-test data and exposures of the bright earth, or “Earthflats”, taken early
during the on-orbit life of WFPC2 in 1994. These on-orbit flatfields provided a significant
improvement over the ground-test data, not only in terms of reducing the r.m.s. pixel-topixel variations but also by providing flatfields that follow the same optical light path as
the astronomical images.
Copyright© 2001 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report WFPC2 2002-02
Nevertheless, the WFPC2 cameras continue to change their relative positions as a
function of time, and in addition new features have appeared on the chips that are not
taken into account by the first generation of on-orbit flatfields. Therefore, an intensive program of WFPC2 Earthflat observations has been maintained throughout its life, to enable
the creation of updated flatfields that more accurately take into account the gradual
changes in the images.
We present here the principal results from this program during the years 1995 -2001,
including the creation of new updated flatfield reference files, as well as a description of
the major changes that have taken place in the camera over this time, both in terms of the
appearance of new features as well as the long-term changes in existing features.
Observations
Since 1994 there have been a number of consecutive Earthflat calibration programs
aimed at obtaining images of the bright earth in all the narrow-band filters of WFPC2,
together with some of the medium and broad-band filters in the ultraviolet. We note that
most of the optical broad band filters cannot be used, since the count rate for the bright
earth is too high at those wavelengths. In Table 1 we summarize the number of images
obtained in all the optical narrow-band filters (F343N and redwards), for all the Earthflat
calibration programs since 1994, identifying each program by its 4-digit HST Program ID
number.
Table 1. Number of images per optical narrow-band filter obtained during all the WFPC2
Earthflat programs since 1994.
ID #
Dates Covered
F375N F502N F656N F953N F343N F390N F437N F469N F487N F631N F658N F673N
5240 1994/01 - 1994/03
54
54
53
54
-
-
-
-
-
-
-
-
5570 1994/03 - 1994/10
200
200
200
200
-
-
-
-
-
-
-
-
6140 1995/02 - 1995/03
-
30
-
10
-
-
-
-
-
10
-
10
6187 1995/09 - 1996/07
190
190
190
189
42
42
42
42
42
46
46
46
6909 1996/08 - 1997/06
181
180
180
180
48
47
46
48
48
46
46
46
6194 1997/06 - 1997/07
-
200
-
-
-
-
-
-
-
-
-
-
7019 1997/03 - 1997/04
-
108
-
-
-
-
-
-
-
-
-
-
7625 1997/09 - 1998/06
194
194
194
194
50
50
50
50
50
50
50
50
8053 1999/03 - 1999/07
166
166
150
150
46
46
46
30
30
46
46
30
8445 1999/07 - 2000/09
114
114
128
128
30
30
30
44
44
30
30
44
8495 2000/01 - 2000/02
20
20
20
20
-
-
-
-
-
-
-
-
8815 2000/11 - 2001/10
200
200
200
200
50
50
50
50
50
50
50
50
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Instrument Science Report WFPC2 2002-02
In general, the four main filters used were F375N, F502N, F656N and F953N. These
were typically observed more often than the other filters, with each of the above programs
obtaining up to 200 exposures in a given HST observing cycle. The additional filters were
cycled through less frequently, but are nonetheless useful for quantifying filter-dependent
effects and distinguishing these from wavelength-dependent effects.
The exposure times for the various narrow-band optical filters are summarized in
Table 2. They are generally different for each filter, and are intended to provide sufficient
counts in a large fraction of the exposures in order that they may be used to construct flatfields. Depending upon the nature of the features on the Earth that are being observed, and
their illumination by the sun, it is generally the case that some exposures are completely
saturated while others have very few or no counts at all; later we describe in more detail
our selection criteria for retrieving usable flatfields from the archive.
Table 2. Exposure times for the Earthflat exposures in the various narrow-band filters.
Filter
Exp. Time (s)
F375N
14.0
F502N
1.2
F656N
0.35
F953N
3.0
F343N
14.0
F390N
1.6
F437N
2.0
F469N
1.0
F487N
0.8
F631N
0.5
F658N
0.4
F673N
0.3
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Instrument Science Report WFPC2 2002-02
Determining Time-Dependence of Features in the WFPC2 CCDs
The first step in our analysis consisted of determining the overall time-dependent
behavior of features on the WFPC2 chips, covering the entire time period from 1994 to
2001. This was first done by means of examining VISFLAT exposures taken during this
period, to identify spatial changes or new features that appeared over time. We selected all
VISFLAT exposures obtained for the F555W filter since 05/01/1994 (corresponding
approximately to the date of the last flats used in the previous generation of flatfield reference files).
The VISFLAT images were first combined using the IRAF crrej task to remove cosmic rays. These combinations were done for consecutive pairs of images that had
approximately equal exposure times. The first 4 consecutive VISFLAT images were combined into a high S/N "reference image", covering the time period 06/08/1994-08/07/1994.
All the other cr-rejected images were then normalized to units of DN/s and divided by the
reference image. We then normalized these "ratio images" to unity, dividing each one by
the median value in the central 400x400 region of each chip, determined using the IRAF
task imstat.
The most noticeable temporal features in the VISFLATS consist of several new spots,
likely due to dust particles, particularly visible on the WF2 chip. Specifically, these new
spots first appear in the February 1997 image, and are not visible in any of the images up
to and including the October 1996 VISFLAT images. In Figure 1 we show a comparison
between these two epochs, clearly showing the appearance of three new dust spots. The
strongest spots reach levels of 5-10% below the surrounding pixels. Besides these three
obvious spots, there are also numerous weaker spots not apparent in this display.
Figure 1: A comparison between October 1996 and February 1997 VISFLAT images,
with arrows indicating the appearance of several new prominent dust spot features on the
WF2 chip (a number of weaker spots are also present but not evident in this display).
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Instrument Science Report WFPC2 2002-02
The new spots are also described in the analysis of VISFLATs by O’Dea et al (1999).
To better constrain the date at which the dust spots appeared on the WF2 chip, we next
examined WFPC2 INFLATS for the range of dates covering October 1996 - February
1997. The INTFLATS are useful because they are obtained more frequently than the VISFLATS, which are obtained only once every few months. To see the relative change in the
INFLATS over time, we divided each image by a "reference" image taken at the beginning
of the date range in question. The last two INFLAT images without dust spots were
obtained on 10/17/1996 (Shutter B) and 10/24/1996 (Shutter A). The next INTFLAT
images, which showed the dust spots, were obtained on 11/11/1996 (Shutter B) and 11/12/
1996 (Shutter A). This allowed us to further narrow down the time period during which
these new dust spots appeared, to somewhere between 10/24/1996 and 11/11/1996.
Our final constraint on the time and date of the appearance of these dust spots was
obtained by examining earthflat images during this smaller interval, since earthflats are
obtained very frequently (typically at least 10 per week). There were a total of 50 earthflat
images obtained during this time interval. By examining these images, we determined that
at least one of the dust spots appeared sometime between 10/28/1996 02:01 UT and 10/29/
1996 09:37 UT, and all of them were present by 11/04/1996 02:54 UT (see Table 3).
So far it is unclear what might have caused this sudden appearance of multiple dust
spots. The only spacecraft-related activities of interest around this time are that a focus
move of the secondary mirror by +5 microns was carried out on 10/30/1996 at 17:40 UT,
and in an apparently unrelated incident, the spacecraft safed shortly thereafter on 10/30/
1996 at 18:10 UT due to a software sunpoint limit being exceed. This resulted in a safing
of all the Science Instruments including WFPC2, although it was not warmed up. However, since at least one of the dust spots seems to have appeared shortly before these
events, it remains unclear what relationship if any exists between these events and the dust
spots. It is notable however that such strong new features occur relatively rarely.
