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Hubble Space Telescope
ACS STAN, 3 Apr 2003

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| STScI Analysis Newsletter (STAN)
| for the Advanced Camera for Surveys (ACS)
| ACS STAN #4, 3 April 2003
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CONTENTS:

1.  Cycle 12 and ACS

2.  Dealing with hot pixels, and dithering vs CR-SPLIT

3.  A dither pattern for the HRC occulting finger

4.  Charge Transfer Efficiency (CTE)

5.  Exposure time estimates for red targets in F850LP

6.  Reminder on WFC buffer dump overheads

7.  Subarrays for WFC and HRC

8.  CCD Bias calibration and Gains in Cycle 12

9.  ACS Coronagraphy

10. New version of Tiny Tim HST PSF simulation software

11. Pipeline automatically providing flat-fielded images 

12. Calibration Workshop Proceedings

13. Recent ACS publications

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1.  Cycle 12 and ACS

This edition of the STAN provides information that may be 
of particular relevance to proposers about to prepare 
Cycle 12 Phase II observing programs.  The information 
collected here covers developments since the Cycle 12 ACS 
Instrument Handbook release in October 2002, with an 
emphasis on items relevant for proposal preparation. For 
assistance with any of the issues discussed here, please 
send email to help@stsci.edu .

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2.  Dealing with hot pixels, and dithering vs CR-SPLIT

The hot pixel annealing rate for the HRC has been as 
expected (~80%), but for the WFC it has been 
significantly lower than expected (~60%). The growth of 
unannealed WFC hot pixels is progressing at a rate such 
that by the middle of Cycle 12 (January 2004)
they will contaminate as many pixels as do cosmic rays in 
a typical (~1000 sec) exposure. For more details on this, 
see ACS Instrument Science Report 02-09:

http://www.stsci.edu/hst/acs/documents/isrs/isr0209.pdf

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 now use some form of dithering 
instead. For example, a simple ACS-WFC-DITHER-LINE pattern 
has been developed, 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 at  http://www.stsci.edu/public/p2pi.html
(This link is no longer available.)

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 recent development 
of PyDrizzle and Multidrizzle in the IRAF/STSDAS
environment now makes this task easy. There are many 
resources available to guide users in the reduction of 
dithered data, including cosmic ray rejection and hot 
pixel elimination. See for instance:

http://www.stsci.edu/hst/acs/analysis/drizzle

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3. A dither pattern for the HRC occulting finger

Some HRC users have been surprised to see a substantial 
region masked out by the occulting finger in their data. 
The occulting finger is not retractable - it will be 
in every HRC exposure. However, as with any other detector 
feature or artifact, the "lost" data can be recovered by 
combining exposures which were suitably shifted with 
respect to each other. The following link provides more 
information on the HRC occulting finger, and a specific 
dither pattern to allow recovery of data lost behind 
the finger:

http://www.stsci.edu/hst/acs/faqs/finger.html

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4. Charge Transfer Efficiency (CTE)

The first external calibration data taken on ACS to measure 
the effects of CTE degradation on stellar photometry are 
currently being analyzed. Preliminary results match 
expectations. For typical observing backgrounds and stellar 
fluxes, CTE losses on WFC are typically 1-2% far from the 
readout amplifier (parallel read-out) and readily 
correctable. Only in the worst case scenarios (i.e., faint 
sources observed with narrow-band filters) do CTE losses 
reach ~10%-20%. Observers requiring better-than-1% precision 
should consider mitigating CTE by placing sources near the 
read-out amplifier and giving careful consideration to 
backgrounds.  No measurable CTE effects are seen for the 
HRC at this time.

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5. Exposure time estimates for red targets in F850LP

At wavelengths greater than 7500 A (HRC) and about 9000 A 
(WFC) ACS CCD observations are affected by a red halo due 
to light scattered off the CCD substrate. An increasing 
fraction of the light as a function of wavelength is 
scattered from the center of the PSF into the wings.  For 
the very broad z-band, F850LP, filter in particular the 
encircled energy depends on the underlying spectral energy 
distribution; such an effect is not yet incorporated in 
the available (CGI) exposure time calculator (ETC).

