WFPC2 Instrument Science Report 99-03 (rev.)
Summary of WFPC2 SM3A Plans
S. Casertano, S. Gonzaga, J. Biretta, J. Balleza
January 18, 2000
ABSTRACT
We summarize WFPC2 activities and calibrations planned for the emergency third Servicing Mission, currently scheduled for launch in December 1999.
1. Introduction
The third HST Servicing Mission, originally planned for the 1999-2000 time frame, will
consist of two separate flights. The first, SM3A, now planned for the first half of December 1999, is an emergency mission meant primarily to replace the gyroscopes on HST,
three of which have now failed leaving the telescope with the minimum number of gyroscopes for normal operations. Several other hardware items will also be replaced, and the
refurbished FGS2A will be installed. The second, SM3B, is currently planned for the year
2001; at that time the Advanced Camera and the NICMOS Cryocoolers will be installed.
While no astronaut activities are planned for WFPC2, it is very important to ensure that
neither astronaut activities, nor contaminations from new instruments, damage or otherwise alter the performance of WFPC2. This goal was successfully achieved during the
Second Servicing Mission (SM2) thanks to an aggressive plan of contamination verification and control, as well as timely testing of key instrument health and performance indicators. Our plans for SM3A, and the following Orbital Verification phase (SMOV3A),
follow closely those for SM2. Herein we outline the goals of the WFPC2 activities for
SM3A and SMOV3A, and describe specific proposals which implement these activities
and the data analyses which are anticipated.
In an effort to minimize the possibility of photo-polymerization of contaminants, the HST
will be held in a Bright Earth Avoidance (BEA) orientation during the first 12 days after
HST release, with the intention of preventing bright UV light from reaching the HST focal
plane area and other exposed surfaces, such as the WFPC2 Pick-off Mirror (POM). This
orientation places severe restrictions on the targets available at any given time, with each
target remaining available for a period of 8 to 15 days. In addition, the gyroscopes will not
be calibrated until the end of the BEA restrictions, which means that telescope slews will
be limited to a few tens of degrees. As a consequence, the telescope will be unable to
switch from the North to the South BEA regions, or vice versa, until the end of BEA. We
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include a discussion of BEA targets in this document.
2. Goals of the WFPC2 SM3A Plan
The main goals of the SM3A plan are firstly, protecting the health and safety of WFPC2
during and immediately after the service mission, and secondly, evaluating and calibrating
any significant changes in the instrument performance.
2.1 Instrument Health and Safety
The first area of health and safety concern is contamination of exposed surfaces of the
HST/WFPC2 optical train (primary, secondary and pick-off mirror) by substances that
could reduce the UV throughput. The WF/PC-1 showed a very high level of contamination
upon return to the ground, especially at Lyman α wavelengths, where the transmissivity of
the POM was essentially zero. However, after six years in orbit and a servicing mission,
WFPC2 has experienced so far no measurable permanent decrease in the far UV throughput, indicating that the precautions taken (minimization of contaminants in the construction of new components and extensive outgassing prior to launch) have been successful in
preventing significant contamination. SM3A should be quite benign in this respect, as the
main potential source of contamination, the FGS2R, is equivalent to the FGS1R which
was installed during SM2.
Nonetheless, we will follow the procedures developed for SM2 in order to contain possible sources of contamination, and we will monitor the camera properties, especially the
UV throughput, so as to take action as needed should contamination approach dangerous
or unprecedented levels. The maximum contamination considered safe is a throughput
loss of 30% through the filter F170W, the level WFPC2 experiences during its monthly
contamination cycle.
A concern for the POM (and other exposed surfaces) is the possibility of photo-polymerization of molecular contaminants deposited on the mirror surface. The exposure of the
POM to UV light during and immediately after the service mission will be limited by
requiring that the OTA axis remain 2 degrees or more from the bright Earth during the service mission, and for 12 days after HST release (bright Earth avoidance or BEA). This 2
degree avoidance angle is sufficient to ensure that Earth light does not strike the WFPC2
POM. In an effort to evaluate such contamination, the Lyman α throughput will be measured both during and shortly after the BEA period (proposals 8492, 8494 below). If early
contamination is detected, the duration of BEA could be extended to provide further protection, though this might have a significant impact on the SMOV plans of all instruments.
A contingency proposal (8533) is being developed to handle this case.
Another area of health and safety concern is contamination of the cold (-88 C) CCD windows (or field-flatteners). Some contamination of these optics is always present during
normal operation of WFPC2. Typically, contaminants build-up slowly on the CCD windows at a rate corresponding to a 30% per month decline in the F170W throughput. Such
contaminants are removed by a “decontamination” procedure where the CCD windows
are warmed to +22 C for 6 hours. Direct experience thus demonstrates that a 30% contamination (at F170W) can be safely and thoroughly removed without long-term buildup. As
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the contamination rate can be uncertain (and higher) during SMOV, the following precautions will be taken to minimize contamination of the cold optics and to insure that the total
contamination remains always within the safe level of 30%:
(1) WFPC2 will remain in a “protect safe mode” during and immediately after servicing.