Table 3. A subset of the 50 Earthflat images obtained from 10/24/96 to 11/11/96, showing
the earthflats immediately before and after the appearance of the new dust spots on WF2.
Image
Start Time (UT)
New WF2
dust spots?
u3ek5105t
1996-10-25 16:07:16.4
-
u3ek5308t
1996-10-25 18:58:16.4
-
u3ek5208t
1996-10-26 13:04:16.4
-
u3ek5901t
1996-10-28 02:01:16.4
-
u3ek6001t
1996-10-29 09:37:16.4
1
u3ek6105t
1996-11-04 02:54:16.4
3
u3ek6305t
1996-11-08 00:00:16.4
3
u3ek6606t
1996-11-11 17:59:16.4
3
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Instrument Science Report WFPC2 2002-02
A second feature noticeable in the VISFLAT images consists of a diagonal gradient in
amplitude across each chip. This effect appears to be strongest in the WF4 chip, and also
seems to grow in amplitude as a function of time (see Figure 2).
Figure 2: VISFLAT exposures in F555W from September 1994 to April 2001, showing
the gradual increase of a gradient on the WF4 chip. Similar gradients are visible on the
other chips, although WF4 displays the strongest degree of variation. The images in all
cases have been normalized to unity, according to the median counts in the central
400x400 pixels. The greyscale covers a range of +/- 1% variation in amplitude.
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Instrument Science Report WFPC2 2002-02
To quantify in more detail the way in which this large-scale gradient changes with
time, we plotted the difference between the measured median values of the two extreme
corners of the WF4 chip (top-left and bottom-right), using a 200x200 pixel box. This
therefore represents the amplitude of the variation, and is plotted as a function of time in
Figure 3.
Figure 3: Amplitude of the large-scale spatial gradient observed in the WF4 chip, derived
from the difference between the normalized top-left and bottom-right median values of
VISFLAT exposures, plotted as a function of time. Results from both the F555W and
F814W exposures are plotted, indicating that the change is not likely to be specific to a
particular filter.
The observed trend displays a strong change over the time period 1994 - 1996, then
begins to level off in 1997 and remains approximately constant thereafter. While this may
be due in part to changes in the camera geometry, it should also be noted that the VISFLAT lamps themselves display a very similar behavior of initial degradation followed by
subsequent levelling off, on almost exactly the same timescales (O’Dea et al., WFPC2 ISR
99-01). This effect is confirmed by overplotting similar statistics for the F814W filter.
Therefore, it is not clear that temporal changes in the large-scale gradients across the chips
are a reliable indicator of changes in the camera geometry, since non-uniform degradation
of the VISFLAT lamps can easily produce similar spatial changes observed across the
chips.
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Instrument Science Report WFPC2 2002-02
However, another approach to quantifying long-term changes in flatfield features
involves the use of small-scale features on the chips that are present throughout the entire
timeframe under investigation. Any changes in such features would be independent of
changes in the lamps, and are likely to be dominated by changes in illumination resulting
from temporal evolution of the camera geometry. We identified three such features in the
VISFLAT images: 2 small spots (one on WF3 and one on WF4) and one larger extended
“finger”, located near one of the corners of WF4. These are shown in Figure 4.
Figure 4: This image shows three of the features that we used to track long-term changes
in the flatfields resulting from changes in illumination of the cameras. One of the features
is a spot toward the top of the WF3 chip; the second is a spot near the top of the WF4, and
the third is the well-known “finger” feature near the bottom corner of WF4.
Because the images displayed in Figure 4 are simply the ratio between the original reference VISFLAT (obtained from the 1994 data) and each subsequent VISFLAT, we can
directly determine whether these features appear to move over time by measuring the
r.m.s. variation of pixels in a small box centered on each feature. If the features move, then
the r.m.s. should increase with time. In Figure 5 we plot the time evolution of the r.m.s. of
each of these three features.
It can be seen that the r.m.s. of all three features initially increases, indicating that
there does exist a real time evolution of features in the flatfield. The extended “finger”
eventually flattens off, which can be interpreted as a “saturation” effect due to the structure
of the feature: once it has moved by more than a few pixels, the r.m.s. will no longer
increase but simply remain constant with time. The two smaller spots are either moving
more slowly, or otherwise are more extended than the scale over which they have moved,
therefore their r.m.s. continues to increase with time.
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Instrument Science Report WFPC2 2002-02
Figure 5: Time evolution of the r.m.s. variation of pixels in a small box centered on each
of the three features identified in Figure 4. The increase of r.m.s. with time indicates that
the features are moving relative to the reference flat, which all the subsequent flats are normalized to. Since images from the two epochs were divided into each other, the r.m.s. values effectively indicate the fractional change in the features as a function of time.
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Instrument Science Report WFPC2 2002-02
It should be noted that it is highly improbable that the dust particles themselves are
physically moving across the surface to which they are attached. They are likely deposited
onto a surface near the focal plane (for example, the pyramid mirror, or possibly the CCD
windows which are near the second focal plane). Their apparent movement across the
chips is most plausibly produced by time-dependent variations in the illumination geometry of the cameras.
The overall trend observed in all these different types of measurements indicates relatively strong evolution during the period 1994 - 1996, with relatively little evolution from
1997 onward. If we combine this with our knowledge of the sudden appearance of the
strong dust spots in late 1996, then the most practical description of the time evolution of
the WFPC2 flatfields is to divide the time period into two epochs: pre- and post-November
1996, with the division between these two epochs defined by the appearance of the dust
spots. This division also falls at roughly one-half the maximum long-term change seen on
Figure 3 and Figure 5, resulting from shifts in the camera geometry.
Earthflat Analysis
Definition of the Time Interval Epochs
Based on the results from the previous section, we defined two “epochs” for creating
the new reference flatfield files, based on the date after which the dust spots had appeared:
Epoch A represents flatfields from before the appearance of the new spots, and Epoch B
represents the time period after the appearance of the dust spots. In addition, the starting
date for Epoch A was chosen such that it would not include any flats that had been used in
the previous generation of flatfield reference files (mostly 1994 to early 1995); thus, only
flats from Program 6187 onwards were used (starting in September 1995). Finally, some
new severe dust spots have recently appeared in the WF4 chip (as of May 2001). Therefore the flats in Epoch B are chosen only up to the end of April 2001. We anticipate the
future creation of new flatfields for dates after May 2001. Our defined time interval
Epochs are thus summarized in Table 4.
Table 4. Definition of the time intervals for the generation of new reference flatfield files.
Epoch
Time Period
A
09/01/1995 - 10/29/1996
B
10/30/1996 - 04/30/2001
Retrieval of the Earthflats from the Archive
Once we had determined the time periods of interest, the next step involved retrieving
Earthflats from the archive in order to combine them into flatfield files for each filter. The
primary selection criterion for retrieving earthflats from the archive was that they had to
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Instrument Science Report WFPC2 2002-02
have sufficiently high counts to be usable, while at the same time not so high as to have a
significant number of saturated pixels. In practice this corresponded to choosing images
that had a mean value above 500 counts in the central 200x200 pixels of the PC, as well as
a mean value less than 3200 counts in the central 200x200 pixels of each of the WF2,
WF3, and WF4 chips (these values are specified by the DADS catalog WFPC2 secondary
reference keywords ws2_meanc200_1,2,3,4). In addition, we required that the total
number of good pixels in each of the 4 chips be greater than 500,000, in order to avoid
including images that had too many bad or saturated pixels (these values are given by the
DADS primary reference keywords wp2_gpixels_1,2,3,4). Previous experience
has shown that these values for the mean counts and numbers of good pixels result in the
retrieval of a sufficiently large number of usable earthflats, while at the same time rejecting most of the flats that would not be usable. This was indeed verified to be the case for
the current work as well.
