WFPC2 Instrument Science Report 96-01
INTERNAL FLAT FIELD MONITORING
Massimo Stiavelli and Sylvia Baggett
1 Introduction
The study of internal WFPC2 at elds and their temporal behaviour allows
for the monitoring of a number of instrumental features, including ADC
performance, stability of gain ratios, and at elding stability.
The aim of this report is to give an overview of the monitoring of these
various aspects. Most of the results presented are derived from two cycle 5
calibration proposals (ID 6189 and 6190), although we have also made use
of frames from cycle 4 proposals (ID 5561, 5655, 5764).
The visible cal-channel calibration system (VISFLAT mode) is based
upon two Carley bulbs operating at about 2060 K and one Welch Allyn bulb
operating at 2550 K. The Carley bulbs were used previously in WF/PC and
have been reliable. Since less was known about the reliability of the Welch
Allyn (hereafter WA) bulbs, a back up bulb was provided. The backup bulb
is not currently in use but it is activated in the event the primary bulb
fails. In order to obtain the desired at and smooth spectrum, the light
from these lamps was ltered through a combination of BG 38 and BG 24
glass windows. A dierent set of bulbs of the Carley type are also used for
illuminating the shutter blades and obtaininig INTFLATs.
We will address the results obtained from VISFLATs and INTFLATs
separately.
1
2 VISFLATS
Flat elds obtained with the visible cal-channel lamps have been inspected
for all visible WFPC2 lters longward and including F300W. The relative
count rates are given in Table 1. From the count rates we have derived
a spectrum of the VISFLAT light by adopting the instrument eciencies
listed in the WFPC2 Instrument Handbook (Figure 1).
A very signicant pinhole in F300W has been identied just outside the
eld of WF3 (see Figure 2). Although previously not reported and not listed
in the thermal vacuum pinholes table (page 6-49 in TV Report), this pinhole
was also visible in cycle 4 F300W visats and therefore has not developed
recently. It is not visible in the pipeline F300W ats nor in the earth ats
taken during cycle 5, probably due to the dierent optical paths between
VISFLATs and external data. We conclude that the pinhole is unlikely to
aect any science data. Additional pinholes are visible also in the same lter
in WF2 and WF4 at the level of one percent or less, but they are much less
prominent in the pipeline at elds and are also unlikely to aect science
observations.
We checked whether VISFLATs taken with short exposure time showed
any systematic dierence in structure (after applying the shutter correction),
but none has been seen to 0.5 % level. Therefore we conclude that the shutter
corrections are consistent.
Special attention has been devoted to the analysis of the wavelength
dependence of the ratios between VISFLATS and the corresponding pipeline
at elds which were obtained from a combination of ground based data
and on orbit earth calibrations. If this ratio turned out to be independent
of wavelength one would have had the option of an alternative strategy
for producing at elds, by taking VISFLATS and correcting them with a
purely geometrical term.
In order to investigate this eect we rst \at elded" VISFLATS taken
at dierent wavelengths by using the standard pipeline atelds and then
computed their ratio. If the structures in the \at elded" VISFLATs are
purely geometrical and wavelength independent this ratio should be spatially
uniform. As shown in Figure 3 this is not the case. Dierences are present
at all spatial scales from the whole chip down to diraction features. The
typical size of these dierences is a few percent peak to peak for the whole
frame and about 2 per cent in the innermost 400 by 400 pixels area.
In order to verify whether these dierences were more geometrical in nature or dominated by a pixel to pixel variations, we have smoothed the at
2
Filter
F300W
F336W
F380W
F390N
F410M
F437N
F439W
F450W
F467M
F469N
F487N
F502N
F547M
F555W
F569W
F588N
F606W
F631N
F656N
F658N
F675W
F791W
F814W
F953N
F1042M
PC1
0.45
0.82
10.8
0.24
2.9
0.7
16.2
70.0
11.8
1.3
2.1
2.8
167
838.5
385.7
29.9
755.3
31.0
24
33.3
1074
864.3
1052
14.9
37.5
WF2
1.86
3.43
44.6
1.04
12
2.9
68.4
290.4
47.6
5.2
8.8
12
700
1739
1604
122.2
3136
125.1
99
134.3
4432
3575
4333
60.2
160.3
WF3
2.02
3.46
44.3
1.01
12
2.9
69.7
302.7
50.2
5.5
9.0
12
715
1800
1659
130.0
3229
129.0
101
137.4
4545
3715
4485
61.7
162.0
WF4
2.00
3.31
42.5
0.94
11
2.8
65.0
286.3
47.6
5.1
8.5
12
690
1727
1598
126.3
3124
127.9
96
135.9
4417
3646
4405
60.1
157.0
Table 1: Count rates (counts/s) in dierent lters for calibrated VISFLATs.
