ISRs

Full versions of these ISRs can be accessed by contacting the STScI Help Desk.

 
Contamination Correction in SYNPHOT for WFPC2 and WF/PC-1 (96-02)

We have implemented a time-dependent photometric calibration of WFPC2 and WF/PC-1 within synphot based on the stellar photometric monitoring data. This provides an empirical correction for the build-up of uniform contaminants on the CCD faceplates of the WFPC2 and WF/PC-1. Although the contaminant issue is less severe for WFPC2, there is more UV science where the effect is still significant. We present the empirical models of the time-variable throughput decline in WFPC2 and WF/PC-1. To activate the correction within synphot, the keyword `cont#' should be included in the obsmode with the Modified Julian Date as the parameter, e.g: "wfpc2,1,f555w,a2d7,cal,cont#49500.0" or "pc,6,f555w,cal,dn,cont#49219." Note that the automatic pipeline does not include the contamination correction in its computation of the photometric header keywords; the correction must be applied manually by, for example, executing the synphot "bandpar" or "calcphot" tasks off line with the cont# keyword in the obsmode.
S. Baggett, et al.

WF/PC Observed PSF Library (93-03)

Wide Field/Planetary Camera (WF/PC-1) point spread functions (PSFs) are briefly discussed and the contents of the WF/PC-1 PSF Image Library are described. Details of the processing and extraction of PSF images from WF/PC-1 observations are given and the header format is presented. The filter and chip coverage are summarized along with a description fo the original data used as a source for the PSFs. A complete list of all PSFs in the Library is included in the Appendix.
S. Baggett and J. MacKenty

WF/PC Photometric Monitoring Results (93-02)

WF2 and PC6 observations of the photometric standard star BD+75D325 have been taken monthly since May 1991 for photometric monitoring of the WF/PC-1. This report contains the results from this program through the end of 1992. All filters below F785LP show decay in throughput sensitivity which has been restored periodically by scheduled (and unscheduled) decontaminations.
C. Ritchie and J. MacKenty

The Evolution and Treatment of Hot Pixels in the WF/PC (93-01)

We examine the history of hot pixels in the WF/PC-1, and suggest improvements to the present dark calibration scheme. While new hot pixels continually appear (at ~30 per month per CCD), a substantial fraction (10 to 30%) are also lost during safings and decontaminations. For long exposures of faint targets, substantial improvement in dark calibrations can be obtained by using calibration data taken near the time of the science data. Calibrating with dark observations taken within one month of science observations reduces the number of residual hot pixels by a factor ~7 over the present pipeline reduction. Beginning in Spring 1993 dark calibration images will be taken at a rate of ~10 per month, which should give a large improvement (factor ~20) for future science observations where the dark calibration is an important limiting factor. Improvements related to "darktime" calculations and preflash calibration are also discussed.
J. Biretta and C. Ritchie

PSF Calibration Plan (92-13)

This report summarizes past and current proposals which obtain WF/PC-1 PSF images. An outline of future PSF calibration plan is presented.
S. Baggett and J. MacKenty

Currently Available Non-SV Flatfield Calibrations (92-12)

The 3973, 3974, 3977 and 3978 Non-SV Proposals took earth-illuminated flat fields for WF/PC filters used by GOs and GTOs during Cycle 1 which were not included in the SV program of flat fields. These proposals were executed in January 1992 and also in February/March 1992. This report summarizes the success of these proposals.
C. Ritchie and J. MacKenty

The Stability of Measles Features: an Autocorrelation Analysis (92-11)

The amplitude and character of "measles" on the WF/PC-1 have been monitored by studying auto correlation functions for small sub-images. The measles appear to be stable in time after the last decontamination, although with significant changes across decontamination events. The character of the measle profile, although like a diffraction pattern in appearance, does not show the expected wavelength dependence of the far-field diffraction model.
W. Sparks, C. Ritchie and J. MacKenty

Deltaflat Corrections (92-10)