We retrieved earthflats for the 4 main filter sets (F375N, F502N, F656N and F953N) as
well as all the other narrow-band filters for which Earthflats have been obtained during the
time period September 1994 - April 2001. The number of earthflats retrieved (as well as
the total number that were observed and are available in the archive) are shown in Table 5,
for each of the two epochs.
Table 5. Number of Earthflats retrieved for each filter, per epoch.
Filter
Epoch A
09/01/1995 - 10/29/1996
# retrieved (/total)
Epoch B
11/04/1996 - 04/30/2001
# retrieved (/total)
Total disk volume
of retrieved files
(Mb)
F343N
33 (/60)
104 (/190)
3,510
F375N
25 (/212)
83 (/687)
4,140
F390N
37 (/59)
125 (/190)
4,187
F437N
27 (/58)
84 (/190)
2,812
F469N
42 (/60)
112 (/188)
3,964
F487N
26 (/60)
99 (/188)
3,237
F502N
89 (/211)
355 (/994)
14,228
F631N
18 (/66)
40 (/186)
1,456
F656N
62 /(212)
204 (/684)
9,046
F658N
17 (/66)
45 (/186)
2,159
F673N
18 (/66)
61 (/184)
2,033
F953N
51 (/211)
178 (/684)
7,192
We note that the relatively small number of usable earthflats during Epoch A is another
reason for not subdividing it further into sub-epochs, despite the fact that the flats were
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Instrument Science Report WFPC2 2002-02
changing rapidly during that epoch. Rather, we retain it as a single epoch in order that the
resulting updated flatfield reference files would be of sufficiently high S/N so as not to
degrade the data.
Conversion to GEIS format and Recalibration
The flats were read into IRAF and converted to GEIS format using strfits, after which
they were recalibrated with calwp2. The recalibration was necessary since automatic pipeline calibration does not include correction by the flatfield for earthflat data, due to the fact
that they are themselves intended to be used to create flatfield reference files. In this particular case, however, our primary aim is to create flatfields that represent a multiplicative
correction to the flatfield reference files that are already in use in the pipeline. Therefore, it
is necessary to calibrate them by applying the appropriate reference or “old” flatfield; any
features remaining in the image after this calibration would then represent real structure
that is to be incorporated into the final updated flatfield reference files.
The recalibration step also involved retrieving all the other calibration reference files
that were needed to calibrate the Earthflats, including darks, biases, a2d conversion, and
shutter shading correction files as appropriate, since these are not automatically delivered
by the pipeline when retrieving the earthflat data.
Initial Statistical Rejection of Earthflats with Excessive Variation
After recalibration, the script streak.cl was run on each of the earthflat images. This
script creates a small 5x5-pixel binned mosaic version of each earthflat, combining
together all 4 WFPC2 chips into a single image. The binning allows the calculation of
image statistics that are representative of large-scale structure across the chip, rather than
being sensitive to small-scale pixel-to-pixel variations. The image statistics for these
binned images were computed using the imstat task, to determine their rms, mean and
minimum and maximum values. Only those flats with r.m.s. variation less than 3% (and
also peak-to-peak amplitude fluctuations less than 2%) were retained for subsequent analysis, while the rest were rejected.
The reason why certain flats have elevated r.m.s. or high peak-to-peak amplitude variations across the chip is primarily a result of spatially resolved features on the surface of
the earth. At the HST altitude of 600 km, its 0.1” PSF corresponds to a spatial scale of
around 30 cm at the surface of the earth, and each of the WF2,3,4 chips covers about 230m
on a side. Furthermore, given the 90 minute orbital period of HST, these features are moving across the detectors at a speed of about 7 km/s, thus covering the length of each of the
WF2,3,4 chips in about 0.03s. Given the typical integration times of the earthflats (0.16 14s), this means that the flats are generally covered with streaks at an orientation corresponding to the direction of motion of the spacecraft. Depending on what features HST is
observing on the earth, the images could either display extremely non-uniform bright and
dark streaks, or otherwise be completely smooth (e.g., corresponding to ocean or clouds).
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Instrument Science Report WFPC2 2002-02
Clearly uniform earthflats are more desirable, and past experience has shown that levels of
a few percent are sufficient to reject most of the unusable ones while retaining many
images that can be used.
Visual Inspection and Further Rejection of Earthflats with Excessive Variation
After creating a subset of earthflats with good statistical properties, these flats were
examined by displaying each one on the screen, and searching for any remaining flats with
non-uniform features. Although the majority of earthflats at this stage are relatively clean,
there still exist a significant fraction (~20 - 30%) that have non-uniform streaks, or streaks
that stop and start in the middle of a chip, or perhaps cases where most of the chips are
smooth except for one that displays a strong streak. These were also rejected, since they
could otherwise still influence the properties of the final combined earthflats if they are
included.
In particular, the types of features that are rejected at this point are ones that the flatfield combination algorithm would not be able to deal with very well. The algorithm used
for combining streakflats takes into account the direction of motion of the spacecraft and
uses 1-dimensional smoothing and filtering algorithms in an attempt to remove any lowlevel streaks. Thus, streaks that are of relatively low amplitude, or display a relatively uniform pattern across the chip, are more likely to be successfully removed than exposures
with extremely bright streaks on one corner of the chip, or bright features that stop or start
halfway across the chips. In Figure 6 we display examples of a usable streakflat and a
streakflat that was rejected. After visually examining all the streakflats and rejecting
unsuitable ones, the resulting earthflats were all considered “clean” and suitable for combination into a single flatfield.
streak_u3ek1b03t.hhh - STREAK_U3EK1B03T[1/1]
streak_u3ek3105t.hhh - STREAK_U3EK3105T[1/1]
360
360
340
340
0
0
z1=0.95 z2=1.05 ztrans=linear Con=1.46 Brt=0.37 cmap=Grayscale ncolors=181
0
0
0
z1=0.90 z2=1.10 ztrans=linear Con=2.29 Brt=0.26 cmap=Grayscale ncolors=131
1
1
1
0
0
0
1
1
1
Figure 6: (Left) Example of a good, streak-free, usable earthflat exposure; the greyscale
covers +/- 5%. (Right) Example of a bad flat with large, non-uniform streaks that would
not be possible to combine with the others and is therefore rejected; greyscale is +/- 10%.
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Instrument Science Report WFPC2 2002-02
Creation of Flatfields for Each Epoch
After rejection of unsuitable earthflats as previously described, a final set of earthflats
was created for each of the two epochs, for each filter. These were then combined using
the IRAF/STSDAS task streakflat. This task was initially developed by the WF/PC-1
Instrument Definition Team, and was subsequently generalized to be able to work on
WFPC2 data. The task is fully automated, requiring only a list of input earthflats together
with the calibrated data from the archive pipeline (including pipeline products such as data
quality files); it creates a single output file together with a corresponding bad-pixel file.
Note that the F673N filter had too few images (only one usable image in Epoch A) and is
therefore not considered further in this discussion.
The task initially calculates a simple average mean flatfield based on all the input
images, excluding only pixels that are flagged as bad. It then divides each individual earthflat frame by this mean flatfield, and normalizes each image by the average value of the
central 400x400 pixels. The task then uses these images to generate a median earthflat
image, excluding outlying pixels by means of iterative sigma-rejection passes. Each of the
earthflats is then divided by the median flat, to produce an estimate of the streak pattern in
each frame. The streak angle is calculated based on header keywords that contain the
orbital motion of the spacecraft relative to the earth, and a box filter is then used to create
a smoothed estimate of the streak pattern along the streak direction. Each earthflat is then
divided by the smoothed streak pattern, thus removing the streaks to first order. This process is repeated iteratively a number of times, each time using a smaller half-width for the
filtering box. In practice there were 8 iterations, using half widths of 800, 600, 400, 250,
150, 90, 50, and 30 pixels. Past experience has shown these values to be effective at
removing the majority of streaks, and this was also verified in the present case by some
experimentation with different values.
