The 2005 HST Calibration Workshop
Space Telescope Science Institute, 2005
A. M. Koekemoer, P. Goudfrooij, and L. L. Dressel, eds.
WFPC2 Status and Calibration
J. Biretta
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Abstract. We briefly summarize the status of WFPC2 as of late 2005, and review
calibration results from the last few years.
1.
Introduction
The Wide-Field / Planetary Camera II was built at the Jet Propulsion Laboratory in
Pasadena, CA under the leadership of John Trauger. It was installed into HST in December
1993 and is the oldest imager on the spacecraft. With over 50,000 science images to its credit,
it accounts for roughly half the science images taken by HST. Even after the installation
of more modern cameras, it continues to represent about 10% to 15% of the HST science
program. Its capabilities for wide-field UV and narrow band imaging remain unique, as
does its availability for long-term astrometric and monitoring projects. Due to its wide field
of view it also makes significant contributions as a parallel imaging instrument.
After 12 years on-orbit it continues to operate relatively well. A new electronic anomaly
was recently discovered in the WF4 channel, and we discuss initial assessments of its impact
and possible mitigation strategies. Long-term radiation damage causes a number of effects,
including increasing numbers of hot pixels and elevated dark current, but these are not serious issues. Charge Transfer Efficiency (CTE) issues in the CCDs, also caused by radiation
damage, continue to slowly increase and represent perhaps the most significant lien against
the instrument’s performance. We briefly summarize results of calibrations carried out over
the last few years in the areas of CTE, photometry, and geometric distortion. Finally we
discuss potential “Close Out” projects which might further refine calibrations and increase
the archival value of WFPC2 data. Herein we highlight only a few recent developments; for
a more complete picture of the WFPC2 calibration the reader is referred to the WFPC2
Instrument Handbook, the HST Data Handbook, numerous Instrument Science Reports,
and excellent reviews in prior Calibration Workshops (e.g. Whitmore 1997, Koekemoer
2002, Whitmore 2002).
2.
New Anomaly in the WF4 CCD
A new anomaly was recently discovered in the electronics of the WFPC2 WF4 channel. It is
characterized by sporadic images with either low or zero bias level from the WF4 CCD. Since
the calibration pipeline automatically subtracts the bias level, images with low bias levels
are not immediately obvious, however more careful examination will reveal faint horizontal
(X-direction) stripes in the background and low target brightnesses. Images with zero bias
level are much more obvious – these images are mostly blank, although bright pixels such
as cosmic rays, strong hot pixels, and very bright targets are sometimes visible. So far the
other three camera channels appear normal and completely unaffected by this anomaly.
The anomaly was first identified in routine dark calibration frames taken on 23 September 2005, but it is clear that the anomaly has been slowly developing for several years.
Figure 1 shows the bias levels of WFPC2 gain=7 images taken between 2003 and late 2005.
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c Copyright 2005 Space Telescope Science Institute. All rights reserved.
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400
CCD Bias Level (biaseven, DN)
350
300
250
200
150
100
50
0
1/1/2003
PC1
WF2
WF3
WF4
7/2/2003
1/1/2004
7/2/2004
1/1/2005
7/2/2005
Date
Figure 1: Bias levels of WFPC2 images between 2003 and the current epoch at A-to-D
converter gain 7. Each point represents a single WFPC2 image. Bias levels in PC1, WF2,
and WF3 are highly stable, whereas WF4 becomes becomes increasingly unstable and tends
toward low values. (Sporadic high points, mostly in WF2, are highly saturated images.)
As can be seen, sporadic images with bias levels significantly below the normal level ∼311
DN begin during 2004, and by February 2005 the first zero bias images are seen. By the
end of 2005 all images in WF4 have below-normal bias levels, and ∼20% have zero bias
level and are blank. The earliest traces of the anomaly appear in early 2002, near the time
of Service Mission 3B, when WF4 first shows sporadic images with bias levels a few DN
below normal.
Images with low bias level suffer two primary issues. Firstly, the images contain faint
horizontal streaks with an amplitude of about 1 DN RMS. Fortunately, these can usually
be removed with appropriate spatial filtering of the image. Secondly, the photometry is
corrupted in the sense that targets appear too faint. The strongest photometric effect is
seen for faint targets at low bias levels. At gain=7 and for bias levels approaching zero, faint
targets are about 40% too faint. At gain=15 targets can be as much as 70% too faint. This
photometric anomaly appears to be caused by non-linearities in the signal chain at very low
voltage levels, i.e. voltage levels not experienced during normal operations. Fortunately the
photometric anomaly appears to be highly deterministic in the sense that a given parameter
set (i.e. gain setting of 7 or 14 electrons per DN, bias level, and target signal in DN per
pixel) always produces the same error, and therefore should be correctable. Observations
specifically aimed at measuring the photometric effects are being carried out (calibration
program IDs 10772 and 10777). Preliminary corrections for both the background streaks
and photometry are discussed by Biretta & Gonzaga (2005).