In this mode the shutter is closed, and a seldom used F785LP filter is placed in the beam,
to minimize the influx of contaminants from the HST hub area. In addition, the thermoelectric coolers (TECs) are turned off, and the heatpipe heaters are turned on, thereby
increasing the temperature of the camera heads and further minimizing contamination.
(2) The shutter-open time will be minimized during the first month after HST release by
prohibiting WFPC2 parallel observations. In addition, the standard decontamination procedure is modified to limit exposure of the WFPC2 interior to contaminants from the hub
area. The modified procedure (“protect decon”) will be used where in the shutter is closed
and F785LP is placed in the beam, again to minimize the influx of contaminants during
the typical 6 hour procedure. (The “standard” decontamination encourages contaminants
to leave WFPC2 by placing the shutter open and clearing the filter wheel.)
(3) The initial cool-down of WFPC2 after HST release will be accomplished in two steps.
The first is a partial cool down for a two day period at an intermediate temperature of -55
C, achieved at a set point of -43 C with the thermoelectic coolers (TEC) left on, followed
by a protect decon. The second step, cooling the camera heads to their normal operating
temperature of -88 C, will only occur if the intermediate temperature points show no indication of dangerous contamination (see proposal 8491 below). The F170W throughput
will be monitored frequently after cool-down, to measure the rate of contamination buildup and to ensure that the projected contamination at the following decontamination will
not exceed the nominal 30% decline in the F170W throughput (see proposal 8492).
Finally, the basic health and safety of the instrument is monitored by obtaining standard
sets of bias frames, dark frames, internal flats, and K-spot images during orbital verification (see proposal 8498). These are similar to the internal monitors executed throughout
each Cycle. The goal here is primarily to look for unexpected changes in the instrument
which result from the service mission.
2.2 Calibration Verification
The second major objective is to identify, and correct for, any significant changes which
occur in WFPC2 calibration as a result of the Service Mission. To this end, there are proposals which test the status of the WFPC2 focus, flat field calibration, photometric calibration, and PSF properties (proposals 8491, 8495, 8496, 8497 respectively).
The goal of SMOV calibration is not necessarily to return to the same calibration accuracy
which was present before SMOV, but rather to calibrate out major changes so that most
science can proceed. The scope of SMOV is limited, due largely to a desire for an early
return to normal science operations. Hence, it may not be possible to generate corrections
for all possible changes based solely on the SMOV data. For example, a satisfactory new
dark calibration requires in excess of 100 dark frames, but the SMOV calibrations include
only 25 darks. In the event of a significant change in dark calibration, which can easily be
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detected from the SMOV data, these darks would provide sufficient calibration for most
science images until a better calibration could be derived from Cycle 8 calibration data.
2.3 Formal Requirements
Formal requirements for WFPC2 SMOV activities are set forth in Appendix H of the SM3
SMOV Level III Requirements Document (SMO-1000 App. H, June 26, 1998). The document originally addressed a single SM3 mission. A marked-up version setting forth the
modifications needed for SM3A is available at http://www.stsci.edu/smov/
smov3a_requirements_document.pdf. The WFPC2 requirements are:
H.11.1 WFPC II Verification Requirements. The following two assumptions apply to
WFPC II at the point of HST release:
1. WFPC II is in PROTECT–SAFE mode.
2. The WFPC II shutter is closed with the F785LP filter in place.
H.11.1.1 Engineering Activation and Check–Out Requirements
H.11.1.1.1 After release, the instrument shall undergo an active decontamination procedure of at least 12 hours. In the interval between release and the start of the decontamination procedure, the shutter shall remain closed and the Thermal Electric Coolers (TECs)
shall remain off.
H.11.1.1.2 Upon completion of the decontamination procedure, the instrument shall
undergo a contamination verification phase of at least 48 hours which shall be monitored
by the ST ScI. Upon the approval of ST ScI, the instrument shall be cooled to its nominal
operating temperature and the AFMs shall be reset.
H.11.1.1.3 Following TEC turn on, a standard UV stellar monitor shall be scheduled at
least twice during the first week and, starting with the second week, at a declining frequency for the duration of SMOV as part of the WFPC II Cycle 8/9 calibration program.
[The ST ScI will use the UV monitor to determine whether the instrument remains at its
operating temperature or undergoes further decontamination.]
H.11.1.1.4 A short–wavelength (approximately Lyman alpha) stellar observation shall be
performed.