One additional consideration was the fact that the streakflat task is only capable of
combining up to 18 earthflat images at a time, due to limitations in the IRAF software. For
a number of filters we had far more than 18 images per epoch, therefore in these cases we
divided all the input images evenly into several input sets each containing 18 images or
less, and streakflat was then run separately on each of these input sets. The output flatfield
files produced by streakflat were then averaged together into a single flatfield image for
each epoch. In practice, this also provided a useful consistency check to ensure that no
systematics were entering into the generation of the final flatfield file - we took care to
match the separate input sets in these cases in terms of similar coverage across the entire
epoch, as well as a similar distribution of days after decontaminations, so that the output
from streakflat for each input set of 18 or less images should be essentially identical. This
was verified to be the case, to levels well below 0.1%, by taking the ratio between these
output files and displaying the ratios with varying levels of contrast, and also by measuring
the image statistics on the smoothed difference images, which always revealed large-scale
r.m.s. differences to be < 0.02% - 0.03% on the WF and PC chips respectively.
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Instrument Science Report WFPC2 2002-02
Accounting for Contamination
After completing the above steps, we had produced two images per filter, one for
Epoch A and the other for Epoch B. The structure in each of these images represents the
multiplicative difference between the reference flatfield files in the pipeline, and the actual
structure on the chips. Thus, these images can be considered “delta-flats”: multiplicative
corrections to the current pipeline flats for each filter.
In addition to flatfield changes, however, significant levels of contamination are also
present on many images for filters bluewards of F487N. It is difficult to avoid some level
of contamination in these filters while at the same time retaining a sufficiently large number of flats to ensure a high S/N correction image. Thus, our approach involved making
sure that the level of contamination was the same between Epoch A and Epoch B, so that
when a ratio image is calculated between these two epochs, the contamination features
would cancel out.
Therefore, for each filter we computed new flatfields for Epoch A and Epoch B, aiming to match the average contamination levels between the two epochs by firstly including
all earthflats obtained with relatively low contamination (typically within 14 days after a
decontamination), and then iteratively adding or subtracting earthflats with higher levels
of contamination until both epochs had the same level of contamination, and at least one
epoch (typically A) had the maximum possible number of usable input files.
A minor complication was that the average contamination rate during Epoch B tended
to be typically somewhat lower than during Epoch A (presumably because the instrument
has had more time to stabilize and the overall level of outgassing is probably reduced).
Therefore, simply choosing input files based on calculating a similar mean contamination
rate for the two epochs did not suffice to produce resulting flatfields with similar contamination levels. Instead, matching of contamination levels had to be done iteratively by
calculating the mean flatfields each time and examining the ratio between A and B. Typically, if Epoch A was constructed from earthflats up to 14 days after a decontamination,
then Epoch B had to be constructed from earthflats up to around 21 days after a decontamination to produce the same fractional level of contamination.
Our final correction images for Epoch A and Epoch B produced by this method were
very well matched in contamination epoch, thus the ratios between Epoch A and Epoch B
display no detectable signatures of contamination features (above our threshold level of
0.1%.) Two exceptions to this were the filters F469N and F631N, which had too few flats
to permit satisfactory matching of contamination between the two epochs; they are not
used in the final creation of correction images anyway because of their limited S/N.
Filters redward of F631N did not suffer significantly from contamination differences although some slight contamination was evident, the levels are sufficiently low that the
difference in rates was not noticeable between Epoch A and Epoch B and this allowed all
the earthflats to be retained, typically ranging up to 28-30 days after a decontamination.
Again, the ratios display no signatures of contamination differences.
15
Instrument Science Report WFPC2 2002-02
In Figure 7 through to Figure 17 we display the resulting images for Epoch A,
together with the ratio between the Epoch B and Epoch A images, for each of the narrowband filters. In Table 6 we give the final numbers of files used for each filter to produce an
averaged flatfield for Epoch A and Epoch B; in Appendix A we list the actual earthflat
files that were used to produce each of these, also showing how they were divided into
separate input files in cases where there were more than 18 earthflats in a given epoch.
Table 6. Number of Earthflats used to create the final correction flatfields, for each Epoch.
Filter
Epoch A
Epoch B
F343N
14
90
F375N
20
52
F390N
12
60
F437N
10
42
F469N
17
46
F487N
9
45
F502N
36
40
F631N
8
10
F656N
13
34
F658N
4
11
F953N
18
54
Note that two of the filters, F631N and F658N, had too few good remaining images in
Epoch B to permit the creation of a ratio image with sufficient S/N. Therefore we no
longer consider them in this discussion. Furthermore, the F469N filter displays substantial
large-scale changes between Epoch A and Epoch B (Figure 11), therefore we do not consider the F469N filter in the rest of this discussion either. The final set of 8 filters that we
used to create the correction flats are thus F343N, F375N, F390N, F437N, F487N, F502N,
F656N, and F953N.
16
Instrument Science Report WFPC2 2002-02
streakflat_f343n_A_mos - STREAKFLAT_F343N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f343n_B_A_mos - RATIO_F343N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=199
0
0
0
1
1
1
Figure 7: F343N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Both images are
normalized to a mean of 1, and the greyscale covers the range +/- 1%.
17
Instrument Science Report WFPC2 2002-02
streakflat_f375n_A_mos - STREAKFLAT_F375N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f375n_B_A_mos - RATIO_F375N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=197
0
0
0
1
1
1
Figure 8: F375N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
18
Instrument Science Report WFPC2 2002-02
streakflat_f390n_A_mos - STREAKFLAT_F390N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f390n_B_A_mos - RATIO_F390N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 9: F390N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
19
Instrument Science Report WFPC2 2002-02
streakflat_f437n_A_mos - STREAKFLAT_F437N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f437n_B_A_mos - RATIO_F437N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 10: F437N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
20
Instrument Science Report WFPC2 2002-02
streakflat_f469n_A_mos - STREAKFLAT_F469N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f469n_B_A_mos - RATIO_F469N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 11: F469N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
21
Instrument Science Report WFPC2 2002-02
streakflat_f487n_A_mos - STREAKFLAT_F487N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f487n_B_A_mos - RATIO_F487N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 12: F487N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
22
Instrument Science Report WFPC2 2002-02
streakflat_f502n_A_mos - STREAKFLAT_F502N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f502n_B_A_mos - RATIO_F502N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 13: F502N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
23
Instrument Science Report WFPC2 2002-02
streakflat_f631n_A_mos - STREAKFLAT_F631N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f631n_B_A_mos - RATIO_F631N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 14: F631N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
24
Instrument Science Report WFPC2 2002-02
streakflat_f656n_A_mos - STREAKFLAT_F656N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f656n_B_A_mos - RATIO_F656N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 15: F656N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
25
Instrument Science Report WFPC2 2002-02
streakflat_f658n_A_mos - STREAKFLAT_F658N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f658n_B_A_mos - RATIO_F658N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 16: F658N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
26
Instrument Science Report WFPC2 2002-02
streakflat_f953n_A_mos - STREAKFLAT_F953N_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f953n_B_A_mos - RATIO_F953N_B_A_MOS[1/1]
1600
1400
1200
1000
0
500
1000
1500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
Figure 17: F953N. Top: Epoch A; Bottom: Ratio of Epoch B / Epoch A. Image normalization and greyscale are the same as in Figure 7.
