+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | ACS+WFPC2 | 19 April 2004 +----------------------------------------------------------+ CONTENTS: 1. UDF Data Released 2. Cycle 13 Phase II Update 3. HST Pure Parallel Observations 4. ACS/WFC Cross-talk and Gain Settings 5. MultiDrizzle Released in STSDAS 3.2 6. Pixel Area Maps 7. Dithering Necessary for Hot Pixel Removal 8. Pointing Pattern Library 9. Recent Instrument Science Reports 10. Recent Publications +----------------------------------------------------------+ 1. UDF Data Released On March 9, in the presence of MD Senator Barbara Mikulski, STScI Director Steven Beckwith unveiled the Hubble Ultra Deep Field (UDF), the deepest view into the Universe to date. The total exposure time of the ACS/WFC image is 400 orbits in the filters F435W, F606W, F775W and F850LP, reaching to about 30th magnitude, and detecting more than 10,000 objects. The UDF was also observed with NICMOS in F110W and F160W. The Space Telescope Science Institute, in collaboration with the Space Telescope - European Coordinating Facility (ST-ECF), and the Canadian Astronomy Data Centre (CADC) are pleased to release the final reduced images, object catalogs, and ancillary files from the Hubble Space Telescope UDF program. +----------------------------------------------------------+ 2. Cycle 13 Phase II Update This edition of the STAN provides information that may be of particular relevance to proposers about to prepare Cycle 13 Phase II observing programs. This information covers developments since the Cycle 13 ACS Instrument Handbook release in October 2003. The Cycle 13 Phase II deadline is May 14, 2004. +----------------------------------------------------------+ 3. HST Pure Parallel Observations STScI is reducing the amount of pure parallel data taken with HST in order to reduce the number of on/off cycles on the S-band Single Access (SSA) transmitters. While not an immediate threat to the scientific performance of HST, the SSA transmitters are a key subsystem and their failure could ultimately limit the observatory lifetime. The change has gone into effect on April 4, 2004. There is no change with regard to coordinated parallels. They will continue to be executed as in the past. +----------------------------------------------------------+ 4. ACS/WFC Cross-talk and Gain Settings Images obtained with the ACS/WFC are affected by a small amount of electronic cross-talk between the four CCD quadrants that correspond to the four amplifiers of the two detectors. The effect produces mostly negative electronic "ghost" images in a given quadrant that mirror real images recorded on other quadrants, although very faint positive ghosts have also been observed. The stretch in these images was chosen to highlight the ghost images. The counts in the ghost images are typically low by a few (1 to 4 seems typical) DN/pixel, and quantitative analysis (e.g., photometry) is rarely affected at a significant level. A detailed analysis of the cross-talk effect is underway and will soon be reported in an Instrument Science Report. Preliminary analysis suggests that the strength of the cross-talk is significantly weaker in images acquired with gain setting GAIN=2 than in images taken with GAIN=1 (see the aforementioned webpage for examples). Therefore, Cycle 13 ACS/WFC observers who are concerned about the quantitative influence or visual appearance of cross-talk are advised to use GAIN=2 instead of the default GAIN=1. Specifically, GAIN=2 yields better dynamic range than GAIN=1, but has the disadvantage of slightly increased read noise relative to GAIN=1. +----------------------------------------------------------+ 5. MultiDrizzle Released in STSDAS 3.2 The MultiDrizzle script provides automatic image registration, cosmic ray rejection and final image combination using Drizzle. It is available within PyRAF (the Python-based interface to IRAF). The first official release of MultiDrizzle has now taken place, as part of the latest release of STSDAS V3.2 (February 2004). The earlier beta version of MultiDrizzle is still available for download to support those who have older installations of STSDAS, although it is strongly recommended to upgrade to the latest STSDAS release if possible. +----------------------------------------------------------+ 6. Pixel Area Maps When HST/ACS images are flatfielded by the CALACS pipeline the resultant FLT files are flat if the original sky intensity was also flat. However, because there is very significant geometric distortion in such images, the relative photometry of point sources in FLT images cannot also be correct because the pixel areas on the sky vary around the field. To help users who may wish to do photometry on the FLT (undrizzled) images we are providing a relative pixel-area map (PAM) for both the HRC and WFC channels of the ACS as well as an example script which will allow the construction of such a PAM when required. The flux of an object on an FLT file should be multiplied by the PAM value at the appropriate pixel position before the published zeropoints are applied.The PAM for the WFC is close to unity at the center of the WFC2 chip, close to 0.95 near the center of the WFC1 chip and close to 1.12 near the center of the HRC. +----------------------------------------------------------+ 7. Dithering Necessary for Hot Pixel Removal Hot pixels continue to accrue, especially on the ACS/WFC. We therefore remind Cycle 13 observers that while the standard CR-SPLIT approach allows for cosmic-ray subtraction, without additional dithering it will not eliminate hot pixels in post-observation processing. Hence, we recommend that observers who would have otherwise used a simple CR-SPLIT use some form of dithering instead. For example, a simple ACS-WFC-DITHER-LINE pattern is available, based on integer pixel offsets, which shifts the image by 2 pixels in X and 2 in Y along the direction that minimizes the effects of scale variation across the detector. The specific parameter values for this pattern are given in Section 8.4.3 of the Phase II Proposal Instructions. However, any form of dithering providing a displacement of at least a few pixels can be used to simultaneously remove the effects of cosmic ray hits and hot pixels in post-observation processing. The developments of PyDrizzle and MultiDrizzle in the IRAF/STSDAS environment now makes this task easy. These tasks are described in detail in Chapter 4 of the ACS Data Handbook. Further details on the issue of hot pixels for ACS can be found in the ACS Instrument Science Report 02-09. +----------------------------------------------------------+ 8. Pointing Pattern Library The use of dithering and mosaicing patterns for ACS is described in Chapter 8 of the Phase II Proposal Instructions. We have now also posted an annotated "library" of pointing patterns on the web. The library is intended to give ACS users a quick way to identify and use, with confidence, a carefully designed pointing pattern which suits their observational goals. Users can always design their own freelance patterns, but for most, the patterns presented in the library should eliminate the need to calculate pointing parameters oneself. +----------------------------------------------------------+ 9. Instrument Science Reports since the last STAN: Results of UV Contamination Monitoring of the Advanced Camera for Surveys (ISR 04-05) Elevated temperature measurements of ACS charge transfer efficiency(CTE) (ISR 04-04) Best Gyroscope Usage to Maximize the HST Mission Lifetime (ISR04-03) Lossy Compression of ACS images (ISR 04-02) ACS CCD Gains, Full Well Depths, and Linearity up to and Beyond Saturation (ISR 04-01) +----------------------------------------------------------+ | To subscribe or unsubscribe to this STAN, send a message | to majordomo@stsci.edu with a blank subject line and | the following in the body: [un]subscribe acs_wfpc2_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