H.11.1.2 Science Verification Requirements
H.11.1.2.1 The baseline instrumental performance and optical alignment and focus of the
WFPC II shall be measured from an initial set of PSF measurements, photometric calibrations and internal calibrations (dark frames, bias frames, K–spot images, and internal flat
fields). Subsequent to this initial set of measurements, all WFPC II calibrations will be
performed as part of the Cycle 8 WFPC II calibration program.
3. Calibration Proposals
The WFPC2 SMOV activities and calibrations are carried out via eight proposals, summa-
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rized in Table 1 and described individually below. One of them (8515) is a pre-SMOV proposal which obtains baseline data on special calibrators (UV bright stars) which are
needed during the Bright Earth Avoidance (BEA) phase of SMOV. The others will be executed through the SMOV period to collect information on the behavior of WFPC2 and to
maintain its health. As usual, complete up-to-date copies of each proposal are available
through the PRESTO WWW page at http://presto.stsci.edu/public/
propinfo.html.
Table 1: WFPC2 SMOV3A Proposals (approximate order of execution)
ID
Proposal Title
Formal
Requirements
Dependencies
-
None
Time
(hours)
Pointed
Orbits
8515
WFPC2 Pre-SM3A UV Baseline
Observations
8491
Part 1
Transition to Hold Mode; Cool
Down
H.11.1.1.1,
H.11.1.1.2
8515, PCS enabled
8492
Contamination Monitor
H.11.1.1.3,
H.11.1.1.4
8491/1, 8515
8491
Part 2
Protect Decontaminations
H.11.1.2.1
8492, end of BEA
8494
Lyman α Throughput Check
H.11.1.1.4
8492, end of BEA
8495
Flat Field Calibration
H.11.1.2.1
8492, end of BEA
8496
Relative Photometry Check
H.11.1.2.1
8492, end of BEA
4
8497
Point Spread Function Verification
H.11.1.2.1
8492, end of BEA
10
8498
Internal Monitor
H.11.1.2.1
8492, end of BEA
18
0
8533
Lyman α Contingency Test
-
Failure of Lyman α
test in 8492.
48
3
168
53
Totals (including contingency):
5
9
68
6
15
24
4
2
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Proposal ID 8515: WFPC2 Pre-SM3A UV Baseline Observations
Purpose: Obtain baseline photometric data for UV bright stars placed in the BEA-allowed
region, to be used as special UV calibrators during BEA phase of SMOV.
Description: Take far UV images of special BEA calibrator stars, essentially duplicating
SM3A exposures in proposals 8492 and 8494. Targets are needed to cover a range of
launch times. At any time there are two BEA-allowed regions, one in the North and the
other in the South, surrounding the instantaneous poles of the HST orbit; due to the HST
orbit precession, potential targets remain available only for a short time (typically 7 to 15
days each). Unlike SM2, HST pointing during BEA will be restricted to one of the two
BEA zones, determined according to the beta angle at release. Therefore targets in either
the Northern BEA or the Southern BEA zone are needed, depending on the launch
(release) date. Coverage is needed from 4 days after release, when WFPC2 is first turned
on, until 12 days after release, when the BEA period ends. The nine targets observed cover
all possible 1999 launch dates, from the original target of October 14 through December
15. Observations consist of exposures in filters F555W, F122M, F122M+F130LP,
F160BW, F160BW+F130LP, and F170W. UV filters crossed with long-pass filters are
used to estimate the contribution of the red leak (particularly strong in F122M) to the total
signal. The BEA targets and their visibility properties are listed in Table 4; all but
GRW+70d5824, which is the WFPC2 primary standard, are observed in this proposal.
One orbit per target, except for the faint WD 0710+741 which requires two orbits. The primary targets for a December launch and a Southern release are WD 0446-789, available in
BEA December 8-21, and WD 0047-524, in BEA December 18-29.
Dependencies: None.
Contingency: If observations fail, re-observe before SM3A. [All observations successfully obtained as of November 15.]
Analysis: Combine images to remove cosmic rays; perform aperture photometry. Measure
F122M, F160BW, and F170W throughputs; cross with F130LP to make corrections for
red leaks. Quick verification of data validity is needed within 3 days of observations to
check for failure; full analysis within 7 days after observation.
Special Requirements: Must execute 0 to 7 days after a decontamination, so as to limit
contamination and obtain a better UV measurement. Original version (targets covering up
to mid-November launch) to execute before end of July 1999. Additional targets to cover
launch delay observed in early November.
Accuracy: Measure F170W throughput to 3% or better accuracy. Measure Lyman α
throughput to 20-40% accuracy (limited by flat field accuracy and red leak corrections).
Products: Pre-SM3A count levels for BEA targets.
Telescope Resources: 9 orbits pointed time.