27
Instrument Science Report WFPC2 2002-02
Applying the Flatfield Corrections to the Entire WFPC2 Standard Optical Filter Set
The next step involved applying our corrections, based on the narrow-band filters, to
the entire WFPC2 optical filter set. We examined the reference files of all the WFPC2 filters and determined that the WFPC2 filters redward of F300W displayed very similar
features, therefore we discuss updates to all those filters. The bluer UV filters have substantially different characteristics and are not discussed further here. The corrections that
we have calculated consist of two parts:
1. A correction from the previous WFPC2 flatfield references files to Epoch A;
2. A correction from Epoch A to Epoch B.
We examined several different possibilities for carrying out the first correction, from
the previous references files to Epoch A. Each of the Epoch A flatfields that we have created for each filter represents directly the mutiplicative difference that would need to be
applied to the reference flatfield for that filter in order to create a current reference flatfield
file for Epoch A. In order to apply these corrections to the remainder of the filter set, however, we need to be extremely careful that we do not introduce any filter-dependent
artifacts into the correction flat. For example, to create a corrected flatfield file for F555W
we would have to be careful not to introduce any fringing or other artifacts that are peculiar to the narrow-band filters used here to create the flatfields for Epoch A.
Upon examination of all the narrow-band Epoch A flatfield correction files, we discovered that almost all of them possessed some degree of structure that was dependent upon
the particular narrow-band filter in question. The only exception to this is the F502N filter.
One possible reason for this is the fact that F502N was used in generating all the previous
pipeline correction flats, therefore any artifacts would cancel out when we created the
Epoch A flatfield file. It is also quite plausible that the F502N filter has very few intrinsic
artifacts, such as the fringes or pinholes that are evident on some of the other filters, therefore this is likely the reason why it was chosen to help create the previous pipeline flats in
the first place.
Therefore, we base our correction from the previous pipeline flats to Epoch A primarily on the changes in the F502N filter. The most prominent changes are that a number of
the small-scales features have evidently moved, and that our F502N flat is of sufficiently
high S/N that we are able to resolve and correct for the small-scale spatial fluctuations that
are apparent in the difference images between the new flats and the previous pipeline flats.
These fluctuations are invariant from Epoch A to Epoch B, and are possibly the result of
some slight smoothing that was carried out when the previous pipeline flats were created.
We explore this possibility by examining the measured r.m.s. in the central 400x400 pixels
of each of the four WFPC2 chips, for Epoch A, Epoch B, and for the ratio between Epoch
A and Epoch B (see Table 7). We also compare these values with the level of r.m.s.
expected based on photon statistics, using the mean count levels for the Epoch A and
28
Instrument Science Report WFPC2 2002-02
Epoch B F502N earthflats in the PC and WF2,3,4 chips, together with the fact that the data
were obtained with a gain of 15. We calculate that the expected photon-based r.m.s. noise
should be ~0.2% for the PC and ~0.1% for the WF chips, in Epoch A and Epoch B separately (which have 36 and 40 separate earthflat images contributing to them). Instead, we
see in Table 7 that the measured r.m.s. for these epochs is substantially higher, about 0.7%
in the PC and about 0.5% in the WF chips. Moreover, for the ratio image between Epoch
A / Epoch B, the measured r.m.s. decreases by factors of two to three, to around 0.4% for
the PC and 0.2% for the WF chips. This confirms that our individual Epoch A and Epoch
B correction images are of sufficient S/N to account for fluctuations of the order 0.4-0.7%
present in the previous flatfields. The new correction flats thus are able to measure pixelto-pixel variations down to an intrinsic (photon-limited) accuracy of ~0.25% for the PC
and ~0.1% for the WF chips, for each epoch separately. This effect can be seen in
Figure 18, where we show in detail the correction image for the WF2 chip in F502N for
Epoch A, compared with the ratio between Epoch B and Epoch A - in the ratio image,
much of the small-scale structure disappears, thus the r.m.s. of the ratio image is lower.
Table 7. Measured and predicted r.m.s. noise properties of the new correction flats for
Epoch A and Epoch B, with the F502N filter.
Epoch A
Epoch B
Epoch B/A
36
40
-
386
379
Predicted r.m.s.
0.22%
0.20%
0.30%
Measured r.m.s.
0.66%
0.66%
0.39%
1902
1873
-
Predicted r.m.s.
0.096%
0.091%
0.13%
Measured r.m.s.
0.51%
0.52%
0.19%
Number of input images
PC:
Mean counts (DN)
WF2,3,4:
Mean counts (DN)
For our second correction, from Epoch A to Epoch B, we note that wavelength dependent effects associated with the small-scale dust spot features are significant (changing by
a few percent). This is because the spots are relatively small and close to the focal plane,
and therefore experience significant diffraction effects. Hence it is desirable to include
wavelength dependent effects, which is possible since the filters used to create our correction flats cover a wide range in wavelength. For each narrow-band filter we created a file
that represented the ratio between Epoch B and Epoch A; this largely cancels out all the
filter-dependent features in each narrow-band flat (pinholes, fringes etc.) since they are
generally stable over time, and the only remaining effects are time-dependent changes
from Epoch A to Epoch B, with associated color dependence.
29
Instrument Science Report WFPC2 2002-02
f502n.3/streakflat_A3_final.r6h[2] - STREAKFLAT_A3_FINAL[2/4]
800
700
600
0
500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=201
0
0
0
1
1
1
ratio_f502n_B_A.2.hhh - RATIO_F502N_B_A.2[1/1]
800
700
600
0
500
z1=0.99 z2=1.01 ztrans=linear Con=1.00 Brt=0.50 cmap=Grayscale ncolors=173
0
0
0
1
1
1
Figure 18: Top: Epoch A, F502N, WF2 chip, showing the pixel-scale structure (greyscale
+/- 1%). Bottom: Ratio of Epoch B / Epoch A for the same filter and chip, showing how
the pixel-scale structure disappears, leaving only the temporal changes in the flatfield.
30
Instrument Science Report WFPC2 2002-02
To account for the color dependence and to increase the S/N of the correction flats, as
well as remove any residual low-level filter-dependent features, we averaged together the
ratios of Epoch B / Epoch A for the various narrow-band flatfields into four general wavelength regimes, as listed in Table 8. We tabulate here the total number of input images that
have been used to produce the Epoch A and Epoch B flatfield files for each of the combinations, as well as the predicted photon-noise r.m.s. for the PC and WF chips based on the
average counts in each case. In addition, we tabulate the measured values of the r.m.s. in
the central 400x400 pixels, for the ratio image (Epoch B / Epoch A).
Table 8. Noise properties of the averaged correction flatfields.
Total Number of
Input Images
Measured
r.m.s.
Predicted r.m.s.
Epoch A
Epoch B
Epoch B/A
Epoch B/A
PC:
0.190%
0.091%
0.211%
0.30%
WF2,3,4:
0.085%
0.041%
0.094%
0.15%
PC:
0.158%
0.094%
0.184%
0.28%
WF2,3,4:
0.071%
0.042%
0.082%
0.14%
PC:
0.170%
0.118%
0.207%
0.33%
WF2,3,4:
0.076%
0.053%
0.092%
0.16%
PC:
0.232%
0.138%
0.270%
0.37%
WF2,3,4:
0.104%
0.062%
0.121%
0.18%
F343N+F375N+F390N
F390N+F437N+F487N+F502N
F487N+F502N+F656N
Epoch A
Epoch B
46
202
67
58
F656N|+F953N
31
187
119
88
It can be seen that the measured r.m.s. values for these averaged correction flats for
Epoch B / Epoch A are around 0.3% for the PC and about 0.15% for the WF chips. In all
cases they are ~50% higher than the predicted values based on the photon noise, suggesting the presence of additional low-level pixel-to-pixel fluctuations on levels ~0.05-0.1%.
