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S T A N / W F P C 2 - Number 12, February 1996


Updating the WFPC2 Instrument Handbook

by John Biretta

We are currently in the process of updating the WFPC2 Instrument Handbook in preparation for the Cycle 7 proposal solicitation. Some of the areas we plan to update are the exposure time estimation, strategies and performance for dithered observations, and improvements that have been made to calibration based on experience with the Hubble Deep Field. Please send any suggestions for modifications to the Handbook to John Biretta (

Rotation of Field in WFPC2 Data Taken Before April 11, 1994

by J.C. Hsu and Brad Whitmore

Observations taken before April 11, 1994 have PRELIMINARY plate scales and reference pixel locations in their image headers. Thus, the pixel locations for a target taken in an image before this date could be nearly 3" different from its position taken after this date, due to aperture updates. This is discussed in the WFPC2 Instrument Handbook (version 3.0, June 1995) in section 2.9.

Users should also be aware that the 3" shift of the reference pixel is not just a parallel shift. There is a 0.8 deg ROTATION for WF2, and smaller rotations for the other chips (0.28 deg in PC1, 0.46 deg in WF3, and 0.06 deg in WF4) after the April 11, 1994 update. Since the STSDAS task METRIC uses WF2 as the reference chip, a 0.8 degree discrepancy will be present between data obtained before and after April 11, 1994, in addition to a shift.

A new STSDAS task UCHCOORD will be available in the next STSDAS release to update the header group parameters to reflect the plate scale, shift, and rotation changes. This task will supersede the task UCHSCALE which can currently update only the scale, NOT the rotation or shift.

We thank Phil Massey and Deidre Hunter for bringing the rotation problem to our attention.

S/N Estimates for WFPC2 Exposures

by Anatoly Suchkov and Stefano Casertano

A clear understanding of how the signal-to-noise ratio, S/N, is calculated is important because in many cases the observer uses the S/N estimate to determine how many orbits his/her observation needs. The observer may use different ways to derive the point source flux and noise estimates from the exposure data. Different approaches may yield different results in terms of signal-to-noise estimates, sometimes far from optimal.

The formula given in the WFPC2 handbook (equation 6.2) for the background-limited case implies optimal weighting of the data points, which is equivalent to PSF fitting. This method yields a signal-to-noise ratio for the background-limited case of:

   S/N = I * sqrt {Sum [P(i,j)^2] / B}         (1)

which is the same as Equation (6.2) in the Handbook. Here, I is the total number ofphotons (i.e. the "signal") expected from the source, B the square of the background noise per pixel (including read noise); I and B are in electrons. P(i,j) is a representation of the PSF at pixel (i,j) from the center of the image. The quantity Sum [P(i,j)^2], also called the "sharpness" of the image, is an estimate of the reciprocal of the "effective" number of pixels used in the PSF fitting; note that Sum [P(i,j)] = 1 by definition. For those interested in more details, an explicit derivation of Equation (6.2) can be found in the WFPC2 WWW pages, in the segment "WFPC2 Exposure Time Estimation Guides" - or contact the authors.

A rough estimate of the S/N can also be obtained by comparing the expected signal from the source within some predefined region centered at the source with the total noise (due to read noise, background, and photon noise from the source) within the same area. This corresponds to aperture photometry performed without weighting. If the area includes n pixels, then the noise will be:

    N^2 = f*I + n*B                             (2)

where f is the fraction of the total source signal I within the area used. As before, I, N, and B are in electrons. The signal-to-noise ratio in the background-limited case is then:

    S/N = f*I / sqrt (n*B)                      (3)

This can yield substantially lower signal-to-noise ratio than the optimal estimate of Equation (1), especially if a large area is used (thus n is large). For example, consider a star at the center of a WF camera around 6000 A; the sharpness is sum(P_ij)^2= 0.128 (Handbook, Table 6.4). If a circular area with radius 3 pixel is used in the aperture photometry, n = 28.3 and f = 0.87. Thus the (S/N) estimate from equation (1) is larger than the estimate based on equation (3) by a factor of:

    a = sqrt (0.128 * 28.3) / f = 2.2           (4)

which translates into a factor of 5 longer observation to reach the same nominal signal-to-noise ratio in this background-limited case.

In the photon-limited case, when the signal-to-noise ratio is high, one has, of course:

    S/N = sqrt (I)  and    S/N = sqrt (f*I)     (5)

for PSF fitting and aperture photometry, respectively, which yields for the same f as above:

    a = sqrt(1/0.87) = 1.07.                    (6)

The formula used in the WFPC2 Exposure Time Calculator, which is available on the WFPC2 WWW pages, reflects the signal-to-noise which is accurate in both the photon-limited case the background-limited case with optimal weights, and is a reasonable representation of the intermediate case when the optimally weighted background noise is comparable to the Poisson noise of the target.

