WFPC2 Proposing FAQs

General:

Q: Is the TAC likely to frown on deep imaging with WFPC2 given the presence of ACS?

A: The TAC will be told to select the best science that can be done with existing instruments in Cycle 16. Of course the TAC has some freedom to do what they want. There is always some argument to use what we have now to its fullest, since we don't know what will happen in the future.

Scheduling and Overheads:

Q: What factors affect the overhead times?

A: The overheads primarily include time to select filters and readout the CCD arrays. In addition, most activities involving the WFPC2 must occur on 1 minute clock events to assure synchronization between HST computers and the WFPC2's internal microprocessor.

Q: Why is there inconsistency between the WFPC2 Instrument Handbook and the Phase 1 Proposal Instructions regarding overhead times?

A: The Phase 1 instructions should be used in the generation of Phase 1 proposals. They reflect past experience of average overhead times. The Handbook explains some of the causes of the overheads but does not necessarily include all possible overheads. In Phase II, APT (Astronomers Proposal Tool) will impose the correct overhead times.

Q: What is the maximum number of WFPC2 exposures obtainable per orbit?

A: This is limited by the tape recorder capacity or the sum of the overhead times. Approximately 20 4-chip readouts can be stored on the onboard tape recorders at any one time. However, several orbits may elapse before the tape recorders are downloaded to the ground so it is generally not practical to obtain 20 exposures every orbit for a long sequence without special effort. In principle it is possible to have <= 2 minutes of overhead for each exposure if the filters are not changed. Phase 2 proposers considering complex exposure sequences should consult with an Instrument Scientist to consider the details of their program.

Q: What is the overhead time for SAM's (Small Angle Maneuvers; ie, moving the pointing of the telescope slightly while on the set of guide stars)?

A: The overhead time is minimal (~10 sec) unless new guide stars are required. In that case, a new acquisition is required. In Phase I, the overhead time for SAMs can be ignored.

Photometry:

Q: I have heard that there is a lot of cosmic ray damage to WFPC2 so that the sensitivity is down, and the ramp effect is larger. Is this true?

A: The sensitivity is the same as always within 1 or 2%. It is actually increasing in the far-UV as some contaminants slowly out-gas.

Most of the evidence for increasing ramp effect is for faint targets in 14 sec. exposures. For those exposures there is an increased ramp. But it is not at all clear what happens for a deep image at say 1200 sec. Our expectation is that the sky background will greatly reduce the ramp effect in longer exposures in broad filters, so you may or may not see the increased ramp. We expect that most observations will reach 3% - 5% photometric accuracy, and perhaps greater accuracy once corrections for the ramp are applied. We are also working on various on-orbit solutions; some of these might make the ramp go away completely.

Q: Which filters are routinely monitored and/or best calibrated?

A: All visible light filters have been calibrated and are relatively stable with time. The UV response varies with time due to the buildup of contamination (currently removed every 49 days). The core filter set for monitoring includes F160BW, F170W, F218W, F255W, F336W, F439W, F555W, and F814W. Both temporal and spatial performance is being tracked. When choosing a filter, the observer should consider the requirements of their scientific program, the transformability of the filters selected (see The Photometric Performance and Calibration of WFPC2, by Holtzman et. al.), the exposure time efficiency, and the integration of their observations into other HST observation datasets.

Q: What is the recommended "I" filter?

A: The F814W filter is generally considered to be the standard WFPC2 "I" filter. F814W transforms very nicely into the Cousins I band (but not into Johnson I).

Q: Why is the F439W filter recommended despite the higher throughput of the F450W filter?

A: The F439W filter is a closer match to the Johnson B filter.

Q: Which V filter should I choose, F555W or F606W?

A: While F606W has about 55% higher throughput than F555W, F555W will generally give better photometry. This is for several reasons:

1) F555W is likely to have better photometric calibration since wedo routine photometric monitoring in F555W. F606W could be tied to monitoring in nearby filters, but connecting it to monitored filters probably introduces systematic errors at 1 or 2% level.

2) The shape of F606W is less well behaved than F555W. The response curve of 606 has some sharp bumps where the response varies by 40% over a few hundred Å. Hence it might give odd colors (at several x 0.01 mag) for objects with strong emission/absorption features.

3) Because the filter is very broad, the image will be more sensitive to color-dependent flat fielding errors. That is, the response curve of the system will be a stronger function of the CCD response curves, which are slightly different from chip to chip. Differences between the spectral properties of light source used to take the flats and the target spectrum will be more important, and lead to flat fielding errors.