3
Figure 1: The VISFLAT light spectrum derived from the count rates in the
individual lters corrected for the eciencies listed in the WFPC2 Instrument Handbook.
4
Figure 2: F300W VISFLAT on WF3. A pinhole aecting counts at more
than 10 per cent level is visible in the top part of the frame. The pinhole
center is outside the eld of view.
5
Figure 3: Ratio of the VISFLATS on WF3 in F555W and F814W after each
has been atelded using the corresponding pipeline at.
6
eld ratios with a 3 by 3 pixel gaussian box. The variations remain essentially identical. Once the smoothed frame is subtracted from the original the
residual frames appear dominated purely by noise. Therefore pixel-to-pixel
variations seem to be properly corrected by the pipeline at elds.
3 FLAT FIELDING STABILITY
The availability of VISFLATS taken regularly allows us to monitor both the
at eld stability and the stability of the cal-channel lamp. The previously
detected slow degradation of the cal-channel lamp is conrmed and its status
continues to be monitored. In Figure 4 we show the ratio of mean uxes in
VISFLATS taken about 1 year apart. Each measurement has been carried
out on the innermost 400 400 pixels of a cosmic ray substracted mean of 6
VISFLATS, and is characterized by rather low noise. We observe signicant
variations at F555W and F675W and smaller variations in the other lters.
The ux variations that we detect are attributed mostly to a degradation of
the cal-channel lamp since the photometric monitoring does not show any
similar trend.
Alternatively, one could monitor these variations by plotting the relative
intensity as a function of time, number of exposures, number of lamp cycles,
or number of hours of lamp \on" time. The clear result is that the degradation appears to be dependent on lamp usage. Although from an engineering
point of view the number of lamp cycles should be the relevant quantity, our
data do not denitively favour the correlation with number of lamp cycles
over the one with total number of hours of usage.
In Figure 5 we show the relative intensity as a function of lamp cycles. The cycles are counted as number of cycles since launch, however the
systematic monitoring program only started after about 100 cycles. In addition, about 100 cycles were used during ground testing of the camera. The
data for the dierent CCDs have been tted independently with a second
order polynomial, but the resulting coecients are comparable for all chips.
Although our data are not adequate to rmly establish the validity of the
second order interpolation, the presence of an increase in slope with time
is well established. The good agreement of the tted coecients for the 4
chips gives us enough condence to attempt an extrapolation of the ts.
By extrapolating the available data we expect to cross 80 % eciency in
about 200 more cycles and reach zero eciency in less than 1000 cycles, i.e.
less than a factor 2.5 below the expected lifetime. Better data would be
7
Figure 4: Time degradation of the cal-channel lamp over a one year period
for dierent lters.
8
required to improve our throughput estimates. The best interpretation to
date is that the Welsh Allyn bulb is deteriorating and becoming dimmer and
hotter; as a result, the ux is mostly lost at intermediate wavelengths (cfr
Figure 4). More detailed models would be required to rmly identify the
WA bulb as responsible for the overall decrease in throughput. To extend
the lamp lifetime, VISFLAT usage will be reduced starting from February
1996.
Despite the slow degradation of the lamp, we can determine how stable
the at elds are spatially. A rst estimate is obtained by checking the mean
uxes in the innermost 400 by 400 area on each chip. In Figure 6 we show,
using dierent symbols for each chip, how the ratios have varied during 19941995 for the dierent lters. In order to correct for the lamp degradation
eect, the uxes have been normalized to the ux in the chip WF3. All
variations are signicantly smaller than 1 per cent, suggesting that WFPC2
at elds should be spatially very stable. More detailed studies were carried
out for each chip by examining image ratios and measuring the amount of
structure. In Figure 7 we show how the ratio of minimum to maximum
mean count rates in areas of 50 by 50 pixels vary for the WF3 chip for the
dierent lters. Similar results have been found for the other chips, thus
conrming the geometrical stability of the WFPC2 at elds.