Observations taken after a decontamination procedure and processed with flatfield reference files created from data obtained prior to that decontamination will require a deltaflat correction. A deltaflat can also be used to reduce, although not eliminate, the effects of the 'measles' contamination. Deltaflats, ratios of pre- and post-decontamination internal calibration lamp exposures (INTFLATS), are currently being generated and installed into the Deltaflat Library which has been established in the Calibration Data Base (CDB). This report presents a snapshot of the contents of this library as well as lists of useful INTFLATs from which the user may wish to generate additional deltaflats. This memo is updated on STEIS as future additions are made to the library.
S. Baggett and J. MacKenty

Absolute Efficiency of the WF/PC (92-09)

The WF/PC-1 absolute sensitivity has been investigated. Revised DQE curves have been produced for synphot, and an additional component introduced. We allow for alternative uses of synphot, 1. as an observation simulator and predictor of exposure times, and 2. to provide an absolute photometric calibration for data obtained with the camera. The data from the WF/PC-1 IDT Final OV/SV Report and the Cycle 1 calibration photometric sweep are shown to be consisted with one another and transformation between the WF/PC-1 photometric system and the synthetic photometry and flux system are described.
W.B. Sparks, C. Ritchie, J. MacKenty

Numbers and Characteristics of PC "Measles" Features from February through April 1992 (92-08)

We report preliminary results of a study of the number and types of measles seen in PC earth flat images taken from February through April 1992.
S. Baggett and J. MacKenty

WF/PC UV Calibration Following Decontamination (92-07)

The WF/PC-1 contains contaminants that limit throughput at UV wavelengths. The 1992.034,035 decontaminations were scheduled decontaminations to remove these contaminants, thereby temporarily restoring UV throughput and increasing throughput at longer wavelengths. The 1992.035 decontamination was followed by a UV observing campaign.
C. Ritchie and J. MacKenty

92-06 not used
 
A Library of Observed WF/PC Point Spread Functions (92-05)

As an aid in the deconvolution of WF/PC-1 observations, a library of WF/PC-1 point spread function (PSF) images has been established in the STSCI Calibration Database (CDB). Rather than storing entire WFPC datasets which already reside in the HST archives, the library consists of smaller, typically 256x256, sub-sections usually centered on the PSF star. Any of the extracted PSF images in the library may be requested from the STScI User Support Branch (USB) following the same procedures used for requesting calibration data. A complete PSF image name consists of the PSF rootname as provided in DATA_FILE column of the tables in this memo plus the extensions '.r7h' for the ASCII header and '.r7d' for the binary data file.
S. Baggett and J. MacKenty

WF/PC Measles Contamination and Compensation with Delta Flats (92-04)

The WF/PC-1 camera heads were decontaminated on Day 1992.034 (Feb 3), following a long period of continuous cold operation of the WF/PC-1 CCD detectors during which many GO/GTO sciences observations were obtained. The decontamination procedure restored UV transmission (temporarily), blue performance, and reduced the internal scattered light. It also made the expected small changes in the flat field structure but did not re-introduce QEH.
J. MacKenty and S. Baggett

WF/PC Reference Files Currently in the Calibration Data Base(92-03)

This report contains a listing of all full-mode WF/PC-1 reference files, grouped by type, that are presently available in the Calibration Data Base (CDB) System. These tables are intended to inform observers as to the quality of the calibration applied to their data by the PODPS pipeline processing and to provide an aid in selecting appropriate reference files for the re-calibration of WF/PC-1 observations. An on-line version of this report is maintained on the Space Telescope Electronic Information System (STEIS). The datafiles described in this report may be requested from the STScI User Support Branch (USB) in the same fashion as any other non-proprietary data products.
S. Baggett and J. MacKenty

Estimation of the Current Status of the WF/PC UV Flood (92-02)