All these effects are at the few percent level. If you need 1-2% photometry, then use F555W. If you are limited by photon statistics at the few % level, then F606W might be better choice. If it is important to tie the observations to standard photometric scales (Johnson V, etc.), then F555W is better choice.

UV Observations:

Q: Have the WFPC2 Quantum Efficiency and Redleaks been characterized in the UV?

A: The QE appears to be considerably less than pre-deployment expectations. This is thought be to due to the calibration of the reflectivity of the internal mirrors and is not expected to degrade further. The Instrument Handbook throughputs reflect the measured on-orbit QE. The redleaks have not yet been characterized on orbit. There are indications that the F218W filter has a gradient in its redleak of a factor of 2 over the WFPC2 field of view.

Special Observing Modes and Strategies:

Q: Is there any advice for Proposal Strategies during Phase II?

A: Yes! See the Resources for Phase II Proposal Development page, as well as the chapter on Observation Strategies in the WFPC2 Instrument Handbook.

Q: What are reasonable dither strategies for cosmic rays, warm pixels, undersampling?

A: There is no single observing strategy that is entirely satisfactory in all circumstances for WFPC2. One must consider cosmic rays, warm pixels (i.e. pixels with high and time variable dark count), spatial undersampling of the image, and trading signal-to-noise for ability to recognize and deal with these features. The optimal strategy chosen depends crucially on the scientific question: is the underlying structure totally unknown, is spatial resolution of paramount importance, is photometric accuracy the most crucial aspect, etc.?

1) Cosmic Rays: The best way to deal with cosmic rays is to CR-SPLIT the exposures (take multiple exposures at a FIXED image location). Note that even with two exposures taken at a fixed position there will be some cosmic rays that overlap. As an example, for an observer that has two 2000s exposures, about 1000 pixels per chip will be unrecoverable because they have been hit in both images. Furthermore, because CR events typically affect ~7 pixels per event, these pixels will not be independently placed, but rather will frequently be adjacent to other unrecoverable pixels.

2) Warm Pixels: There are three ways to deal with warm pixels: correct using "dark frames" that bracket the observation (presently obtained weekly), obtain a second image (or pair of images to best reject cosmic rays) shifted by a small amount spatially (e.g. about 5 pixels), or use a program such as 'cosmicrays' in IRAF to filter out the obvious warm pixels.

3) Undersampling: In order to maximize spatial resolution, an observing strategy that is being used by a number of observers is to shift images by sub-pixel amounts. In principle, the information provided by this method can be used to minimize the problems of undersampling and obtain a higher spatial resolution than from a single location image.

4) Sensitivity Variation: There is a variation in the sensitivity across each individual pixel. Since the PSF is undersampled, this can limit the photometric accuracy (and also explains why optimal cosmic ray reject is not consistent with sub-pixel shifting).

For related articles on dither strategies, please see the new Dithering Handbook. Also, see the January, 1995 issue of the WFPC2 Space Telescope Analysis Newsletter and the February, 1995 issue of the ST-ECF Newsletter. Also, the following two FAQ items address the issue of dithering.

Q: I want to dither my exposures by exactly integral pixel amounts. What is the exact relationship between the POS TARGs we specify in the proposal and the CCD rows and columns?

A: The POS TARG axes run exactly along the CCD rows and columns on the specified aperture. For example, if you specify aperture WF3, the POS TARG axes will run *exactly* along the rows and columns on WF3. For the other CCDs the POS-TARGs will run only approximately along rows and columns, since there are small rotations (<0.5 degree) of the CCDs from their nominal alignments. Note that if WFALL is specified, then the rotation for WF3 is used. For small dithers (<0.3 arcsec.) the rotations between CCDs are unimportant, as they imply pixel registration errors less than 3 milliarcseconds, which is roughly the nominal pointing and guiding stability. But such small dithers do not allow integral pixel stepping simultaneously on both the WFC and PC. A dither of 0.5 arcseconds (5 WFC pixels or 11 PC pixels) gives near-integral stepping on both the WFC and PC, though the CCD rotations will then introduce registration errors up to 5 milliarcseconds. For more detailed information please see the report Dithering: Relationship Between POS TARG's and CCD Rows/Columns.

Q: How accurate are dithers between observations?

A: For observations within a single visit of less than 8 orbits, the dither accuracy is about 3 milliarcseconds. For programs exceeding 8 orbits, or for different visits to the same target, position errors up to 500 milliarcseconds and field rotations up to ~0.1 degree can occur, although experience indicates the errors are typically tens of milliarcseconds.