Images with zero bias level are not correctable. Most of the image pixels will fall below
the “zero” level of the analog-to-digital converter, and hence will register as zero DN. A few
bright pixels may still be visible (cosmic rays, very bright target pixels, etc.) if they rise
above the A-to-D converter zero value.
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WFPC2 Status and Calibration
WF4 Bias Level (DN)
Bay 1 Temp (deg C) x 10
RPLHTR (ON=50/OFF=40)
WF4 Bias Level & Bay 1 Temp
350
300
250
200
150
100
50
0
17-Jul-2005
18-Jul-2005
Date
19-Jul-2005
Figure 2: WF4 bias levels plotted along with Bay 1 temperature and Replacement Heater
(RPLHTR) activations between July 17 and 19, 2005 to illustrate typical behavior. Note
correlation of low bias levels with peaks in temperature. Temperature peaks are driven by
activations of the Replacement Heaters which are indicated on the plot by RPLHTR values
of 50.
The root cause of the anomaly is not yet understood at this time, but investigations
are focussing on a circuit in the WF4 camera head that samples and amplifies the signal
from the CCD. Scenarios involving both component failure and radiation damage induced
failure seem possible.
Incidences of the anomaly appear to be strongly correlated with peaks in the temperature of the WF4 camera head electronics. The anomaly is most severe near peaks in
the temperature (Figure 2), and hence a possible mitigation strategy is to reduce the temperatures and temperature swings of the electronics. The temperature swings are caused
by cycling of the Replacement Heaters inside WFPC2, which serve to maintain the optical bench near 12 degrees C. The heaters are crudely controlled by comparing the Bay 1
temperature against a pair of software set points at 11 and 15 degrees C. The heaters go
on when Bay 1 falls to 11 degrees C, and go off when Bay 1 reaches 15 degrees C. By reprogramming these set points, it should be possible to reduce the temperature of the WF4
camera head electronics where we suspect the problem lies. Obviously some attention must
also be given to the optical bench stability. On-orbit tests of this strategy are planned for
January and February 2006.
[Note added in proof: In January 2006 a test was made with the upper set point of
the Replacement Heater reduced from 14.9 to 12.2 degrees C. This strategy was extremely
successful. The range of WF4 bias values went from 0 to 240 DN, to about 150 to 270 DN.
Most importantly there are no longer zero bias or blank images. No significant changes in
the optical alignment or optical image quality were seen. Immediate plans are to leave the
temperatures at these new settings, and to explore further temperature reductions.]
More detailed information on the anomaly can be found in Biretta & Gonzaga 2005, as
well as later postings on the WFPC2 website. An Anomaly Review Board has been formed
(chaired by Ed Cheng of GSFC) and a detailed report is expected a few months into 2006.
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Number of Hot Pixels per CCD .
4000
3500
3000
PC1
WF2
WF3
WF4
2500
2000
1500
1000
500
0
1994
1996
1998
2000
2002
2004
2006
Year
Figure 3: Number of hot pixels per CCD vs. year. Here we define a hot pixel as having a
dark current in excess of 0.005 DN per second a gain 7, which is roughly the level at which
a hot pixel represents a significant error in a typical 300 second science exposure.
3.
Radiation Damage: Hot pixels and Dark Current
Given WFPC2’s twelve years on-orbit, radiation damage is an obvious concern. Typical
manifestations of damage include increased numbers of hot pixels, increased dark current,
and charge transfer issues.
Figure 3 shows the number of permanent hot pixels in each CCD as a function of time.
As can be seen, all the CCDs show a roughly linear growth in the number of hot pixels
over time. Even at the current epoch these hot pixels represent a very small fraction of the
pixels, and hence have minimal impact on science observations. Moreover, given the small
fraction of hot pixels, dithering of the target position is highly effective in removing them.
Approximately once per month the CCDs are warmed to +20 degrees C both to clear
the cold optical windows on the CCDs (i.e. decontaminate the windows), and to remove the
hot pixels via annealing of the CCD. The plotted data in Figure 3 are from 1800 second
dark frames taken immediately after each of these DECONs, and hence these represent
permanent hot pixels. Near the beginning of 2002 all the CCDs show a sudden drop in the
number of hot pixels. This drop appears to be associated with two events where the CCDs
were left in a warm state for several days (an HST safing and SM3B). Apparently these
events were able to correct additional hot pixels beyond what is typically annealed by the
6 hour DECONs.