In order to obtain correct estimated count rates for red 
targets observers are advised to use the Synphot package 
in IRAF/STSDAS for which a proper integration over 
wavelength has now been incorporated for encircled energy.
To quantify this new synphot capability, we compare ETC 
results with synphot for a set of different spectral 
energy distributions and the observation mode 
WFC,F850LP. In the following table, the spectral type 
is listed in the first column. The fraction of light with 
respect to the total integrated to infinity is listed in 
the other two columns, for the ETC and synphot calculations 
respectively. These values are derived for a 7x7 pixel box 
for the ETC calculations and for a 0.2 arcsec aperture 
for synphot:

  Sp. Type   ETC      Synphot

    O        0.76       0.74
    M        0.76       0.7
    L        0.76       0.68
    T        0.76       0.6

The ETC results are off by 3% (O star), 9% (M star), 
12% (L star), and 27% (T star). If this small effect is 
relevant to particular observations, then the Synphot 
software package  can be used. A new keyword has been 
implemented to call for the encircled energy tables. 
The keyword is "aper". The user is allowed to select an 
aperture (radius in arcsec) and indicate this value by 
typing "aper#value". Currently, the following apertures 
are supported: every tenth of arcsec between 
0. and .6 arcsec, 0.8, 1., 1.5, 2. and 4. arcsec.
When calling "aper#0." the user will obtain the number of
counts in the brightest pixel, i.e. the peak counts of the
source centered at that pixel.

Arbitrary aperture sizes are also permitted but are not 
recommended, because Synphot provides only a linear 
interpolation between supported apertures, which is a poor 
approximation, especially at small apertures. A typical 
obsmode would now read like: acs,wfc1,aper#0.2,f850lp. 
From the command line such an obsmode should be entered 
within quotes, or synphot complains:

cl> calcphot "acs,wfc1,aper#0.2,f850lp"

GOs can always "epar" the program they want to run, or use 
Pyraf instead of cl. Users can also provide their own input 
spectrum, which has to be in the same format as the user's 
provided spectrum for the ETC.  Please refer to the ETC help 
pages for more specific information about this format. The 
file should be placed in the directory synphot is run from.

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6.  Reminder on WFC buffer dump overheads

Users of the WFC should be reminded that it takes a long 
time (nearly six minutes) to read out and transfer ACS/WFC 
data for each exposure. For WFC exposures 338 seconds is a 
magic number; for longer exposure times the next readout 
and data buffer dump will be hidden in the next exposure; 
for shorter exposure times the telescope and ACS will sit 
idle for an extra six minutes after the exposure to deal 
with the data. This is discussed well in the ACS Handbook 
(Chapter 9), but thought worth pointing out here. Somewhat 
new for Cycle 12 is improved support of subarrays which can 
further help ameliorate buffer management overheads, see 
the next item for details on new subarrays.

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7.  Subarrays for WFC and HRC

In cycle 12, selection of general subarrays will be 
available, and in some cases, supported. In this context, 
supported means that bias frame calibrations will be 
supplied by STScI for a predefined set of subarrays. See:

http://www.stsci.edu/hst/acs/faqs/subarrays.html

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8.  CCD Bias calibration and Gains in Cycle 12

The ACS Instrument Handbook for Cycle 12 included
details on readout noise and full well depths for the two 
CCD instruments (e.g. see Chapter 7). The supported GAINS 
in Cycle 12 are HRC = 2, 4 electrons/DN; WFC = 1,2. The 
Cycle 11 calibrations were concentrated for the default 
GAINS (WFC 1, HRC 2). Experience has shown that calibration
reference files are independent of gain selected with the 
important exception of biases which have unique 
gain-dependent structure.  We will provide equally good 
calibration reference files at both of the supported GAINs 
for HRC and WFC in Cycle 12. Users should therefore feel 
free to choose the best gain for their science independent 
of calibration program considerations.