Note that large dithers will incur other errors. The camera distortion increases with strength toward the CCD corners, and alters the image scale by about 2% at the corners. Hence a 1.993 arcsecond dither will be 20.0 WFC pixels at the field center, but suffer a 0.4 pixel error at the CCD corners. The individual CCDs are misaligned by up to ~0.5 degrees from their nominal orientations, and again, this implies errors when attempting to dither by certain pixel amounts. A POS TARG = 1.993, 0.000 arcsecond dither in X on WF3 would cause spurious motion in Y of 0.17 pixel on WF4, due to the rotation. Large dithers may also require a different set of guide stars, and then the pointing accuracy is only that of the guide star catalog (~1 arcsecond).

Q: What is the status of the Linear Ramp Filters?

A: WFPC2 contains four "linear ramp filters" which provide a narrow band imaging capability (bandpass FWHM ~ 1.3% of central wavelength) at all wavelengths in the range 3710 to 9762 Å. These filters, known as FR418N, FR533N, FR680N, and FR868N, are essentially narrow band filters whose central wavelength varies as a function of position on the filter. To use these filters, observers merely specify filter and aperture names "LRF" and the desired central wavelength in their proposals.

Scheduling of LRF observations by STScI requires an accurate mapping from desired wavelength to target placement in the WFPC2 field of view. This wavelength/aperture position calibration was completed in early May 1995. The results are based largely on pre-flight JPL tests, which give the run of central wavelengths on the individual filters, and on March 1995 on-orbit observations where flat fields were taken through linear ramp filters crossed with narrow band filters, so as to define the registration of the filters within the WFPC2 field of view. The final product is a mapping from wavelength to position in the WFPC2 field of view. The current mapping (August 1995) uses all four filters with four partial rotations (-33, -18, 0, +15 degrees). This mapping allows observation at all wavelengths from 3710 to 9762 Å (i.e. without gaps in the wavelength coverage).

We note that a few small ranges in wavelength are effectively offset from the center of the filter passband; these wavelengths would otherwise fall off the CCD edges and be unobservable. The primary impact of these offsets is a slight loss in throughput (up to about 10% loss) at the affected wavelengths.

The unvignetted field of view is only ~10 arcseconds, and observers should be aware of this limitation when planning observations. All four CCDs (including the PC) are used at various wavelengths - while most wavelengths are observed on the WFC, a few small ranges are observed on the PC. The final wavelength mapping is described in section 3.3.2 of the the current version (v. 9.0, October 2004) of the WFPC2 Instrument Handbook.

Q: Under which circumstances should an observer request LOW-SKY (or SHADOW)?

A: By consulting the Instrument Handbook, a determination can be made as to whether the sky background will dominate the total measurement error, or whether the other two sources of background (instrument and particle dark, and source photon noise) will dominate. If sky background noise dominates the other two sources of noise, then the LOW-SKY or SHADOW special requirements should be used.

Q: Can the roll angle be varied during a single visibility period?

A: Any change in orientation requires a new visit and new guide star acquisition. It is possible to fit more than one roll into a single visibility period. For example, in an average visibility period of 55 minutes, the following could be accomplished:

    Guide Star Acquisition at 1st orientation    12 min
    Short WFPC2 exposure                          5 min
    Small Angle Maneuver                          3 min
    Guide Star Acquisition at 2nd orientation    12 min
    Short WFPC2 exposure                          5 min

In this example, 2 different orientations were accomplished in only 37 minutes, but remember that a new guide star acquisition is required to accomplish more than one orientation per visibility period.

Q: Can the WFPC2 be used in parallel with other science instruments?

A: WFPC2 may be used in parallel with ACS, NICMOS, and STIS.

Q: Should I use CLOCKS=YES or CLOCKS=NO for my exposures?

A: For most circumstances, we recommend CLOCKS=NO. The reasons for this recommendation are as follows:

1) CLOCKS=YES is not widely used, so anomalies may exist or develop that we are not aware of. Also, this mode is not as well calibrated as CLOCKS=NO (although we expect the calibration to be independent of the state of the clocks).

2) The shutter open time when CLOCKS=YES can have significant errors. In this mode, there are delays of up to 0.25 seconds in opening the shutter, which are not present when CLOCKS=NO. This means that for exposures less than ~20 seconds, there may be photometric errors greater that 1% unless special precautions are taken in the data reduction.