The low-level or background dark current which is present in all pixels is illustrated
in Figure 4. This shows an interesting and complex behavior. The different CCDs initially
have values that are nearly a factor of three different, with WF2 being the lowest, and
PC1 being the highest. All the CCDs initially show a linear increase, but around 2000
the increase is slower. Part of the complexity is caused by the fact that some of the dark
current is not electronic, but rather originates in the CCD windows, perhaps as a result of
scintillation. This optical component of the dark current correlates with the cosmic ray flux
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WFPC2 Status and Calibration
Dark Current (DN / 1800s)
2.5
2
1.5
1
PC1
WF2
0.5
WF3
WF4
0
1994
1996
1998
2000
2002
2004
2006
Year
Figure 4: Low-level dark current vs year at gain 7.
in the images, and is strongest in PC1 and weakest in WF2, and probably accounts for the
different dark currents in the different CCDs. Some of the apparent leveling-off after 2000
may also be related to the long warm events near the beginning of 2002. But even after
these considerations, it is not apparent why the dark current increase is slower at current
epochs than after the initial installation of WFPC2. One speculative cause is CTE effects,
which might trap some of the dark current at late epochs. In any case, the dark current
does not appear to have become a significant issue for WFPC2 image quality.
Both Figures 3 and 4 show WF4 having low data points in late 2005. This is due to
the WF4 anomaly producing low counts and sometimes blank images. In fact the anomaly
was first discovered while updating Figure 4.
4.
Radiation Damage: Charge Transfer Issues
The most significant radiation-induced issue for WFPC2 are problems with the charge
transfer efficiency (CTE) in the CCDs. The CCDs are read out by transferring the charge
in each pixel across the CCD chip to a single readout amplifier; CTE problems arise when
some portion of the charge is left behind during this readout process. The problem is wellknown and there are many references (e.g., Whitmore, Heyer, & Casertano 1999; Dolphin
2000). CTE is especially troublesome because it disproportionately impacts faint targets,
and hence will ultimately limit the ability to detect faint objects. It also increases with
time as radiation damage grows. While it was once only a significant issue for faint targets
on faint backgrounds, it now has a significant photometric impact on all brightness levels.
There has not been a detailed trending of its photometric progress during the last
few years, but we believe the previous relations and web-tools are still appropriate (e.g.,
see http://www.stsci.edu/instruments/wfpc2/wfpc2 cte/wfpc2 cte calc.html). For
example, at the end of year 2005 we predict that a moderately bright target (2000 electrons
total) in a typical 300 second F555W exposure near the CCD center will lose about 10% of
its counts or require a correction of 0.10 magnitudes. Even a very bright target approaching
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Figure 5: Average hot pixel tail at epoch 2002. Corresponding hot pixel brightness is 310
DN.
saturation (55,000 electrons total) loses about 5% of its counts, and will require correction
for accurate photometry. The situation is most serious for faint targets – for the same
example a target near the detection limit (66 electrons total) will lose about 40% of its
counts during readout. A faint background further compounds the troubles – if we had
used the F502N filter instead of F555W, the last case would suffer losses of about 90%
during readout.
Most CTE work has focussed on direct measurement of the photometric effects – that
is, measuring target brightness as a function of distance from the readout amplifier. A
new report (Biretta & Kozhurina-Platais 2005) has taken a slightly different approach and
attempted to identify and quantify the charge which is left behind. They have examined
apparent “tails” on hot pixels in the images, and used these to study CTE effects. Hot
pixels are chosen as they are bright single pixels, and provide a simple probe of any charge
which is left behind or which trails behind during readout. Figure 5 illustrates the average
profile of many hot pixels from year 2002, and shows that there is a tail approximately 200
pixels long behind the hot pixel. While the surface brightness of the tail is low, it contains
approximately 9% of the total charge and is equal to the expected photometric losses – i.e.,
this 200-pixel long tail appears to represent nearly all the photometric loss. Information
about the shape and intensity of the tail should be useful in creating an image-based CTE
correction.
One curious result from this study, is that there seems to be significant tails on hot
pixels even near row zero on the CCD. Figure 6 illustrates the integrated intensity in the
tails as a function of CCD Y position. Data are plotted for both epoch 1995 and 2001.
WFPC2 Status and Calibration
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2001
1995
Figure 6: Measured hot pixel tail intensity (integrated) intensity as a function of CCD row
at epochs 1995 and 2001. Note that the tail intensity is non-zero at Y = 0.