A method of assessing the impact of decontamination procedures on the UV flood and of estimating the likelihood that the next decontamination will cause a return of QEH has been developed. Application of this methodology to the WF/PC-1 is presently limited both by the absence of a suitable pre-flood calibration and by some inconsistencies in the existing data. Although there is a strong possibility that systematic effects may dominate the present analysis and produce a completely misleading answer, the existing data suggest that the ~10 decontaminations carried out to date have removed only ~50% of the UV flood applied in December 1990 and that another ~5 flash style decontaminations are unlikely to reintroduce the QEH problem.
J. MacKenty and C. Ritchie

Determination of the Position Dependent Zero Point Magnitude Correction for the Core Aperture Photometry with WF/PC (not WFPC ISR but TS 92-02)

The method for developing a position dependent zero point magnitude correction is outlined. In general, it has been found that the magnitude obtained using Core Aperture Photometry tend to increase toward the edges of the FoV, and magnitudes are brightest near the center of the chip. It is shown that the magntiude variation can be very easily corrected using a linear relation delta mu = a_1 r + a_o, where r is the radial distance from the most sensitive part of the camera. The magnitude correction makes it possible to measure magnitudes to an accuracy of 0.05 mag or better. This improvement is re- assuring, especially considering that the flats for the WFC were not accurate enough for this calibration, and were not used in determining the correction. It is expected that the correction will reach even higher accuracy once improved flats are obtained.
Ellyne Kinney and Roberto Gilmozzi

Position Changes for Standard Star Observations (92-01)

Observations of the photometric standard star BD+75D325 are made monthly with the WF/PC-1. Over time, there has been a shift in star location on the CCDs causing the star center in PC observations to fall near a blocked column despite a PDB aperture change for PC 6 in late June 1992 to prevent this from occurring.
C. Ritchie

Exposure Times for G200L Images of AGK+81D266 during UV Flood (91-08)

During the UV Flood procedure executed 25-29 Dec 1990, UV grism (G200L) exposures of AGK+81D266 (a hot circumpolar subdwarf O star and UV (not optical) flux standard) were taken on all 8 WF/PC chips. The purpose of these grism exposures was to verify the presence of UV sensitivity (i.e. absence of contamination) following high-temperature decontaminations and the UV flood. The relevant grism images extracted from the DMF show that the 5s (WF) and the 23s (PC) exposures slightly saturate the order 0 image, and weakly expose (20-40DN/pixel) the order -1, +1, and +2 spectra of the target.

In the new UV flood procedure, which will become available in early 1992, the JPL exposure times of 40s (WF) and 160s (PC) will nicely increase the exposure level of the dispersed spectra, but will over-saturate the order 0 image causing charge to bleed down the columns. On most chips this bleeding will not seriously interfere with the order -1 and +1 spectra, from which the UV sensitivity is most easily gauged. However, on WF1 and WF3 the grism dispersion is roughly parallel to the columns. We therefore recommend using the JPL exposure times for all chips except WF1 and WF3, for which 10s exposures should be used to prevent bleeding.
K. Horne and J. MacKenty

Analysis of Stellar Monitor Proposal #3173 (91-07)

WFC and PC observations of BD+75D325 have been taken for 3 consecutive months through the Stellar Monitor proposal 3173. Aperture photometry shows stable sensitivities to a few percent in filters F439W, F555W, and F785LP. F336W is stable to 5-10 percent and has returned to the originally observed sensitivity since the Flash decontamination on July 5. The sensitivity is not stable in the UV, but has increased since the Flash decontamination.
C. Ritchie

91-06 not used.
 