Despite this, we do advise CLOCKS=YES if you expect star images to be so saturated that a significant amount of charge will bleed off the chip during the exposure. This would mean that you expect much more than 10^8 electrons from at least one star in the exposure (more than 1000 pixels would be saturated). One advantage of CLOCKS=YES is that the overhead time is 1 minute less for exposures longer than 180 seconds. This can be significant if you have a large number of exposure times in the 3 to 10 minute range. Also, unlike the original WF/PC, we do not see a significant variation of WFPC2 dark level with CLOCKS=YES.

In summary:

0.11 - 20 sec   Use CLOCKS=NO (or make photometric 
                corrections during the analysis of 
                the data)
    
20 - 180 sec    Use CLOCKS=NO unless more than 10^8 
                electrond from a single star are expected

180+ sec        Use CLOCKS=NO unless more than 10^8 
                electrons are expected or you need to 
                save 1 minute of overhead per exposure 

WFPC2 PSFs:

Q: Where can I get a description of the WFPC2 PSF?

A: In addition to the discussion about the PSF in the Instrument Handbook, the TinyTim Software can calculate model psfs. With the addition of on-orbit mirror maps into this software (versions 3 and 4) the quality of these PSFs is very high.

Q: When searching for companions of bright targets, at what M(target)-M(QSO) does it become necessary to use PSF subtraction?

A: The table below gives the brightness of "object-like" features in the PSF, expressed as a delta-magnitude from the bright target. The third column gives the delta-magnitude detection limit, if one assumes "detection" means features which are three times brighter than the PSF features. At some radii, the OTA diffraction spikes are important. In those cases we give a range where fainter limits can be reached if the OTA diffraction spikes are avoided; this can be done by observing at several different spacecraft roll angles. These results are derived from TinyTim models of the WFPC2 PC PSF at F555W.

Observers with M(target)-M(QSO) larger than the limiting values below should be prepared to use PSF subtraction.

Radius       Brightness of PSF      Limiting M(target)-M(QSO)
from QSO     "features" expressed   magnitude (3 sigma)
on PC CCD    as delta magnitude
---------    ----------------       ------------------------
0.1"         3.2 mag                2.0 mag
0.3"         6.9                    5.7 
1"           8.5-10.1               7.3-8.9
3"           11.1-11.9              9.9-10.7

Q: How well can the PSF be subtracted? When searching for faint companions to bright targets, what is the limiting M(target)-M(QSO) magnitude in PSF subtractions?

A: Changes in the PSF due to OTA breathing limit the accuracy of PSF subtractions within the first few arcseconds. The table below gives the brightness of "object-like" features (meaning size ~2x2 pixels) in the difference between an in-focus PSF, and a PSF which is 5 microns out of focus. Five microns is the typical range of focus errors due to OTA breathing.

Observers with M(target)-M(QSO) larger than the limiting values below will have serious difficulties.

Radius       Brightness of       Limiting M(target)-M(QSO)
from QSO     "features" in       mag (3 sigma) in
on PC CCD    PSF subtraction     typical PSF subtraction
---------    ----------------     ------------------------
0.1"         4.7 mag             3.5 mag
0.3"         8.6-9.1             7.4-7.9
1"           11.4-11.9           10.2-10.7
3"           13.2-14.1           12.0-12.9

Q: When does saturation/blooming of the bright source become a problem when searching for faint companions of bright targets?

A: Saturation and blooming is usually not the limiting factor. As a rule of thumb, blooming will occur for (assuming typical broad band filters):

        WF              PC
V<      exp. time.      exp. time.
----    ----------      ---------- 
20      5000
19      2000            6000
18      800             2400
17      300             1000
16      120             360
15      50              150
14      20              60

Even when saturation occurs, it will tend to wipe-out pixels only in the vertical direction. If they are exposing more about 30 times longer than limit given in table, then they will wipe-out only 3 by 10 pixel ellipse. The direction of the saturation bloom can be controlled by rolling the spacecraft.

For further information see the WFPC2 PSF Subtraction page.

Other Questions:

Q: Are there documents other than the WFPC2 Instrument Handbook that would provide useful information for preparing a proposal?

A: The paper: Performance and Calibration of WFPC2 (Post Script, 5.0 Mbytes) describes the WFPC2 IDT's (Instrument Definition Team) understanding of the instrument as of July, 1994. It discusses many instrumental effects including the charge transfer problem (which can affect photometry), the hot pixel growth rate, and astrometric calibration. The IDT has also prepared the document: The Photometric Performance and Calibration of WFPC2 (Post Script, 4.2 Mbytes), which describes conversions to the Landolt UBVRI photometric system and the time dependency of the UV throughput, and provides a photometric calibration "cookbook". Also see recent issues of the Space Telescope Analysis Newsletter, which provides updated WFPC2 information on a monthly basis.