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9.  ACS Coronagraphy

The new developments for this may be found in Instrument 
Science Report ACS 2002-11, "ACS Coronagraph Update for 
Cycle 12 Proposers," by John Krist:

http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf

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10. New version of Tiny Tim HST PSF simulation software

Version 6.1 of Tiny Tim will be released in mid-April 2003. 
This will be the first update since ACS was put into orbit
and includes significant enhancements to its ACS PSF 
modeling capabilities. On-orbit aberrations are included as 
well as the most recent geometric distortion coefficients.  
Field-dependent aberrations have also been updated. 
Field-dependent CCD charge diffusion is now also included, 
for both the WFC and HRC (this is the largest factor in the 
variation of the PSF FWHM over the field of view). Tiny 
Tim is available at:

http://www.stsci.edu/software/tinytim

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11. Pipeline automatically providing flat-fielded images 

As of 12 March 2003, the ACS calibration pipeline (CALACS) 
has begun automatically and always providing the individual
flat-fielded images (*flt.fits) which were used to produce 
the combined flat-fielded products, such as the cosmic-ray 
cleaned (*crj.fits) and drizzled (*drz.fits) images.
Previously, these files were only provided for some types 
of observations, so users who desired them had to set 
EXPSCORR=PERFORM and re-run CALACS themselves. Since these
individual flat-fielded images are the input for 
MultiDrizzle, they have become essential for the reduction 
of most ACS observations, regardless of how the exposures 
were defined and executed. For information on MultiDrizzle,
please see the STSDAS Web page.

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12. Calibration Workshop Proceedings

Proceedings from the October 2002 HST Calibration Workshop
are now available online:

http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2002/

Archived webcasts of the presentations are also 
available for viewing. 

 

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13. Recent ACS publications

This listing was generated with STEPsheet:
http://stepsheet.stsci.edu/


ANDERSON, J. "Astrometry with the Advanced Camera: 
PSFs and Distortion in the WFC and HRC" 
2002 HST Calibration Wksp. 13-22, 2003

BOND, H.E.; HENDEN, A.; LEVAY, Z.G.; PANAGIA, N.; 
SPARKS, W.B.; STARRFIELD, S.; WAGNER, R.M.; CORRADI, R.L.M.; 
MUNARI, U. "An Energetic Stellar Outburst Accompanied by 
Circumstellar Light Echoes" Nature 422: 405-408, 2003

CASERTANO, S.; GOODS TEAM; LAIDLER, V.; CAPAK, P.; 
COWIE, L.; HUBBLE HIGHER-Z SN TEAM; HICKEN, M. 
"Supernovae 2002kh, 2002ki, and 2003N" IAUC 8052: 1, 2003                 

CLAMPIN, M.; SIRIANNI, M.; BLAKESLEE, J.P.; GILLILAND, R.L. 
"Status of the Advanced Camera for Surveys" 
2002 HST Calibration Wksp. 3-12, 2003

COX, C.; GILLILAND, R.L. "The Effect of Velocity 
Aberration on ACS Image Processing" 
2002 HST Calibration Wksp. 58-60, 2003

DAHLEN, T. "Supernovae 2003az, 2003ba, 2003bb, 2003bc, 
2003bd, 2003be" IAUC 8081: 1, 2003

DE GRIJS, R.; LEE, J.T.; MORA HERRERA, M.C.; 
FRITZE-V. ALVENSLEBEN, U.; ANDERS, P. 
"Stellar Populations and Star Cluster Formation in 
Interactivng Galaxies with the Advanced Camera for Surveys"
NewA 8: 155-171, 2003 

FAN, X.; STRAUSS, M.A.; SCHNEIDER, D.P.; BECKER, R.H.; 
WHITE, R.L.; HAIMAN, Z.; GREGG, M.; PENTERICCI, L.;
GREBEL, E.K.; NARAYANAN, V.K.; LOH, Y.-S.; RICHARDS, G.T.; 
GUNN, J.E.; LUPTON, R.H.; KNAPP, G.R.; IVEZIC, Z.;
BRANDT, W.N.; COLLINGE, M.; HAO, L.; HARBECK, D.; PRADA, F.; 
SCHAYE, J.; STRATEVA, I.; ZAKAMSKA, N.; ANDERSON, S.; 
BRINKMANN, J.; BAHCALL, N.A.; LAMB, D.Q.; OKAMURA, S.; 
SZALAY, A.; YORK, D.G. "A Survey of z > 5.7 Quasars in the 
Sloan Digital Sky Survey. II. Discovery of Three Additional 
Quasars at z > 6" AJ 125: 1649-1659, 2003