July 6 Decontamination Affected QE Slightly (91-05)

The WF/PC-1 CCD's were decontaminated on July 6 after spacecraft conditions forced a temporary shutdown of WFC coolers. This decontamination operation reduced slightly the QE of PC8, but did not affect it as much as a similar decontamination executed in May.
S. Ewald

WF/PC Photometric Calibration during Jan-May 1991 (91-04)

Observations of 3 spectrophotometric standard stars taken between 31 Jan. 1991 and 23 May 1991 are used to investigate the sensitivity of the WF and PC cameras relative to the pre-launch baseline predictions. The analysis considers all spectrophotometric standard stars observed between the UV flood on 26-28 Dec. 1990 and the latest observation on 23 May 1991, which occurred after the deep safing event of 02 May 1991. The main results are summarized in Figures 3a and 3b, which show the ratio of observed to predicted count rates as a function of wavelength in the WF and PC camers respectively. Figures 4a and 4b compare the baseline and corrected quantum efficiency curves for WF2 and PC6 respectively.

At optical wavelengths the measurements indicate stable sensitivites in WF2 and PC6 to a few percent in F555W, and F785LP and 5-10 percent in F336W. In the UV the sensitivity is not stable.

Comparison of the observed and predicted counts indicates that the ground-based sensitivity calibrations differ significantly from on-orbit performance. The sensitivity in F785LP is down by 35-40 percent in SF2 and PC6,7, confirming the result of our earlier analysis of Omega Cen data. At F555W, WF2 is up by 10 percent while PC6 and 7 are down by 10-20 percent. At F439W, WF2 is up by 20 percent, PC6 is down by 10-15, and PC7 is down by 35 percent. At F336W, WF2 is up 70-90 percent while PC6 is down 45-50 percent. F284W is up by 45-60 percent in WF2, and the other UV filters are down by 20-97 percent and unstable as noted above.

The issue of flat field normalization in relation to photometric calibrations is briefly discussed. A reccomendation is made that flat fields be normalized to the median value in a 200 by 200 pixel box centered on the prime chip (WF2 or PC6) rather than to a mean over all 4 chips.
K. Horne, L. Walter, C. Ritchie

WF/PC Contamination Control (91-03)

The prediction of contaminant deposition on the cold (-100C) charge coupled device (CCD) sensors of the Wide Field Planetary Camera (WF/PC-1) due to sources internal to the instrument is crucial to the evaluation of expected performance and to the assessment of approaches for improvement. An integral component in such predictions is a model of the transport from the internal sources to the CCD's. In the present work, the model used is based on the Jet Propulsion Laboratory (JPL) Contamination Analysis Program (CAP).

A CAP model comprises a geometric multinodal representation of the instrument, internodal shape factors for line of sight transport, nodal contaminant sources and the necessary deposition and re-emission kinetics. IN CAP, indirect transport by way of intermediate nodes (of considerable importance to the internal problem) is explicitly calculated as a diffuse reflection for each internodal exchange. This leads to rather long computer run times for each required 30 day prediction for separate sources and for various internal modifications to reduce the accumulation on the CCD sensors. A method has been developed to pre-calculate the effective total transport factors from each source node to each receiving node (including the CCD sensors) and from each re-emitting node to each receiving node. The effects of this preprocessing calculation are to sharply reduce the number of nodes, to increase the allowable time step in the transient CAP analysis, and to greatly reduce the run time.

Some results of this work to date are presented and the interpretation of the results are discussed. The discussion includes the implications of the results for other space instruments.
J. Barengoltz, J. Millard, T. Jenkins, D. Taylor

Spacecraft Jitter: Its Effect on the HST PSF and on the `Breathing'?) (91-02)

The effect of the spacecraft jitter on the HST Point Spread Function is analyzed, with special reference to the terminator oscillations. It is found that the jitter may substantially affect the shape of the PSF, seriously impairing the chances of accurate photometry. However, the true PSF can be modelled by convolving the uperturbed PSF with the jitter PSF obtained from the FGS pointing information, provided the observations were conducted in fine lock. The best results are obtained for high telemetry rates (better sampling of the jitter). The true PSF so obtained can then be used to deconvolve WF/PC-1 images.
R. Gilmozzi

Post-Flood PC quantum Efficiency (91-01)

PC frames of [Omega] Centauri taken on January 2, 1991 through 10 filters have been used to determine the post-flood quantum efficiency of the PC. The [Omega] Cen frames were taken shortly after the UV flood and subsequent safeing event. Observed count rates were obtained by aperture photometry with 60 and 10 pixel radii. Ground-based photometry was used to estimate the spectra of the [Omega] Cen targets. The spectrum for each target was then used with WF/PC-1 passbands from the CDBS database (which are those presented in the 1990 WF/PC-1 Handbook) to calculate the predicted total count rates for each WF/PC-1 filter.