While the strength of the tails increase with Y and epoch as expected from photometric
studies, the Y=0 intercept is non-zero. Apparently there is a significant amount of charge
which is trapped at the position where the image is exposed, and then quickly released
during the readout process. This component of photometric loss is not properly treated
in the unusual CTE models which have zero loss at Y=0. Presumably there is nothing
special about the Y=0 location on the CCD, and similarly charge is everywhere trapped at
the position where the image is exposed. This trapping would produce an effect similar in
nature and size to so-called Long vs. Short Effect (Casertano & Mutchler 1998; Whitmore
& Heyer 2002) where images in short exposures are too faint relative to longer exposures of
the same target.
5.
Photometric Calibration
The photometric calibration of WFPC2 continues to be relatively stable. Long-term observations of the standard star GRW+70D5824 are plotted in Figure 7 for the F555W filter.
There is a long-term decline in the sensitivity in all CCDs due to CTE effects, and this decrease is now approaching 4% to 5%. Aside from this long-term decrease, the photometry
is stable to about 1.5% RMS.
The throughput in the UV continues to be subject to fluctuations caused by the slow
build up of contaminants which are then rapidly removed during decontamination procedures. Contaminants slowly accumulate on the cold CCD windows and decrease the UV
throughput. These contaminants are removed by warming the CCD assemblies to +20 ◦ C
for six hours on a regular basis. Early in the WFPC2 mission these decontaminations were
carried out every 30 days, but more recent years the interval has been increased to 45 or
60 days. This increased interval is in response both to an overall reduction in contaminant
levels in the camera, and also represents an effort to increase HST schedule efficiency. The
UV throughput loss per day due to contaminant accumulation varies somewhat between
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1.1
Relative Counts
1.05
1
0.95
PC1
0.9
WF2
WF3
WF4
0.85
1994
1996
1998
2000
2002
2004
2006
Year
Figure 7: Relative counts observed for standard star GRW+70D5824 in filter F555W as
function of time. The long-term sensitivity decrease is due to CTE effects which have not
been corrected in this plot.
CCDs, but for example, in the PC1 channel early in the mission the F170W sensitivity
decreased by about 0.56% per day, whereas the decrease is now about 0.32% per day.
The photometric zero points were recently reviewed by Heyer, et al., 2004. They
compared zero points derived by different groups including STScI, Holtzman 1995, Dolphin
2000, Steston, and Saha. Data included both Omega Cen and NGC 2419. The zero points
from different targets and different workers in the F555W and F814W filters were found to
agree to about 2% RMS. But larger differences of 3% to 4% RMS were found in F439W
and F336W, with the largest differences being about 6%. These differences are much larger
than the Poisson errors, and reflect some unknown systematic error in the photometric
calibrations.
6.
Geometric Distortion
WFPC2 images suffer from mild geometric distortion which is mostly caused by refractive
field flatters located immediately in front of the CCDs (these in fact act as protective
windows for the CCD detectors). At intermediate wavelengths, for example in the F555W
filter, the distortion reaches a total of about 6 pixels (or about 2% by pixel size) in the
corners of the CCDs. As these are refractive elements, the geometric distortion will be a
function of wavelength. Kozhurina-Platais, Anderson, & Koekemoer (2003) have empirically
measured the wavelength dependence of the geometric distortion. They find the distortion
at F300W is ∼3% greater than at F555W, and at F814W it is ∼1% less. These variations
are consistent with those expected from the wavelength dependence of the refective index of
MgF2. It is expected that the distortion will continue to increase into the UV. For example,
the distortion at F255W is expected to be about 5% greater than that at F555W, but this
has yet to be verified observationally.
WFPC2 Status and Calibration
7.
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Calibration Plans
Our current plans for Cycles 13 to 15 are to continue the routine monitors that have
been established over the years (bias frames, darks, internal flats, Earth flats, standard
star monitors, CTE monitors). Many of the WFPC2 filters have not had photometric
calibrations in a few years, so we plan to make standard star observations in all the filters
during the next year or so. We also plan to take observations of very red stars which
may be useful in performing cross-calibrations between WFPC2 and ACS. No doubt many
additional observations will be made to calibrate the WF4 anomaly, and in particular the
low photometric results that accompany low bias levels.
8.
Close-Out Calibrations
We anticipate that WFPC2 will be de-orbited near the end of 2007 during HST service
mission SM4. Due to management concerns, there is also a desire to cease all WFPC2
support around that time. These two constraints together provide impetus for carefully
planning our remaining WFPC2 activities, and beginning that work now. During these
next two years we will embark on a number of activities to finalize and “complete” the
calibration of WFPC2.