This could be due to residual noise in the calibration darks/biases, or alternatively could
represent real changes over time in the pixel-to-pixel structure on levels of 0.1%.
In Table 9 we tabulate which averaged Epoch B/A correction flats we have applied to
the various filters in the WFPC2 Standard Optical Filter Set (i.e., all the filters redward of
F300W, excluding polarizers, quads and linear ramp filters). The Epoch B/A correction
flats are chosen such that their wavelength coverage at least overlaps that of each filter to
which they are applied. In Table 10 we show the filenames in the CDBS pipeline system
for the new flatfield reference files, corresponding to Epoch A and Epoch B.
31
Instrument Science Report WFPC2 2002-02
Table 9. Filters Used in Creating all the Updates from Epoch A to Epoch B for the entire
WFPC2 Optical Standard Filter Set (redwards of F300W).
Filter
Earthflat Filters Used
Total Number
of Files used
for Epoch A
Total Number
of Files used
for Epoch B
F300W
F343N+F375N+F390N
46
202
F336W
F343N+F375N+F390N
46
202
F343N
F343N+F375N+F390N
46
202
F375N
F343N+F375N+F390N
46
202
F380W
F343N+F375N+F390N
46
202
F390N
F343N+F375N+F390N
46
202
F410M
F390N+F437N+F487N+F502N
67
187
F437N
F390N+F437N+F487N+F502N
67
187
F439W
F390N+F437N+F487N+F502N
67
187
F450W
F390N+F437N+F487N+F502N
67
187
F467M
F390N+F437N+F487N+F502N
67
187
F469N
F390N+F437N+F487N+F502N
67
187
F487N
F390N+F437N+F487N+F502N
67
187
F502N
F487N+F502N+F656N
58
119
F547M
F487N+F502N+F656N
58
119
F555W
F487N+F502N+F656N
58
119
F569W
F487N+F502N+F656N
58
119
F588N
F487N+F502N+F656N
58
119
F606W
F487N+F502N+F656N
58
119
F622W
F487N+F502N+F656N
58
119
F631N
F487N+F502N+F656N
58
119
F656N
F487N+F502N+F656N
58
119
F658N
F487N+F502N+F656N
58
119
F673N
F656N+F953N
31
88
F675W
F656N+F953N
31
88
F702W
F656N+F953N
31
88
F785LP
F656N+F953N
31
88
F791W
F656N+F953N
31
88
F814W
F656N+F953N
31
88
F850LP
F656N+F953N
31
88
F953N
F656N+F953N
31
88
F1042M
F656N+F953N
31
88
32
Instrument Science Report WFPC2 2002-02
Table 10. Reference file names for the old and new pipeline reference flatfields, and USEAFTER dates, for the entire WFPC2 Optical Standard Filter Set (redwards of F300W).
Filter
Old Flatfields
Filename
USEAFTER
New Epoch A Flatfields
Filename
USEAFTER
New Epoch B Flatfields
Filename
USEAFTER
F300W
g6409256u 1994-04-23
m341819ou 1995-09-01
m3c10042u 1996-10-30
F336W
g6i1148hu 1994-04-24
m341819pu 1995-09-01
m3c10043u 1996-10-30
F343N
g6i1148lu 1994-04-24
m341819qu 1995-09-01
m3c10044u 1996-10-30
F375N
g640925bu 1994-04-23
m341819ru 1995-09-01
m3c10045u 1996-10-30
F380W
g6i1148nu 1994-04-24
m341819su 1995-09-01
m3c10046u 1996-10-30
F390N
g7c14030u 1994-04-24
m341819tu 1995-09-01
m3c10047u 1996-10-30
F410M
g7c14034u 1994-04-24
m3418200u 1995-09-01
m3c10048u 1996-10-30
F437N
g6i1148qu 1994-04-24
m3418201u 1995-09-01
m3c10049u 1996-10-30
F439W
g6i1148tu 1994-04-24
m3418202u 1995-09-01
m3c1004au 1996-10-30
F450W
g640925gu 1994-04-23
m3418203u 1995-09-01
m3c1004bu 1996-10-30
F467M
g7c14037u 1994-04-24
m3418204u 1995-09-01
m3c1004cu 1996-10-30
F469N
g6i11492u 1994-04-24
m3418205u 1995-09-01
m3c1004du 1996-10-30
F487N
g6i11494u 1994-04-24
m3418206u 1995-09-01
m3c1004eu 1996-10-30
F502N
g640925ku 1994-04-23
m3418207u 1995-09-01
m3c1004fu 1996-10-30
F547M
g7c1403bu 1994-04-24
m3418208u 1995-09-01
m3c1004gu 1996-10-30
F555W
g640925nu 1994-04-23
m3418209u 1995-09-01
m3c1004hu 1996-10-30
F569W
g6h0944pu 1994-04-25
m341820au 1995-09-01
m3c1004iu 1996-10-30
F588N
g6h0944tu 1994-04-25
m341820bu 1995-09-01
m3c1004ju 1996-10-30
F606W
g640925ru 1994-04-23
m341820cu 1995-09-01
m3c1004ku 1996-10-30
F622W
g6h09453u 1994-04-25
m341820du 1995-09-01
m3c1004lu 1996-10-30
F631N
g6h09457u 1994-04-25
m341820eu 1995-09-01
m3c1004mu 1996-10-30
F656N
g6h0945cu 1994-04-25
m341820fu 1995-09-01
m3c1004nu 1996-10-30
F658N
g6h0945fu 1994-04-25
m341820gu 1995-09-01
m3c1004ou 1996-10-30
F673N
g6h0945iu 1994-04-25
m341820hu 1995-09-01
m3c1004pu 1996-10-30
F675W
g6h0945lu 1994-04-25
m341820iu 1995-09-01
m3c1004qu 1996-10-30
F702W
g6q1912hu 1994-04-24
m341820ju 1995-09-01
m3c1004ru 1996-10-30
F785LP
g7c1403eu 1994-04-24
m341820ku 1995-09-01
m3c1004su 1996-10-30
F791W
g6q1912ju 1994-04-24
m341820lu 1995-09-01
m3c1004tu 1996-10-30
F814W
g6409260u 1994-04-23
m341820mu 1995-09-01
m3c10050u 1996-10-30
F850LP
g7c1403iu 1994-04-24
m341820nu 1995-09-01
m3c10051u 1996-10-30
F953N
g7c1403lu 1994-04-24
m341820ou 1995-09-01
m3c10052u 1996-10-30
F1042M
g7c1403pu 1994-04-24
m341819nu 1995-09-01
m3c10041u 1996-10-30
33
Instrument Science Report WFPC2 2002-02
The final, corrected flats for each filter were delivered to the CDBS system in March
2002 and are available for pipeline calibration as the standard reference files. All the flatfields derived from the Epoch A data have a USEAFTER date of 09-01-1995, while all the
corrections based on the change between Epoch A and Epoch B have a USEAFTER date
of 10-30-1996.
References
Biretta, J., 1995, “WFPC2 Flatfield Calibration”, in “Calibrating Hubble Space Telescope: Post Servicing Mission”, eds. A Koratkar and C. Leitherer, p. 257
Hester, J., 1993, “Flatfield Characteristics”, in “WFPC2 Science Calibration Report”,
ed. J. Trauger, p. 5-1.
