Instrument Science Report WFPC2 96-07
WFPC2 Throughput Stability in the
Extreme Ultraviolet
John W. MacKenty and Sylvia Baggett
May 29, 1996
ABSTRACT
We present a measurement of the stability of the WFPC2 throughput in the Ly-α passband
during its first year in orbit. We find that the throughput has remained unchanged with an
uncertainty of 10 to 20%. In light of the near total loss of Ly-α reflectivity of the WF/PC1’s pickoff mirror during its stay in orbit, this suggests that WFPC2 POM is not likely to
develop the same problem unless a new source of contamination is introduced during
future servicing missions.
1. Introduction
Analysis of the WF/PC-1 front end optics following its return from space have shown that
the reflectivity of the pick off mirror (POM) located in the HST hub area (external to the
WF/PC-1) was coated with a layer of polymerized contaminants ~450Å thick which
reduced the Ly-α reflectivity to less than 1% of its pre-launch value (see Figure 1). The
external surface of the WF/PC-1 aperture plug (a component not present in the WFPC2)
was also significantly contaminated. The heavy contamination present on the cold windows immediately in front of the WF/PC-1 CCDs precluded observations at Ly-α during
the WF/PC-1 mission and the contamination situation of the external optics was therefore
not recognized until the returned elements were analyzed.
The contamination present of the WF/PC-1 optics appears to have required both the presence of molecular material and sufficient UV light to photo-polymerize that material. The
Failure Review Board commissioned to investigate this situation has concluded that a
combination of outgassing from the FGS instruments and UV light reflected from the
earth is the most likely cause of the contamination.
An important consideration is the future performance of the WFPC2 is the potential degradation of the UV reflectivity of its POM. To measure any changes which may have already
occurred and to provide a baseline for future monitoring and testing subsequent to upcom-
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ing servicing missions, we have obtained observations of a spectrophotometric standard
star in the Ly-α passband. This report describes these observations and their implications.
Figure 1: Pre- and Post-Flight POM Reflectivity
2. Experimental Method
Contamination Spectrum
A property of most thin layer contamination situations is that the transmission spectrum
appears as a smoothed discontinuity with very little transmission at wavelengths shortwards of the transition wavelength and essentially normal transmission at longer
wavelengths. As the layer increases in thickness, the transition wavelength increases. The
observed performance therefore shows an initial decline at the shortest wavelengths followed by the spread of the problem to longer wavelengths over time. Therefore
measurements designed to detect relatively small amounts of contamination and to recognize a problem at its onset should be conducted at the shortest available wavelengths.
Standard Star Observations
We obtained observations of the standard star BD+75D325 (an O5 star) in the F122M (Ly
α filter) at two different epochs. This star was first observed with WFPC2 in February
1994 and March 1994 approximately 2 months after WFPC2 was installed into HST
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(SMOV 4763: Trauger). It was re-observed in December 1994 and January 1995 (CY4/
CAL 5778: MacKenty). This provides a measurement baseline covering WFPC2’s first
year in orbit. This star is a strong UV source and, having been used extensively as a photometric standard by HST, is known to have constant flux over time. The particulars of the
observations are listed in Table 1
Table 1. Observations of BD+75D325
Date
Dataset Name
ExpTime
Detector
Filter1
Filter2
1994 Feb 27
u23e5408t
1.4
PC1
F122M
1994 Feb 27
u23e5401t
1.4
WF3
F122M
1994 Mar 13
u23e6108t
1.4
PC1
F122M
1994 Mar 13
u23e6101t
1.4
WF3
F1224M
1994 Dec 22
u2170101t
3.0
PC1
F122M
1994 Dec 22
u2170102t
3.0
PC1
F122M
1994 Dec 22
u2170105t
3.0
WF3
F122M
1994 Dec 22
u2170106t
3.0
WF3
F122M
1994 Dec 22
u2170103t
5.0
PC1
F122M
F130LP
1994 Dec 22
u2170104t
5.0
PC1
F122M
F130LP
1994 Dec 22
u2170107t
5.0
WF3
F122M
F130LP
1994 Dec 22
u2170108t
5.0
WF3
F122M
F130LP
1995 Jan 06
u2170201t
3.0
PC1
F122M
1995 Jan 06
u2170202t
3.0
PC1
F122M
1995 Jan 06
u2170205t
3.0
WF3
F122M
1995 Jan 06
u2170206t
3.0
WF3
F122M
1995 Jan 06
u2170203t
5.0
PC1
F122M
F130LP
1995 Jan 06
u2170204t
5.0
PC1
F122M
F130LP
1995 Jan 06
u2170207t
5.0
WF3
F122M
F130LP
1995 Jan 06
u2170208t
5.0
WF3
F122M
F130LP
Redleak Compensation
Since the F122M filter has a large red leak, we also obtained observations of the same star
during the second epoch observations (late 1994 and early 1995) with the F122M filter in
combination with the F130LP filter. The F130LP effectively blocks all flux shortwards of
130 nm. This permits a direct measurement of the redleak under the assumption that the
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long wavelength throughput of the WFPC2 has not changed. Slightly more than have of
the observed flux in the F122M filter is found to be redleak.