There is little agreement among the count rate in the UV filters, varying from 2 times lower to 2 times higher than predicted. These results are not currently understood. The count rates for F439W, F547M, F555W, F702W, and F718M are about right, and about 2 times lower than predicted for F785LP and F889N.
K. Horne and C. Ritchie

Technical Report on Wide Field Camera Observations of 2237+0305 (SV 3068) (90-10)

Our analysis of five Wide Field Camera observations of 2237+0305 yielded the following results: 1) The standard calibration performed by the Space Telescope Science Institute's software satisfactorily repairs the analog-to-digital conversion errors in the raw data. 2) The standard calibration software overestimates the bias plus preflash level by nearly one electron. 3) The use of flat fields taken during the vacuum testing of the camera has the potential to produce misleading results. 4) The noise properties of the images can be characterized by a readout plus preflash noise of ~1.9 electrons and poisson statistics in the sky. 5) Centroids of well exposed objects (several thousand photons in the central pixel) can be determined to an accuracy of a few hundredths of; a pixel. 6) The preflashs appear to remove low light level charge transfer inefficencies. 7) Relative positions of objects measured from Wide Field Camera and Faint Object Camera data agree to a few milliarcseconds over an area a few arcseconds across.
D. Schneider & J. Bahcall

WF/PC Internal Molecular Contamination During System T-V Test (90-08)

During a recent System Thermal-Vacuum (STV) test of the Wide Field/ Planetary Camera (WF/PC-1), instrumentation was added to characterize the internal molecular contamination using a temperature controlled Quartz Crystal Microbalance (TQCM) and several ultra-violet (UV) light sources. Correlation of this data with detailed temperature data for various elements in the WF/PC-1 provided significant insight into the sources and the effect of extremely low levels of internal contaminants affecting the far UV instrument throughput. As a result, the WFPC design was modified to ensure on-orbit capability to temperature from -80 to 100 degrees C, and provide guidance for on-orbit operations during UV observations.
R. Griffith

"Core" Aperture Photometry with WF/PC (90-07)

The effect of the variable, structured PSF of HST on aperture photometry is investigated. It is shown, both theoretically and practically, that the best results are obtained using extremely small apertures (0.15" radius). The choice of a "background" annulus of 0.2" width and 0.3" radius (i.e. well within the PSF) allows the photometry of moderately crowded fields with accuracies only slightly worse than those obtained by PSF fitting techniques. With this approach the effect of varying PSF is taken into account by a position-dependent zero point calibration whose determination procedure is outlined. This technique is applied ot WFPC data on NGC 1850, using standard IRAF packages. It is shown that V photometry accurate to 20% (r.m.s.) can be achieved on mv = 24 [approximately] stars in [on the order of] 1000 seconds.
R. Gilmozzi

A Pre-Flood Checkup on WF/PC Quantum Efficiency Using the STSDAS SYNPHOT Software Package (90-06)

We describe a recent attempt to verify the throughput and quantum efficiency of the WF/PC relative to the pre-flight baseline calibration, using the following filters: F230W, F555W, and F785LP. Using aperture photometry, the observed count rates were obtained and the aperture corrections applied were based on a mean encircled energy curve. The targets used are not spectrophotometric standards because the WF/PC had not yet measured such standards. Therefore the spectrum of each target is based on ground based photometry and, in some cases, knowledge of the spectral type. The predicted total count rates for the filters were computed from the target spectrum.