4 DECONTAMINATION
VISFLATs have been taken regularly before and after each decontamination.
The data allow tracking of the eciency variations due to contaminants
and their removal by the decontamination. This eect is shown in Figure
8 where for various lters we show the ratio of counts before and after
decontamination. The removal of contaminants appear to slightly increase
the eciency in the UV and to slightly decrease it in the optical. Note
however that the contaminants also introduce spatial dependencies in the
at elding. The results of decontamination on the photometric accuracy
will be addressed in a separate report. Here, we simply intend to note that
contamination has eects smaller than 1 per cent on the mean at eld
uxes. Since the photometric monitoring program shows a larger eect we
conclude that contaminants are mostly causing a scattering of light to large
angles, so that the photometry of point sources is more aected than the
at elds.
9
1
1
0.98
0.98
0.96
0.96
0.94
F555W,PC1
100
200
0.94
300
100
Cumulative Lamp Cycles
200
300
Cumulative Lamp Cycles
1
1
0.98
0.98
0.96
0.96
0.94
F555W,WF4
F555W,WF2
100
200
0.94
F555W,WF3
100
300
Cumulative Lamp Cycles
200
300
Cumulative Lamp Cycles
Figure 5: Relative intensity of the visual cal-channel lamp for the F555W
lter as a function of the number of lamp cycles for the dierent CCDs. The
relatively larger scatter observed for WF2 and WF3 is probably due to the
presence of the most signicant contamination worms in these cameras. The
solid lines are ts to the data.
10
Figure 6: Global spatial stability of the WFPC2 VISFLATs. We plot the
ratios of the mean values of CRREJ combined images in the innermost 400
by 400 pixels. The lamp degradation has been subtracted by normalizing
the uxes to the mean pre and post decontamination ux in WF3.
11
Figure 7: Time stability of at eld structures for the WF3 chip. We plot
the ratio of minimum to maximum counts for six 1994 exposures divided by
the same ratio for six 1995 exposures.
12
Figure 8: Ratio of the mean uxes within the central 400 by 400 pixels
measured before and after decontamination for various lters. Contamination appears to decrease the UV eciency and increase the eciency in the
optical.
13
5 COMPARISON OF VISFLATS, PIPELINE FLATS
AND EARTH FLATS
Our major concern was not the present of structures in the at eld ratios
but their wavelength dependence. In fact, if the ratios of at elds turn out
to be independent of wavelength, one can easily obtain one from the other
with very high accuracy. In practice, we nd that the product of earth ats
with pipeline ats (we use the product since the pipeline ats are actually
inverse ats) and the product of VISFLATs and pipeline ats depend on
wavelength. Consequently we have also checked whether the wavelength
dependence is similar for VISFLATS and earth ats. This would indicate
that VISFLATS could be used to produce improved at elds if securing
earth ats became not feasible in the future.
Since earth ats in optical broad band lters (F439W, F555W, F675W,
F814W) saturate and an initial test of dark earth observations result in
very low S/N images, we use as reference ats earth calibrations obtained
with F336W,F390N, and F953N, and also the F606W and F814W sky at
elds kindly provided by the Medium Deep Survey group. These lters were
chosen so as to be at very dierent wavelengths from the better tested 5000
A region. Our earth ats were obtained by inspecting few tenths of recently
acquired images and then coadding those without noticeable streaks and
with the best S/N. Four to eight dierent frames were used for each at
eld.