One should keep in mind that the above considerations do not take into account instrumental effects like OTA breathing whose effect on signal- to-noise ratio is more complicated. These and spatially-variable PSFs are the source of systematic errors, but this is beyond the scope of the present discussion.

Calibration of Linear Ramp Filter Data

by John Biretta and Keith Noll

A WWW page has been created to assist observers in calibration of data obtained through the WFPC2 Linear Ramp Filters. In summary, we are recommending the observers flat field their data using narrow band filter flats at a nearby wavelength. Target locations can be derived using the on-line LRF calculator tool. Photometric calibration can be obtained either via the WFPC2 Exposure Time Calculator program, or via a new SYNPHOT LRF throughput table which may be downloaded from STScI. For further details, please see the above mentioned WWW page.

Secondary Mirror Movement Planned for March 14

by Stefano Casertano

The next semi-annual focus adjustment of the Optical Telescope Assembly (OTA) is planned for 14 March (day 74) around 19 hours UT. The OTA secondary will be moved outwards by 6 micron, to compensate for the continuing desorption in the OTA metering truss. This will result in an average focus position about +3 micron from nominal, compared to the current position of -3 micron derived from trending data. A compensating movement of the COSTAR DOB will be carried out at the same time.

Since the shape of the PSF in the PC changes with the secondary mirror movement, observers requiring accurate PSF, for example to identify faint sources near bright point sources in the PC, should avoid combining observations before and after the secondary mirror movement. The encircled energy distribution of the PSF, and thus aperture photometry, should not be affected in a measurable way. The continuing drift in focus position can affect aperture photometry in very small (2-3 pixel) aperture in the PC at the level of a few percent; an investigation of this effect is ongoing.

If desorption continues at the current rate, the next movement of the OTA secondary mirror will take place around October 1996.


by S. Baggett, W. Sparks, C. Ritchie, J. MacKenty

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.

by Massimo Stiavelli and Sylvia Baggett

The WFPC2 camera has two different options for producing internal flat fields. The first option is to use the VISFLAT calibration channel designed to produce a uniform illumination of the chip. The alternative option is to use a set of internal lamps illuminating the shutter blades when the shutter is closed (INTFLAT). Since both these options are fully internal, they allow for a detailed monitoring of the flat fielding without any impact on the telescope efficiency as they can be carried out while some other primary instrument is being used or during occultation.

By analyzing data collected during 1994 and 1995 we have found evidence for an increase of the VISFLAT lamp degradation rate and have reduced its usage accordingly. The VISFLAT exposures have also allowed us to establish that WFPC2 flat fields are geometrically stable to better than 1 percent. By comparing VISFLATs taken before and after decontaminations, we have found that the effect of contamination is much smaller on the VISFLAT count rates than it is on the photometry of standard stars, implying that contamination affects stellar photometry mostly by scattering light at large angles rather than by absorption.

By analyzing INTFLATs we have been able to verify to within 1 percent the gain ratios determined by the WFPC2 IDT. If one allows for a possible overall shift due to variations in the lamp flux the agreement improves to 0.1 percent except for the PC, for which, however, the data also have larger statistical errors. The gain ratios have been stable to within 0.1 percent since 1994.


We draw your attention to these papers, based on WF/PC and WFPC2 data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list.

Hubble Deep Field observations"  Science with HST 2

HU, E.M.; MCMAHON, R.G.; EGAMI, E.  "Detection of a Lyman
alpha emission-line companion to the z = 4.69 QSO
BR1202-0725"  ApJ 3-10-96 astro-ph/9512165

GLAZEBROOK, K.; VAN DEN BERGH, S.  "Galaxy morphology to I
= 25 in the Hubble Deep Field"  MNRAS accepted

"MERLIN and HST observations of the jet in 3C273"  IAU
Symp. 175 - in Fanti preprint

"Galaxy clustering around nearby luminous quasars"  ApJ

GROTH, E.J.; HOLTZMAN, J.A.  "The intermediate-mass
population in the core of the R136 star cluster"  ApJ
459: L27-L30, 1996

MACCHETTO, F.  "A new HST image of Cygnus A"  A&A accepted

"Seyfert galaxies. IV. Nuclear profiles of Markarian
Seyfert galaxies from HST images"  ApJ accepted


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