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Proposal 8491: Transition to Hold Mode; Cool Down; Protect Decontaminations
Purpose: Transition WFPC2 to operate and perform initial decontamination and preliminary cool down to intermediate temperature; perform protect decontaminations throughout
SMOV.
Description: This proposal consists of two parts. Part 1 starts approximately three days
after release by powering up the WFPC2 mechanisms for first time after Service Mission.
The camera is brought from the Protect Safemode to Hold mode, followed by a 16 hour
Protect Decontamination (shutter closed, F785LP in place). The camera is then cooled
down to the intermediate temperature of -55 C. Three observations of the BEA UV
standard, at 24 hour intervals, are used to verify the absence of abnormal contamination.
Focus measurements and internal flats are also taken. The camera then enters a new Protect Decontamination that carries through a scheduling (SMS) boundary, after which it
will be cooled to its normal operating temperature of -88 C and normal operations start
(Proposal 8492). Part 2 consists of two additional Protect Decontaminations, to be carried
out two and four weeks after the initial cooldown to -88 C, with the attendant internals and
photometric monitor.
Dependency: 8515, pointing system enabled for Part 1. 8492, end of BEA for Part 2.
Contingencies: If anomalous contamination is detected, alternate SMS is activated that
leaves WFPC2 in a decontamination state and does not include WFPC2 cooldown to normal operating temperature. Situation will be assessed in real time.
Analysis: Fast turnaround analysis of photometric and focus data in Part 1 is needed to
assess contamination status and activate contingency if required.
Special Requirements: Part 1 external data to be downloaded immediately and transmitted directly for analysis without standard OPUS processing (Quicklook mode). Note that
special commanding is required for protect decons; this proposal contains all the WFPC2
special commanding needed through SMOV3A.
Products: Contamination rates at intermediate temperature. Validation for safe cooldown
to operating temperatute. Focus measurement.
Accuracy: Contamination rate to be measured to better than 3% in F170W. Internal error
of 1 micron for focus measurement.
Telescope Resources: 10 pointed orbits, 92 hours total non-pointed time.
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Proposal ID 8492: Contamination Monitor
Purpose: Monitors UV throughput frequently throughout the first two weeks of WFPC2
activity in SMOV and carries out first Lyman α test.
Description: Take far UV images of special BEA calibrator stars, duplicating pre-SM3A
baseline exposures in proposal 8515. Targets are needed to cover a range of launch times;
see Section 6 for a discussion of BEA pointings. Note that, unlike SM2, HST pointing
during BEA will be restricted to either the Northern or the Southern BEA zone, depending
on the HST orientation at release. [Release will be in the Southern BEA region for launch
on December 6.] Coverage is needed from 4 days after release, when the first WFPC2
observation is planned, until 12 days after release, when the BEA period ends. The nine
targets planned (see proposal 8515) cover possible 1999 launch dates, from the original
target date of October 14 through December 15. UV observations will begin immediately
after cool down to -88 C, and then follow +3, +6, +12, +18, +24, +36, +48 hours, +3, +4,
+5, +8 days after cool down. Observations will switch to GRW+70d5824 after end of
BEA.
Three Lyman α observations, consisting of exposures in filters F555W, F122M,
F122M+F130LP, F160BW, F160BW+F130LP, and F170W, will be taken in the first three
days; UV filters crossed with long-pass filters are used to estimate the contribution of the
red leak (particularly strong in F122M) to the total signal. Results will be used to measure
early Lyman α contamination; if severe, the possibility of extending the BEA period will
be considered. One orbit per target, except for the faint WD 0710+741 which requires two
orbits.
Dependency: Completion of 8515 and 8491 Part 1.
Contingency: In case of anomalously high UV contamination, introduce additional
decontaminations. In case of anomalous Lyman α contamination, consider extending BEA
period and executing Lyman α contingency proposal 8533.
Analysis: Aperture photometry of all exposures, comparison with baseline data.
Special Requirements: Need fast turnaround; data will be processed by OPUS and sent
directly by FTP to analysis team. Analysis done within 12 hours of data receipt.
Accuracy: Expect 2-3% on UV photometry, 20-30% on Lyman α contamination.
Products: Contamination curves.
Telescope Resources: 15 orbits pointed time.
8
Proposal ID 8494: Lyman α Check
Purpose: Measure far-UV contamination of pick-off mirror.
Description: Observation of primary standard (GRW+70d5824) in F122M, F160BW
alone and crossed with LP filters. Measure throughput compared with pre-SM data. See
8492 for more details on Lyman α observations.
Dependencies: Completion of 8492, end of BEA period.
Contingency: None.
Analysis: Aperture photometry in each UV and crossed filter; comparison with baseline
data.
Special Requirements: None.