Calibration observations: obviously any and all data must be collected prior to deorbit. An important aspect of this is to try to anticipate data which might be needed
or desirable in the future. For example, data that might be useful to cross-calibrate with
future instruments, or data that might be useful to resolve long-standing issues such as
CTE corrections for extended sources or complex scenes.
Primary calibrations: basic calibrations should be finalized and brought up-to-date to
reflect the status of WFPC2 at the end of its mission. This might potentially include new
super-bias and dark reference files, new flats, and current photometric calibrations. Trends
in long-term effects such as CTE should also be updated.
Documentation: final versions of the WFPC2 Instrument Handbook, Data Handbook,
and various web resources must be created to capture the latest information, and ensure it
will be accessible into the future.
Enhancements to WFPC2 legacy: WFPC2 will likely represent a significant fraction
of the total output of HST, so it is interesting to consider projects that might enhance its
value, or make the data more accessible. Some of these projects might include:
1. Image-based CTE corrections. These would properly handle complex and extended
targets, and eliminate the need for future archival users to perform manual CTE
photometric corrections.
2. Photometric history of WPFC2 and connection to other instruments.
3. Enhanced archival products. For example, cosmic ray rejected mosaicked images with
proper photometric calibration.
There are also many areas where the calibration could be improved or made more
secure. Selecting among these will obviously be a question of priorities:
1. Improved photometric zero-points, particularly in the blue and UV.
2. Better photometry for narrow-band and ramp filters. There is sometimes evidence for
errors in the 5% to 10% range whereas ideally the errors should be closer to those for
broad filters.
3. Narrow band filter stability: there is a potential for filter bandpasses to change over
time, and this has not been addressed in any complete fashion for WFPC2.
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4. Calibration of red-leaks: the blue and UV filters have significant leaks, but this has
not been calibrated on-orbit in a thorough manner.
5. Sky flats for primary filters: these might provide some improvement or at least a test
of the accuracy of the existing flats. Given the enormous quantity of data available,
it might be possible to make reasonably noise-free sky flats.
6. Sub-pixel and focus effects on photometry: some early efforts were made, but it
probably remains a significant source of uncertainty.
The HST observer community is welcomed to make suggestions for enhancements,
and moreover, to actively participate in Close-Out calibration activities. Funding as well
as observing time to support such work are potentially available to outside observers via
the Calibration Outsource program and Calibration Proposals. We would greatly welcome
outside participation in any of these projects.
References
Biretta, J., & Kozhurina-Platais, V. 2005, Instrument Science Report WFPC2 2005-01,
available through http://www.stsci.edu/instruments/wfpc2/wfpc2 top.html
(Baltimore: STScI)
Biretta, J., & Gonzaga, S. 2005, Instrument Science Report WFPC2 2005-02 (Baltimore:
STScI)
Casertano, S., & Mutchler, M. 1998, Instrument Science Report WFPC2 1998-02
(Baltimore: STScI)
Dolphin, A. 2000, PASP, 112, 1397
Heyer, I., Richardson, M., Whitmore, B., & Lubin, L. 2004, Instrument Science Report
WFPC2 2004-01 (Baltimore: STScI)
Heyer, I., & Biretta, J. 2005, WFPC2 Instrument Handbook (Baltimore: STScI)
Holtzman, J., et al. 1995, PASP, 107, 1065
Koekemoer, A. 2003, in Proc. 2002 HST Calibration Workshop, ed. S. Arribas, A. Koekemoer, & B. Whitmore (Baltimore: STScI), p. 291
Kozhurina-Platais, V., Anderson, J., & Koekemoer, A. 2003, Instrument Science Report
WFPC2 2003-02 (Baltimore: STScI)
Mobasher, B., et al., 2002, HST Data Handbook (Baltimore: STScI)
Whitmore, B. 1998, in Proc. 1997 HST Calibration Workshop, ed. S. Casertano et. al.
(Baltimore: STScI), p. 317
Whitmore, B., Heyer, I., & Casertano, S. 1999, PASP, 111, 1559
Whitmore, B. 2003, in Proc. 2002 HST Calibration Workshop, ed. S. Arribas, A. Koekemoer, & B. Whitmore (Baltimore: STScI), p. 281
Whitmore, B., & Heyer, I. 2003, in Proc. 2002 HST Calibration Workshop, ed. S. Arribas,
A. Koekemoer, & B. Whitmore (Baltimore: STScI), p. 359