Internal Contamination Compensation
Since WFPC2 contains sufficient internal contaminants to reduce the 170 nm throughput
by approximately 1% per day, the 122 nm throughput is presumably attenuated by at least
that much. These contaminants also effect the redleak to some extent. Rather than attempting to directly calculate the effects of the internal contaminants, we elected to control for
them by obtaining the differential 122 nm measurements with the internal contaminants in
the same state. This is possible because WFPC2 is decontaminated on a 28 day cycle. The
decontamination removes the material built up on the cold (-88o C) windows mounted
immediately in front of each CCD detector and appears to fully restore the UV
performance.
The first epoch observations were obtained on dates 5 and 19 days subsequent to a decontamination. The 22 December 1994 and 6 January 1995 observations were therefore
matched to the same point in the decontamination cycle.
It is important to note that the decontamination cycle has undergone exhibited some
changes over WFPC2 lifetime. During the first several months in orbit there was a gradual
decline in the rate of contaminant buildup. Also, during the first epoch observations the
CCD operating temperature was set at -78o C. The warmer setpoint could conceivable
effect both the rate of growth of the contaminant layer and its composition. We believe
that these effects are small and do not change our final conclusions.
Detector Response Changes and Compensation
The WFPC2 CCDs have a charge transfer efficiency problem which was discovered after
its installation into HST. The problem was largely circumvented by lowering the operating
temperature of the CCD detectors from -78o C to -88o C on 23 April 1994. However, since
the first epoch 122 nm observations were obtained prior to this date, we must correct for
the signal loss which was present in these observations. Photometric monitoring in the
F170W, F218W, and F255W passbands showed an increase in sensitivity at the center of
PC1 of 5% to 8% and approximately 5% for WF3 (with considerably more scatter) when
the operating temperature was reduced.
Flux Measurements
Each frame was manually edited for cosmic ray events and the flux in a 5 pixel radius
aperture was measured. The small radius was selected since contamination might cause
scattering as well as absorption. The mean of an annulus centered on the star was subtracted to correct for any residual background removal errors. The measured fluxes,
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expressed as count rates (DN s-1) are given in Table 2. Observations with potential problems are marked with an “:”.
3. Results
Table 2. Observed Count Rates
Date
PC1
F122M
WF3
F122M+F130LP
F122M
F122M+F130LP
1994 Feb 27
1248
1580
1994 Dec 22
1454
712
1594
890
1994 Dec 22
1471
717
1643
886
1994 Mar 13
1172
1995 Jan 06
1159
548:
1147
603
1995 Jan 06
1173
550
1163
616
1443
As expected, the redleak flux declines from day 5 to day 19 in the decontamination cycle
since the F122M redleak includes a significant component in the UV when observing a hot
star.
Averaging the observations obtained on the same dates, increasing the first epoch count
rates by 5% in both for both PC1 and WF3 (to account for the CTE improvement from
lowering the CCD operating temperature), and subtracting the measured redleaks, we find
the results shown in Table 3.
Table 3. Corrected Count Rates
Epoch
Decon Phase
PC1
WF3
1
+5
595
771
2
+5
748
731
+26%
-5%
Change
1
+19
623
834
2
+19
617
547
-1%
-34%
Change
It is interesting to note that the decline with decontamination phase expected in the 122
nm passband is 18% in PC1 and 25% in WF3 for the second epoch observations but that
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this decline is not present in the first epoch observations. This casts some doubt on the
assumption that the decontaminations are sufficiently repeatable to make a high precision
of the 122 nm throughput.
4. Conclusions and Recommendations
To the limits of these measurements, the 122 nm throughput of the WFPC2 has remained
constant during its first year in orbit. It appears likely that this test would detect a decline
in reflectivity in the POM comparable to that experienced by WF/PC-1 during its first year
in orbit even if we assume that the WF/PC-1’s contamination had been deposited at a uniform rate during its 3.5 years in orbit. Since it is very likely that the WF/PC-1 external
contamination deposition rate decreased with time we may still have the contamination
situation present when the WF/PC-1 was removed.
The detection of a clear signal at 122 nm implies that the surface of the POM has not been
significantly degraded since the installation of the flight mirror at JSC.
It would be prudent to repeat these measurements annually. A test in late 1995 or early
1996 should determine if the apparently large systematic errors arise from the internal
contamination environment during the early phases of the WFPC2’s deployment (which is
our suspicion).
It would also be prudent to repeat this test before and immediately following each future
servicing mission in conjunction with a more extended monitoring of the UV performance
of WFPC2 (e.g. F170W). Such monitoring is necessary both to access the state of the
POM and the internal contamination on the CCD windows.
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