Holtzman, J., et al., 1995, “The performance and calibration of WFPC2 on the Hubble
Space Telescope”, PASP 107, 156
O’Dea, C. P., et al., 1999, “Internal Flat Field Monitoring II. Stability of the Lamps,
Flats, and Gains”, WFPC2 Instrument Science Report 99-01
34
Instrument Science Report WFPC2 2002-02
Appendix A
The final flatfield file for each filter was created from anywhere between 9 and 90
input earthflat exposures (see Table 6). In this Appendix we summarize the input files used
for the 8 final filters, listing also the number of days after a decontamination for each file.
F343N
Epoch A:
Input File
Epoch B:
Input File 1
Epoch B:
Input File 2
Epoch B:
Input File 3
u2xk0701t
14
u3ek4702t
0
u3ek5201t
8
u3ek5202t
8
u2xk0702t
14
u3ek1m02t
8
u3ek1r01t
18
u3ek1r02t
18
u2xk0m01t
6
u3ek8201m
14
u3ek8202m
14
u4944702r
0
u2xk0m02t
6
u4945201r
6
u4944701r
0
u4947201r
11
u2xk0r01t
14
u4945202r
6
u4947202r
11
u4941r02r
17
u2xk0r02t
14
u4947701r
17
u4941r01r
17
u4949702r
4
u2xk1201t
0
u59m0202r
18
u4949701r
4
u59m1702r
6
u2xk3h01t
3
u59m6201r
16
u59m0702r
20
u5jf0202r
17
u2xk3h02t
3
u5jf7201r
4
u59m1701r
6
u59m8702r
7
u3ek1201t
7
u5jf1m02r
7
u5jf0201r
17
u5jf6201r
15
u3ek1202t
7
u6910r01r
11
u5jf7202r
4
u5jf5701r
20
u3ek1701t
12
u6911b01r
23
u5jf1m01r
7
u5jf1h01r
4
u3ek1702t
12
u6913f01r
20
u5jf1c01r
5
u5jf1c02r
5
u3ek3202t
6
u6912l02m
11
u6910r02r
11
u6912b02r
10
u6913p02r
3
u6912v01r
21
u6912v02r
21
Epoch B:
Input File 4
Epoch B:
Input File 5
Epoch B:
Input File 6
u3ek7701t
16
u3ek7702t
16
u3ek1m01t
8
u3ek1w01t
21
u3ek1w02t
21
u4941202r
5
u4940701r
3
u4940702r
3
u4946202r
17
u4945701r
14
u4941m02r
8
u59m0201r
18
u4945702r
14
u59m0701r
20
u59m2702r
19
u4941m01r
8
u59m9702r
6
u59m1c02r
21
u59m9701r
6
u5jf6202r
15
u59m3702r
8
u59m2701r
19
u5jf8701r
1
u59m6202r
16
u59m8201r
1
u5jf8201r
4
u59m8701r
7
u5jf1h02r
4
u5jf7702r
5
u5jf5702r
20
u5jf7701r
5
u6910j01r
9
u5jf8702r
1
u6910302m
7
u6911r01r
7
u5jf8202r
4
u6910j02r
9
u6912102r
17
u6911r02r
7
u6912101r
17
u6913k02r
12
u6912l01r
11
u6913f02r
20
u6913u02r
19
u6913p01r
3
35
Instrument Science Report WFPC2 2002-02
F375N
Epoch A:
Input File 1
Epoch B:
Input File 1
Epoch A:
Input File 2
u2xk0101t
4
u2xk1601t
10
u2xk1g01t
20
u2xk2102t
20
u2xk4b01t
3
u2xk4b02t
3
u2xk5h01t
3
u2xk5402t
17
u2xk5i01t
3
u2xk5h02t
3
u2xk5g01t
7
u2xk5j01t
3
u2xk6601t
16
u2xk5g02t
7
u2xk6d02t
4
u2xk6602t
16
u2xk6b02t
5
u2xk6f02t
4
u2xk7301t
17
u3ek0601t
25
Epoch B:
Input File 2
Epoch B:
Input File 3
Epoch B:
Input File 4
u3ek2f02m
6
u3ek2g02m
6
u3ek2i01m
6
u3ek2k01m
10
u3ek2l02m
10
u3ek2m01m
10
u3ek2n02m
10
u3ek3f01m
10
u3ek3f02m
10
u3ek2r01m
11
u3ek3l02m
12
u3ek3m01m
12
u3ek3n01m
12
u3ek3n02m
12
u3ek3o01m
12
u3ek3v02m
19
u4942p02r
6
u4940101r
20
u4942f01r
3
u4946102m
18
u4943k02r
18
u4943i01r
10
u4943m02r
18
u4944q02r
11
u4941q01r
16
u4943i02r
10
u4944p02r
11
u59m2o02r
12
u59m4k02r
11
u4943h02r
12
u4944s02r
18
u59m2x02m
6
u59m2u02r
3
u4941q02r
16
u4944k01r
13
u59m1l01r
1
u59m4102r
14
u59m2v02r
6
u59m2601r
18
u5jf3r01r
6
u59m3f02r
2
u59m5102r
3
u59m2w02r
6
u5jf7101r
6
u5jf3t02r
12
u5jf3s01r
6
u59m3z01r
9
u5jf4h01r
12
u5jf1q02r
14
u5jf4r02r
14
u5jf4t02r
1
u5jf4i02r
14
36
Instrument Science Report WFPC2 2002-02
F390N
Epoch A:
Input File
Epoch B:
Input File 1
u2xk0203t
4
u2xk0204t
4
u2xk0703t
14
u2xk0704t
14
u2xk0r04t
14
u2xk1c03t
15
u2xk2c03t
7
u2xk2c04t
7
u2xk2h03t
15
u2xk3704t
16
u3ek1204t
7
u3ek3203t
6
Epoch B:
Input File 2
Epoch B:
Input File 3
Epoch B:
Input File 4
u3ek4703t
0
u3ek7703t
16
u3ek5703m
18
u3ek7704t
16
u3ek0703m
21
u3ek0704m
21
u3ek1c04m
21
u3ek6703m
21
u3ek6704m
21
u3ek7204m
3
u3ek8203m
14
u3ek8204m
14
u4941203r
5
u4941204r
5
u3ek8703m
2
u3ek8704m
2
u4941704r
13
u4947204r
11
u4945203r
8
u4945704r
14
u4942704r
6
u4949703r
4
u4946703r
3
u4947704r
17
u59m2203r
12
u59m0704r
20
u4947203r
11
u59m3704r
8
u59m1c03r
21
u59m2703r
19
u4948203r
19
u59m7704r
6
u59m1w03r
6
u59m4204r
14
u59m0203r
18
u5jf0204r
17
u5jf6204r
15
u59m4704r
21
u59m4203r
14
u5jf7204r
4
u5jf9203r
7
u5jf0203r
17
u59m4703r
21
u5jf1h04r
4
u5jf1r03r
14
u5jf9703r
12
u59m1w04r
6
u6910j04r
9
u6910j03r
9
u5jf7703r
5
u5jf7704r
5
u6911b03r
23
u6912103r
17
u6910303r
7
u6912104r
17
u6913k03r
12
u6913u04r
19
u6913k04r
12
u6912g03r
10
u6912v04r
21
37
Instrument Science Report WFPC2 2002-02
F437N
Epoch A:
Input File
Epoch B:
Input File 1
Epoch B:
Input File 2
Epoch B:
Input File 3
