Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/acs/performance/cte/AAS_2003_CTE.pdf Дата изменения: Fri Jul 25 18:30:18 2003 Дата индексирования: Sun Dec 23 01:08:54 2007 Кодировка: Поисковые слова: 47 tuc

SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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CCD charge transfer efficiency of the Advanced Camera for Surveys
Adam Riess (Space Telescope Science Institute) Marco Sirianni (Johns Hopkins University) Max Mutchler (Space Telescope Science Institute) Michael R. Jones (Goddard Space Flight Center)


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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Abstract
The CCDs of the Advanced Camera for Surveys (ACS) will suffer from declining charge transfer efficiency (CTE) as radiation damage from the space environment accumulates. We present preliminary results of our CTE monitoring efforts, now that ACS has been in orbit for about 10 months. While CTE is not expected to have a significant effect on Cycle 11 ACS science observations, we describe plans to mitigate and calibrate the effects of CTE loss as it becomes more severe with time.


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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CTE becomes a significant issue as flight CCDs age. This has been well-documented for Hubble Space Telescope instruments:
http://www.stsci.edu/hst/acs/performance/cte_workgroup/cte_papers.html

Monitoring CTE provides a forecast of the scientific "shelflife" of flight CCDs -- important for prioritizing the science program of this limited resource. Calibrating CTE for science data becomes essential to extending the useful lifetime of flight detectors. We plan to develop a robust CTE calibration for ACS science data, e.g. some function of pixel location (x,y), sky background, date, etc.


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

4

We obtain internal EPER and FPR data at many signal levels every 6 months, with a monthly check at only one signal level. CTE is also measured from the tails of cosmic rays and hot pixels in dark frames. The data presented here is from early inflight internal data, which is useful for monitoring the relative degradation of the detectors over time: just a first look at the small but already measureable CTE loss in the ACS detectors. We have external observations of 47 Tuc planned for early 2003, which will help to characterize the effect of CTE loss on astronomical objects (photometry, astrometry, etc). This data will lead more directly to a CTE calibration for science data.


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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CTE monitoring
Extended Pixel Edge Response (EPER), and First Pixel Response (FPR) tests Measuring the "deferred charge" tails of cosmic rays and hot pixels Effect on stars (photometry, astrometry, tails): observations of 47 Tuc planned for early 2003 Correlation of preflight Fe55 data, and inflight ~1620esignal level data


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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Extended Pixel Edge Response (EPER) test
A dark frame with a special clocking pattern that produces extra (75) trailing physical and virtual overscans, where "deferred charge" (trapped then released) is reclaimed to measure both parallel and serial CTE with one exposure. WFC EPER 4195 x 2123 (only chip 2 shown here)

virtual overscan physical overscan
4096 x 2048 pixels

HRC EPER 1118 x 1099

1024 x 1024


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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Serial First Pixel Response (FPR) test
A dark frame with a special clocking pattern. A quick readout of the first half of the chip, then a normal readout of the full chip creates an electronic "knife edge" in the center of the science array, where deferred charge is measured. WFC serial FPR 4144 x 2068 (only chip 2 shown here)

HRC serial FPR 1062 x 1044

fast readout fast


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

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Preliminary results
The following plots show our preliminary inflight CTE measurements for the various internal CTE tests we have conducted to date. A declining CTE trend is clearly evident, which we will continue to monitor. CTE loss is most significant at low signal levels, where the trapped charge represents a larger fraction of the total charge. The EPER plots show measurements made at various signal levels during preflight thermal vacuum testing at Ball Aerospace (upper), inflight data taken during Servicing Mission 3B Orbital Verification (SM3B/SMOV) in April 2002 (middle), and data taken during October 2002 as part of the routine Cycle 11 calibration program (lower). The CTE measurements based on "deferred charge" tails of hot pixels and cosmic rays were made with inflight dark frames obtained since SM3B (March 2002).


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

9

Early CTE trend for WFC from tails of hot pixels and cosmic rays


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS 10 in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

Early CTE trend for HRC from tails of hot pixels and cosmic rays.


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS 11 in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

WFC CTE per pixel (transfer) from early EPER tests
pre-flight April 2002

October 2002

pre-flight April 2002

October 2002


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS 12 in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

Mitigation of CTE effects
We will soon obtain EPER and FPR data while the temperature of the ACS CCDs is raised from -77 C to -67 C. Determining CTE as a function of temperature will allow us to assess how much improvement can be expected from a change in the operating temperature of the detectors. Pre-flashing the CCDs reduces the effect of CTE by essentially pre-filling the charge traps just prior to a science exposure. Special apertures near the CCD serial registers can also be defined and used, which minimize the clocking (number of pixel-to-pixel transfers) that each charge packet undergoes. These techniques are available to help mitigate the effect of declining CTE as the ACS detectors age.


SPACE TELESCOPE SCIENCE INSTITUTE

201st Meeting of the AAS 13 in Seattle, 7 January 2003 Adam Riess, Marco Sirianni, Max Mutchler, Michael Jones

For more information...
To follow our CTE monitoring and calibration efforts, bookmark the ACS website:

http://www.stsci.edu/hst/acs/
Also, subscribe to the ACS e-mail newsletter (STAN) by sending a message to majordomo@stsci.edu with a blank subject line, and in the body type: [un]subscribe acs_news ACS e-mail newsletters (STANs) are also archived at: http://www.stsci.edu/hst/acs/documents/newsletters