Accuracy: About 15-20% on Lyman α contamination, primarily limited by the red leak.
Products: Throughput measurement in UV filters and UV crossed with long-pass filters
(to determine red leak contribution) will indicate any drop in throughput at Lyman a,
which could be due to contamination in pick-off mirror. Not a prerequisite to science
observations.
Telescope Resources: 2 orbits pointed time.
9
Proposal ID 8495: Flat Field Check
Purpose: Verify that flat field is unchanged at ~1% level.
Description: Take 120 Earthflats in F502N to check OTA+WFPC2 flat field. INTFLATS
will be taken in filters F336W, F439W, F555W, F675W, F814W, and F502N to provide a
quick check for wavelength-dependent effects; four are taken in each filter at gain 7,
except for F555W where four are taken in each gain. VISFLATS are taken in F336W,
F502N, F555W, F675W, and F814W (two each) to test for wavelength-dependent effects
in calibration-channel optics. Observations will begin after CCDs are operating at -88 C
and after Bright Earth Avoidance ends, and will extend over several weeks. Ideally, execution would begin about 12 days after HST release, at the end of the BEA period, and be
completed over next 4 to 6 weeks.
Dependencies: Completion of 8492, end of BEA period.
Contingency: None.
Analysis: The F502N Earthflats will be flattened with pipeline flats; ones which are too
bright or dark, or with streaks stronger than ~3% peak-to-peak will be discarded. The
remaining ones combined with STREAKFLAT, and examined for agreement with previous flats in this filter. If necessary, a correction image will be generated and used to update
the calibration flats for the pipeline. The INFLATs and VISFLATs will be combined to
remove cosmic rays, and compared against Cycle 8 data from proposal 8457.
Special Requirements: None.
Accuracy: Measure any changes in flat field to ~1% accuracy.
Products: Revised calibration flat fields, if necessary.
Telescope Resources: No pointed time; 10 hours non-pointed time.
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Proposal ID 8496: Relative Photometry Check
Purpose: Verify that photometry is unchanged at 1% to 2% level.
Description: The WFPC2 primary standard GRW+70d5824 is exposed in a wide range of
filters (F160BW, F170W, F185W, F218W, F255W, F300W, F336W, F439W, F555W,
F675W, and F814W) in each CCD (one orbit per CCD).
Dependencies: Completion of 8492, end of BEA period.
Analysis: Aperture photometry is performed on the images. Results are compared with
similar pre-SMOV data in Cycle 8 proposal 8441.
Special Requirements: Execute shortly after a decontamination.
Accuracy: Measure any changes in photometry to 1% to 2% accuracy.
Products: Relative photometric throughput by comparison with pre-SMOV data. Revised
photometric calibration, updates to the photometric tables in the pipeline and SYNPHOT
database as needed. Not a prerequisite to science observations.
Telescope Resources: 4 orbits pointed time.
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Proposal ID 8497: PSF Check
Purpose: Verify that PSF is unchanged across field-of-view and verify OTA focus.
Description: Separate observations for PSF and focus check.
The PSF check consists of two visits. One is a sequence of dithered exposures of the primary standard GRW+70d5824 with a range of exposure times, so that both the core and
the wings of the PSF can be measured. The other is a sequence of dithered observations of
a medium-density stellar field in ω Cen (alternate target NGC 2419) on a 4x4 grid on the
CCD using 0.375 arcsec offsets. The resulting fractional step size is 0.025 arcsec, which
will yield critically sampled PSFs over most the field of view. This is a subset of the PSF
measurements in Cycle 5-8 calibration programs, which use the same field. The proposal
also allows a check for subpixel phase effects on the integrated photometry.
The focus check consists of seven one-orbit focus measurements. Two series of three measurements each will take place shortly before and after the planned opportunity for secondary mirror move at the end of BEA, and will be used to confirm the quality of the focus
attained. A single measurement will take place late in SMOV to verify focus stability.
Each measurement consists of eight exposures on the currently available standard, divided
into two 4-point box dithers, and covers most of an orbit, thus providing a focus point
independent of orbital variations.
Dependencies: Completion of 8492, end of BEA period.
Contingency: If ω Cen is unavailable at the required time, use alternate target NGC 2419.
Analysis: Extract PSFs and assemble critically sampled “composite” PSF using dither
software. Compare against pre-SMOV PSFs. Measure focus via phase retrieval; command
secondary mirror move if needed, ensuring that it does not interact with other instruments’
plans.
Special Requirements: None.
Accuracy: Measure any changes in PSF to 5% to 10% accuracy. Measure focus with a
typical error of 1 µm of secondary mirror despace..
Products: Update PSF library. Also updates to TIM and TinyTIM, if needed.
Telescope Resources: 10 orbits pointed time.