The results are in agreement with earlier results from S. Faber, WF/PC IDT. For the F230W filter the rates are 4 times less than predicted, roughly correct for the F555W, and approximately 2 times lower for the F785LP. Because the WF/PC has yet to be UV flooded, the low UV sensitivity is expected. However the low infrared response has yet to be understood. One possibility is that the laboratory measurement was in error. This report also details the analysis steps using the new STSDAS.SYNPHOT software package.
K. Horne, E. Wyckoff, and D. Bazell

A Pre-Flood Study of WF/PC Photometric Stability Using Aperture Photometry on NGC 188: Data Taken in August and September (90-05)

A relatively uncrowded field of the Galactic cluster NGC 188 was studied in order to learn about the level of photometric stability of the Wide Field Camera and OTA. Different methods of aperture photometry were applied to images taken a month apart in order to determine which was the most successful at producing repeatability in the data which, as a result of spherical aberration, suffers most dramatically from a position dependent point spread function (PDPSF).

The results depend heavily on the size of the aperture radius and the position and size of the background annulus. The selection of these parameters can create quite different results that range from systematic shifts of several percent to scatter about the fit of up to 45% rms. Of the setups...tried [they] find that the parameter selections that produce the best result are an aperture radius of 0.15", annulus radius of 0.2", annulus width of 0.1", and a mean background determination of the pixels in the annulus. This is based on the assumption that the 'best' result is the one where the line fit to the data has the smallest offset from zero balanced with the smallest amount of scatter about this fit.

A compromise must be made between two major difficulties that affect the quality of aperture photometry that can be done with HST. On one side there is the PDPSF which could be compensated for by using a large aperture to include all flux independent of shape. This, however, is complicated by the fact that the S/N ratio decreases very quickly with distance from the core so large apertures produce noisy measurements. On the other, photometry could be done in the core only where the S/N is high; however, the PDPSF causes a different fraction of the PSF to [be] measured in a small aperture at different positions. [They] believe that this effect can be calibrated using a position dependent zero point for the photometry. Even though the core aperture photometry is the method most sensitive to the variable PSF the fact that [they] find results as good as or better than large aperture photometry implies that the results will be much better when [they] can correct for a variable zero point.
E. Wyckoff, R. Gilmozzi, K. Horne

Sensitivity at 9000A (90-04)

Analysis of WF/PC standard star observations by both the IDT (Faber) and the STScI (Horne et al.) shows that the sensitivity at 5500A is within 90% of the value expected from the Thermal Vacuum Test 6 (TV6) results, while that at 9000A is only 58% of what was predicted (as read from Faber's plot and Figure 1 of the Horne et al. report). No reason for the low response in the near IR is known.
P. Seitzer

The Take Data Flag (90-03)

Analysis of WF/PC standard star observations by both the IDT (Faber) and the STScI (Horne et al.) shows that the sensitivity at 5500A is within 90% of the value expected from the Thermal Vacuum Test 6 (TV6) results, while that at 9000A is only 58% of what was predicted (as read from Faber's plot and Figure 1 of the Horne et al. report). No reason for the low response in the near IR is known.
S. Ewald

Filter F656N Anomaly I (90-02)

The H-alpha narrow band filter in WF/PC-1, F656N, has deteriorated since delivery. A series of concentric rings is visible on exposures taken through this filter. The rings are centered near pixel (300,600) of CCD WF1 and extend across all four chips. Also, CCD WF4 shows fringes and filter F664N probably has a ghost.
S. Ewald

Bias Change During TV6 (90-01)

Analysis of WFPC images taken during JPL thermal vacuum tests has always shown a low level (~0.5 DN) difference of bias level between the odd and the even numbered columns. During TV6 (February - March, 1988) this pattern changed from even columns having the higher bias level to the odd columns being higher. The change occurred steadily over the duration of the TV6 and does not show a correlation with UV Flooding or decontamination (bake-out) activity. The mean bias level of the odd and even columns together shows a steady increase of about two DN per day over the twenty-five days of the TV6 test.
C. Ritchie and S. Ewald

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