u2xk0205t
4
u3ek5706m
18
u3ek5705m
18
u3ek1r06t
18
u2xk0m05t
6
u3ek1c06m
21
u3ek0705m
21
u3ek2705m
5
u2xk0r05t
17
u4943706r
20
u4942205r
24
u3ek1c05m
21
u2xk1c05t
15
u4945205r
8
u4942706r
6
u3ek8705m
2
u2xk2c06t
7
u4946705r
3
u4945705r
14
u4942206r
24
u2xk3205t
10
u4941c05r
12
u4941c06r
12
u4945706r
14
u2xk3705t
16
u59m4206r
14
u59m2705r
19
u4947706r
17
u2xk3h05t
3
u59m1w05r
6
u59m1r06r
21
u59m1h05r
13
u3ek1705t
12
u59m7706r
6
u5jf9205r
7
u59m5706r
8
u3ek1706t
12
u5jf0206r
17
u5jf1r05r
14
u59m1w06r
6
u5jf1h05r
4
u5jf8205r
4
u5jf5706r
20
u5jf1w06r
24
u6910r05r
11
u5jf8206r
4
u6910r06r
11
u6912g06r
10
u6912b05r
11
u6913u06m
19
u6913p05r
3
u6912v06r
21
F487N
Epoch A:
Input File
Epoch B:
Input File 1
Epoch B:
Input File 2
Epoch B:
Input File 3
u2xk0r09t
14
u3ek970at
5
u3ek1h09t
1
u3ek7709t
16
u2xk0r0at
14
u3ek6709m
21
u3ek670am
21
u3ek5709m
18
u2xk1209t
0
u3ek870am
2
u4944709r
0
u3ek8709m
2
u2xk120at
0
u494620ar
17
u494870ar
26
u494670ar
3
u2xk1c09t
15
u4947209r
11
u4941c09r
12
u494c204m
24
u2xk2w0at
6
u494d204r
3
u4947709r
17
u494d703r
9
u2xk3209t
10
u59m420ar
14
u59m570ar
8
u494770ar
17
u2xk320at
10
u59m8209r
1
u5jf620ar
15
u59m820ar
1
u2xk370at
16
u5jf670ar
23
u5jf9209r
7
u5jf5709r
20
u5jf970ar
12
u5jf1m09r
7
u5jf9709r
12
u5jf1w09r
24
u5jf8209r
4
u5jf1c09r
5
u5jfa203r
21
u5jfa703r
23
u5jfb703r
24
u6910c03r
8
u6910s03r
11
u6910c04m
8
u6910k03m
9
u6912109r
17
u6912l09r
11
u6913u09r
19
u6912v09r
21
u6912v0ar
21
38
Instrument Science Report WFPC2 2002-02
F502N
Epoch A:
Input File 1
Epoch B:
Input File 1
Epoch A:
Input File 2
u2xk0103t
4
u2xk1103t
2
u2xk2v04t
6
u2xk3104t
10
u2xk4103t
9
u2xk4403t
11
u2xk4503t
13
u2xk4504t
13
u2xk4b03t
3
u2xk4b04t
3
u2xk4c03t
3
u2xk4e03t
3
u2xk4e04t
3
u2xk4f03t
4
u2xk4f04t
4
u2xk5j03t
3
u2xk5k03t
3
u2xk5g03t
7
u2xk6103t
9
u2xk6104t
9
u2xk6303t
11
u2xk6304t
11
u2xk6403t
11
u2xk6503t
11
u2xk6504t
11
u2xk6b04t
4
u2xk6e04t
4
u2xk6g03t
6
u2xk6g04t
6
u2xk6h03t
10
u2xk6i03t
10
u2xk6k03t
11
u2xk7d04t
2
u3ek1103t
7
u3ek2603t
1
u3ek3104t
5
Epoch B:
Input File 2
Epoch B:
Input File 3
Epoch B:
Input File 4
u3ek1l04t
8
u3sc2302r
6
u3ek4b04m
7
u3ek1b03t
11
u3sc2904r
13
u3sc2402r
7
u3ek4k04m
13
u3sc1503r
12
u3ek2t04n
11
u3ek2e04m
5
u4942p03r
6
u3ek2l04m
10
u3ek4f03m
4
u3ek2y04m
2
u4945604r
10
u3ek3b04m
5
u4942u03r
12
u3ek3q04n
13
u4943p03r
4
u3ek4j04m
7
u4941g03r
4
u4945603r
10
u4944h03r
3
u4946603r
4
u59m2x04r
6
u4943h03r
12
u59m2n04r
12
u4944h04r
3
u59m3k04r
13
u59m3h03r
8
u59m1l03r
1
u59m4q04r
2
u59m3q04r
0
u59m7104r
0
u59m3i03r
9
u59m3l03r
13
u6910903r
5
u5jf4h04r
13
u5jf2b03r
13
u6912k04r
11
39
Instrument Science Report WFPC2 2002-02
F656N
Epoch A:
Input File
Epoch B:
Input File 1
Epoch B:
Input File 2
u2xk0105t
4
u3ek1v06t
20
u3ek2a06m
4
u2xk2v05t
6
u3ek2i05m
6
u3ek2u05m
18
u2xk4105t
9
u3ek2x05m
1
u3ek2x06m
1
u2xk4606t
17
u3ek4h06m
6
u4942b06r
20
u2xk4905t
21
u4943c06r
28
u4943k05r
18
u2xk4c05t
3
u4943k06r
18
u4943o06r
4
u2xk5806t
26
u4944a05r
26
u4944b05r
27
u2xk5b05t
27
u4941g06r
4
u4944m06r
5
u2xk6a06t
18
u4944m05r
5
u4944v06r
25
u2xk6g06t
6
u4944o05r
10
u4944d05r
31
u2xk7505p
17
u4949606r
3
u59m2o05m
12
u2xk0g05t
9
u494c102r
26
u59m3o05r
23
u2xk1v05t
21
u59m4606r
20
u59m4v05m
7
u59m4r05r
9
u59m3z06r
9
u59m3v06r
7
u5jf3v06r
10
u5jf2b05r
13
u5jf1b06r
4
u5jfb601r
24
u6912k06r
12
40
Instrument Science Report WFPC2 2002-02
F953N
Epoch A:
Input File 1
Epoch A:
Input File 2
u2xk1b08t
15
u2xk1108t
2
u2xk5607t
25
u2xk4e08t
3
u2xk5i07t
3
u2xk5g07t
7
u2xk6b08t
4
u2xk6707t
17
u2xk6g08t
6
u2xk6i07t
10
u2xk7208t
15
u2xk7107t
15
u2xk7b07t
3
u2xk7a08t
24
u3ek2107t
20
u2xk1v07t
21
u3ek4107t
23
u3ek3107t
5
Epoch B:
Input File 1
Epoch B:
Input File 2
Epoch B:
Input File 3
u3ek2a08m
4
u3ek2p08m
11
u3ek2r08m
11
u3ek2t08n
11
u3ek2w08m
18
u3ek3k07m
12
u3ek4c08m
3
u3ek4q08m
12
u3ek4v07m
2
u4942h08r
7
u4942l07r
12
u4942l08r
12
u4942m08n
20
u4942z08r
26
u4944108r
26
u4943d08r
28
u4941v07r
23
u4941g07r
4
u4943e07r
5
u4949608r
3
u4944q07r
11
u4944u07r
24
u4943r07r
17
u4949607r
3
u4941b08r
12
u59m4i08r
1
u4943r08r
17
u494b103r
6
u59m2j07r
7
u59m2f08r
21
u59m2e08r
21
u59m3j07r
9
u59m9607r
29
u59m2n08r
12
u59m4v07r
7
u59m1g08r
6
u59m7107r
0
u59m3y07r
26
u59m5607r
9
u59m3z07r
9
u59m4d08r
14
u5jf3k07r
15
u5jf3l08r
15
u5jf4e08r
5
u5jf9107r
5
u5jf1q08r
14
u5jf1b07m
4
u5jf8108r
7
u5jfa604r
23
u5jfb103r
23
u5jfd103r
23
u6910i04r
9
u6913e08r
20
u6912507r
24
41