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Proposal ID 8498: WFPC2 Internal Monitor
Purpose: Verify no change in basic instrument health and internal calibrations.
Description: A variety of internal exposures are obtained in order to provide a monitor of
the integrity of the CCD camera electronics in both bays (gain 7 and gain 15), a test for
quantum efficiency in the CCDs, an internal check on the alignment of the WFPC2 optical
chain, and a monitor for possible buildup of contaminants on the CCD windows.
The regular monitoring observations include: 2 F555W INTFLATs and biases each at
gain=7 and gain=15; 2 K-spot images optimized to WF and PC, respectively; 5 dark
frames of 1500 sec. each. The regular monitoring is run once a week. Note that for decon
weeks, the internal observations are part of Proposal 8491/2. In addition, VISFLATS and
UV flats will be run only once, using the internal calibration channel. Five darks will be
obtained at -55 C to attempt a measurement of CTE via cosmic ray tails.
Dependencies: Completion of 8492. End of BEA period for UV flats.
Contingency: None.
Analysis: Combine frames to remove cosmic rays. Compare with similar pre-SMOV
images. Update calibration reference files if large changes are seen.
Special Requirements: None.
Accuracy: Measure any changes in bias and dark frames to ~1.4 electrons per pixel. Measure any changes in INTFLAT to ~1% accuracy.
Products: Update bias and dark reference files, if large changes are seen.
Telescope Resources: 18 hours non-pointed time. No pointed time is needed, but 2 parallel WFPC2 orbits will be affected by UV flats (need 2 hours continuous time to minimize
lamp use).
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Proposal ID 8533: Lyman α Contingency Plan
Purpose: Joint WFPC2/STIS proposal to diagnose source of Lyman α anomaly and verify
return to safe conditions. Contingency proposal executed in case of anomalous Lyman α
data. Requires extended BEA period.
Description: WFPC2 will repeat the Lyman α exposures of the available BEA target,
sandwiched between protect decontaminations. This will determine whether Lyman α
throughput is returning to normal, allowing end to BEA and beginning of normal operations. The concurrent STIS exposures will differentiate between Lyman α contamination
of the OTA and of the pick-off mirror; the former will affect STIS in the same measure as
WFPC2, the latter WFPC2 only. Exposures will be repated until the throughput returns to
normal and exposure to Earth light is deemed safe.
Dependencies: Only activated if 8492 shows anomalous Lyman α contamination.
Contingency: None.
Analysis: Determine Lyman α throughput with the same method used in 8492.
Special Requirements: Extend BEA period.
Accuracy: About 15% in the measured Lyman a throughput.
Products: Allow return to normal operations.
Telescope Resources: 48 hours non-pointed time for decontaminations; 3 orbits for
Lyman α check.
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4. Timeline for WFPC2 SMOV Activities
Table 2 below summarizes the relative timing of the WFPC2 SMOV activities. Relative
dates are given as R+x, where x is the number of days since telescope release; absolute
dates assuming SM launch 12/06, and release on 12/13. The key events are as follows:
- First WFPC2 activation (R+3): transition to hold, 16 hour decontamination, and initial
cool down to -55 C (R+4).
- At -55 C, verify contamination rate and focus position (2 days), then start decontamination and go through scheduling (SMS) break.
- Verify contamination data quickly; if anomalies are present, activate contingency (i.e.,
do NOT cool the camera to operating temperature). Otherwise proceed with cool down
to operating temperature of -88 C (R+7).
- UV contamination measurements continue at short intervals, using special BEA UV
targets until end of BEA (R+12). First Lyman α check also takes place. Contingency:
add decontaminations in case of anomalous UV contamination; consider extending
BEA if substantial Lyman α contamination is detected.
- After end of BEA and gyroscope calibration, begin normal WFPC2 science and additional calibrations (R+15).
- Additional decons 2 and 4 weeks after first cooldown to -88 C (R+21, R+35).
- WFPC2 SMOV activities end and parallels resume shortly afterwards (R+40).
Interdependencies with non-WFPC2 activities occur in two instances. First, the initial
WFPC2 activation requires enabling the Pointing Control System, including the ability to
carry out small slews (< 40 deg). Second, the start of WFPC2 science and the use of nonBEA targets require large slews, and thus the calibration of gyroscopes. All other WFPC2
activities are largely independent of other instruments or telescope events.
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Table 2: WFPC2 SMOV3A Activity Timeline.
Assumes Servicing Mission launch on 12/19/1999 and HST release (R) on 12/25/1999.