We nd that the wavelength dependence of VISFLATs is dierent from
the pipeline ats and the earth ats. The ratios of these dierent at elds
vary both with position and wavelength. Earth-ats and VISFLATs are \at
elded" by multiplying them by the pipeline (inverse) at. In the case of
the MDS sky at since they have already been inverted we have dened the
\at elded" sky at as the pipeline at divided by the sky at. In Figure
9 we show how the ratios of minimum to maximum counts in 40 by 40 pixel
areas vary as a function of wavelength for the \at-elded" earth-ats and
the \at-elded" VISFLATs. Most of these dierences are across the chip,
while for the innermost 400 by 400 central pixels in each chip, variations tend
to be smaller than 1 %. Once the whole chip area is considered, variations
increase to 6.5 % peak to peak for the F336W and F814W VISFLATS and up
to 16 % peak to peak for the F390N VISFLAT. Discrepancies are typically
larger for the earthats. The comparison of F606W and F814W with the
MDS sky at is instead quite good, with less than 2% and 3.6 % per cent
14
discrepancies peak to peak, respectively, for the WF chips and the PC.
These tests suggest that VISFLATs will not allow us to improve at
elding accuracy beyond that obtained by the latest delivery of pipeline
ats. However, VISFLATs appear adequate to monitor a change in at eld
structures and could probably be used to generate dierential at elds if
any such changes were to be detected.
6 INTFLATs
INTFLATS are calibration images taken with Carley-type bulbs and with
the shutter closed. The light from the INTFLAT lamps is reected o the
shutter and illuminates the CCDs. The dierent reectivities of the two
shutters produce dierent INTFLAT patterns. The INTFLATs have been
analyzed similarly to the VISFLATs. The major dierence between the
two types of at elds is the very large spatial variability of INTFLATs.
Monitoring of the INTFLAT throughput in F555W (which is the one most
routinely monitored) did not show any indication of a decrease in intensity
of the lamps, further indication that the decrease in VISFLAT throughput
may be due to the WA lamp. If anything, we nd hints for an increase in
(the Carley bulbs) intensity which is currently not understood.
Since INTFLATs were taken with both shutter blades and both gain
values, one can check the stability of these ratios. We have focussed on
the time dependence of the ratio between dierent gains. As shown in
Figure 10, the ratios of the two gains are constant with time to a high
degree, guaranteeing that one can safely calibrate one gain and transfer the
calibration to the other. We derive the following gain ratios for the 4 CCD
chips: PC1 1.946, WF2 1.972, WF3 1.980, WF4 1.930. Holtzman et al.
(1995) found, respectively, 1.987, 2.003, 2.006, and 1.955, so that the ratios
of our gain ratios versus the Holtzman et al. values are: PC1 0.979, WF2
0.985, WF3 0.987, WF4 0.987. The absolute agreement is therefore better
than 1.5 % for the WF chips and about 2 % for the PC which is also
characterized by worst statistics. Since our ratios are aected by possible
variations in lamp luminosity during the lamp cycles, one could allow for
an overall correction. By doing so, all the WF chips gain ratios are in
agreement with Holtzman's to within 0.1 %, while PC1 diers by 0.8 %.
Since Holtzman et al. attribute a 1 % accuracy to their measurements, we
consider the agreement between the two measurements to be very good.
15
Figure 9: Ratio of minimum to maximum ux for \at-elded" sky-earth
at elds and VISFLATs. Dierent symbols are used for the individual
chips.
16
1.05
1.05
F555W,PC1,A
F555W,WF4,A
1
1
0.95
0.95
J.D.
J.D.
1.05
1.05
F555W,WF2,A
F555W,WF3,A
1
1
0.95
0.95
J.D.
J.D.
Figure 10: Ratio of whole chip intensities of INTFLATs taken with shutter
A and gain 7 and 15. Results for shutter B are similar. The ratio appears
to be stable and in agreement with the gain ratios determined by Holtzman
et al. The gain 15 exposures are twice as long as the gain 7 ones.
17
7 SUMMARY
Our results from a study of internal at eld images can be summarized as
follows:
1. VISFLATs
a new pinhole has been identied in F300W
the ratios of VISFLATs to pipeline at elds are wavelength de-
pendent at all scales
an acceleration in the lamp degradation has been identied. The
higher temperature Welch Allyn is identied as the most probable
cause of the degradation
at elds appear geometrically stable to better than 1 %
contamination aects mean at eld intensities averaged over 400
400 pixels by less than 1 %
2. INTFLATs
no throughput decrease with time is observed
gain ratios appear stable to better than 0.1 %
8 Acknowledgement
We thank Jesus Balleza for providing us with engineering data.
18