Day
Activity
12/25/1999
(R)
HST RELEASE
12/27/1999
(R+2)
WFPC2 TO HOLD;
WFPC2 TO OPERATE
PROTECT DECON
12/28/1999
(R+3)
Comments
Contingency
(WFPC2 IN PROTECT SAFEMODE)
SHUTTER CLOSED, F785LP,
TEC OFF
COOL DOWN: -55˚C
UV MONITOR: 0
FOCUS CHECK #1
12/29/1999
(R+4)
UV MONITOR: +24 HRS
FOCUS CHECK #2
12/30/1999
(R+5)
UV MONITOR: +48 HRS
FOCUS CHECK #3
UV STAR IN BEA ZONE, F170W,
F555W
ANALYSIS OF UV DATA:
12 HRS
CONTAMINATION RATE <30%
LOSS / 2 WEEKS?
NO: RETURN TO
PROTECT DECON
ANALYSIS OF FOCUS
DATA: 3 DAYS
FOCUS NORMAL?
NO: PLAN FOCUS
MOVE (DAY R+14)
NO: ADD DECON
01/02/2000
(R+8)
COOL DOWN: -88˚C
01/02/2000
(R+8)
UV MONITOR: 0, +3, +6,
+12, +18 HRS
CONTAMINATION RATE <30%
LOSS / 2 WEEKS?
INTERNAL MONITOR
BIAS, DARK, INTFLAT, K-SPOT
LYMAN α CHECK #1
BEA UV STAR, F122M, F160BW,
F130LP. LYMAN α THROUGHPUT NORMAL?
NO: POSSIBLY
EXTEND BEA
(CONTINGENCY
AFTER R+12)
01/03/200001/06/2000
(R+9-R+12)
UV MONITOR: +24, +36
HRS, +3, +4 DAYS
CONTAMINATION RATE <30%
LOSS / 2 WEEKS?
NO: ADD DECON
01/03/200001/05/2000
(R+9-R+11)
SECOND SET OF FOCUS
CHECKS
FOCUS CONSISTENT WITH
EARLY MEASUREMENTS?
NO: REDETERMINE
FOCUS MOVE
01/07/2000
(R+13)
UV MONITOR: +5 DAYS
IF LYMAN α CONTAMINATION NORMAL, END BEA
PERIOD
ACTIVATE LYMAN α
CONTINGENCY
PROPOSAL 8533
16
Table 2: WFPC2 SMOV3A Activity Timeline.
Assumes Servicing Mission launch on 12/19/1999 and HST release (R) on 12/25/1999.
Day
Activity
Comments
01/09/2000
(R+15)
OPPORTUNITY FOR
FOCUS MOVE
CONTINGENCY IF FOCUS
POSITION ANOMALOUS
01/10/2000
(R+16)
* SCIENCE START *
NO PARALLELS
Contingency
UV MONITOR: +8 DAYS
01/15/2000
(R+21)
01/16/2000
(R+22)
FLAT FIELD CHECK
EARTH FLATS IN F375N &
F502N
PSF CHECK
STANDARD STAR AND STAR
FIELD, F555W
FOCUS CHECK
STANDARD STAR, F555W
PRE-DECON CONTAMINATION MONITOR
STANDARD STAR, F170W ALL
4 CCDs
DECONTAMINATION: +
2 WEEKS
PROTECT DECON
POST-DECON CONTAMINATION MONITOR
THROUGHPUT RESTORED?
LYMAN α CHECK #2
STD STAR, F122M, F160BW,
F130LP
PHOTOMETRY CHECK
STD STAR, ALL CCDs
NO: RETURN TO
PROTECT DECON
INTERNAL MONITOR
01/22/2000
(R+28)
INTERNAL MONITOR
01/29/2000
(R+35)
PRE-DECON CONTAMINATION MONITOR
DECONTAMINATION: +
4 WEEKS
01/30/2000
(R+36)
POST-DECON CONTAMINATION MONITOR
THROUGHPUT RESTORED?
INTERNAL MONITOR
02/03/2000
(R+40)
FOCUS CHECK (ONE
ORBIT)
VERIFY FOCUS STABILITY VS.
EARLY SMOV DATA
* END WFPC2 SMOV *
START PARALLEL SCIENCE
17
NO: RETURN TO
PROTECT DECON
5. Data Analysis Plans
Some of the WFPC2 proposals for SMOV3A are time-critical, since their results might
indicate health and safety concerns for the camera (especially excessive contamination)
and/or affect subsequent SMOV activities. Time-critical proposals are assigned both a primary analysis team and a backup, and a firm time limit is given for their analysis. Timecritical proposals with a turnaround time of less than three days also require special data
handling, including dedicated data downlinks from HST and special processing at STScI.
Specifically, the results from proposal 8491 Part 1 must be available 12 hours after the
observations are taken. “Quicklook” processing (direct FTP delivery without OPUS calibration) will make the data available within 6 hours, leaving a 6 hour window for the analysis. Observations for proposal 8492 need to be analyzed within 1 day, and the data will be
delivered directly via FTP after OPUS processing. Non time-critical proposals are only
assigned a single analysis team, and their analysis must generally be completed within
four weeks.
Table 3 identifies data analysis units, with teams, time scales, and backups where needed.
No team is listed for proposal 8492, which will last for over a week and requires roundthe-clock coverage for the first several days. Analysis of proposal 8492 will be split among
several WFPC2 group members, but specific assignments will be made once the execution
dates are final.
Table 3: Data analysis teams and time requirements
Activity
Proposal
Primary
responsibility
Backup
Turnaround Time
(after data acquired)
Transition to hold, decon and
cool down
8491 Part 1
Balleza, Casertano
Biretta
On-call availability in
case of trouble
Contamination at -55 C
8491 Part 1
Casertano, Gonzaga
Baggett, Heyer
12 hours
Focus
8491 Part 1
Lallo
Casertano
3 days
Contamination Monitor
8492
Casertano (coord.)
TBD
1 day
Protect Decontaminations
8491 Part 2
Casertano
Lyman α Throughput Check
8492, 8494
Baggett
Flat Field Check
8495
Koekemoer, Biretta,
Wiggs
4 weeks
Relative Photometry Check
8496
Schultz
4 weeks
PSF Check
8497
Riess, Gonzaga,
Casertano
4 weeks
Internal Monitors
8498
O’Dea
4 weeks
Baseline UV Stars - Pre-SM
8515
Casertano, Gonzaga
3 days quick check
2 weeks analysis
1 week
Casertano
18
8492: 1 day
8494: 1 week
6. Targets for Bright Earth Avoidance
During the first 12 days following HST release, the spacecraft will be held in a Bright
Earth Avoidance (BEA) orientation. This severely limits the region of the sky which is
accessible, and requires special targets to facilitate early calibrations (i.e. UV Monitor /
Cool Down, Focus Check, and Lyman α proposals). The accessible sky region at any one
time during BEA is the region within 22 degrees of the instantaneous poles of the HST
orbit, at declination +/-61.5 degrees. The poles of the HST orbit precess with a period of
56.1 days, thus any target remains within the BEA region for a limited time, up to 2 weeks.
Because the three new gyroscopes will not be calibrated until after the BEA period, HST
will also be limited to small slews during BEA. Thus BEA pointing will be restricted to
either the Northern or the Southern BEA region, depending on the original telescope
release. Depending on the release orientation, either Northern or Southern targets must be
readied for different times. Note that BEA availability is needed between 4 and 12 days
after release.
Ideally, BEA targets must be hot and UV-luminous, to minimize the effect of the red leaks
on the UV and Lyman α contamination measurements. We have identified suitable targets
from the Catalog of Spectroscopically Identified White Dwarfs, 3rd Edition (McCook and
Sion 1987, ApJ Suppl 65, 603), which is available electronically from the NASA Astronomical Data Center at http://adc.gsfc.nasa.gov. We have selected preferentially DA2 and DA3 dwarfs with V magnitude between 12.5 and 15 , resorting to a DA4 in
only one case. Table 4 lists the properties of the identified targets and their availability.
Table 4: Targets for Bright Earth Avoidance Calibrations
Target
R.A.
(J2000)
Declination
(J2000)
V
U-B
B-V
BEA Avaliability Period
(year.day:hour)
Class
Northern targets (for release on days 308-342 and 353-365)
WD 0214+568
02 17 33.2
57 06 47
13.68
-0.92
-0.12
DA2
2000.010:19 - 2000.024:00
WD 0710+741
07 17 02.4
73 56 52
14.97
-0.75
-0.06
DA3
1999.362:21 - 2000.013:12
WD 1104+602
11 07 42.8
59 58 34
13.80
-0.81
-0.02
DA3
1999.355:00 - 2000.003:18
GRW+70d5824
13 38 51.8
70 17 08
12.77
-0.84
-0.09
DA3
1999.347:22 - 1999.363:18
WD 1713+695
17 13 05.9
69 31 36
13.27
-0.70
0.07
DA3
1999.341:04 - 1999.353:18
2000.030:18 - 2000.046:13
WD 2126+734
21 26 57.5
73 38 46
13.09
-0.65
0.02
DA4
2000.020:22 - 2000.036:15
Southern targets (for release on days 294-307 and 343-352)
WD 0047-524
00 50 03.3
-52 08 17
14.20
-0.79
-0.02
DA2
1999.353:07 - 1999.363:11
WD 0446-789
04 43 46.7
-78 51 51
13.47
-1.02
-0.10
DA3
1999.342 - 1999.356
WD 2039-682
20 44 20.6
-68 05 16
13.53
-0.78
0.11
DA3
1999.303 - 1999.317
19