GILLILAND, R.L.; RIESS, A. "Extreme Red Sensitivity of 
ACS/WFC" 2002 HST Calibration Wksp. 61-64, 2003

MACK, J.; BOHLIN, R.C.; GILLILAND, R.L.; VAN DER MAREL, R.; 
DE MARCHI, G.; BLAKESLEE, J.P. "ACS Flat Fields and
Low-Order 'L-flat' Corrections from Observations of 
47 Tucanae" 2002 HST Calibration Wksp. 23-30, 2003

MEURER, G.R.; LINDLER, D.; BLAKESLEE, J.P.; COX, C.; 
MARTEL, A.R.; TRAN, H.D.; BOUWENS, R.J.; FORD, H.C.;
CLAMPIN, M.; HARTIG, G.F.; SIRIANNI, M.; DE MARCHI, G. 
"Calibration of Geometric Distortion in the 
ACS Detectors" 2002 HST Calibration Wksp. 65-69, 2003

MUTCHLER, M.; KOEKEMOER, A.M.; HACK, W. 
"Drizzling Dithered ACS Images - A Demonstration" 
2002 HST Calibration Wksp. 70-73, 2003

PASQUALI, A.; PIRZKAL, N.; WALSH, J.R. 
"The Wavelength Calibration of the WFC Grism" 
2002 HST Calibration Wksp. 38-46, 2003

PIRZKAL, N.; PASQUALI, A.; WALSH, J.R. "Flat-fielding of 
ACS WFC Grism Data" 2002 HST Calibration Wksp. 74-77, 2003

RATNATUNGA, K.U. "Statistical Analysis of ACS Data without 
Covariance in Errors" 2002 HST Calibration Wksp. 78-81, 2003

RIESS, A. "Growth of Hot Pixels and Degradation of CTE 
for ACS" 2002 HST Calibration Wksp. 47-52, 2003

SIRIANNI, M.; DE MARCHI, G.; GILLILAND, R.L.; BOHLIN, R.C.; 
PAVLOVSKY, C.; MACK, J. "On-Orbit Sensitivity of ACS"
2002 HST Calibration Wksp. 31-37, 2003

SIRIANNI, M.; MARTEL, A.R.; JEE, M.J.; VAN ORSOW, D.; 
SPARKS, W.B. "Bias Subtraction and Correction of 
ACS/WFC Frames" 2002 HST Calibration Wksp. 82-85, 2003

SPARKS, W.B. "ACS Calibration Software" 
2002 HST Calibration Wksp. 53-57, 2003

TRAN, H.D.; MEURER, G.R.; FORD, H.C.; MARTEL, A.R.; 
SIRIANNI, M.; BOHLIN, R.C.; CLAMPIN, M.; COX, C.; 
DE MARCHI, G.; HARTIG, G.; KIMBLE, R.; ARGABRIGHT, V. 
"On-Orbit Performance of the ACS Solar Blind Channel" 
2002 HST Calibration Wksp. 86-89, 2003

TRAN, H.D.; SIRIANNI, M.; FORD, H.C.; ILLINGWORTH, G.D.; 
CLAMPIN, M.; HARTIG, G.; BECKER, R.H.; WHITE, R.L.; 
BARTKO, F.; BENITEZ, N.; BLAKESLEE, J.P.; BOUWENS, R.; 
BROADHURST, T.J.; BROWN, R.; BURROWS, C.; CHENG, E.; 
CROSS, N.; FELDMAN, P.D.; FRANX, M.; GOLIMOWSKI, D.A.; 
GRONWALL, C.; INFANTE, L.; KIMBLE, R.A.; KRIST, J.; 
LESSER, M.; MAGEE, D.; MARTEL, A.R.; MCCANN, W.J.; 
MEURER, G.R.; MILEY, G.; POSTMAN, M.; ROSATI, P.; 
SPARKS, W.B.; TSVETANOV, Z. "Advanced Camera for Surveys 
Observations of Young Star Clusters in the Interacting 
Galaxy UGC 10214" ApJ 585: 750-755, 2003 

WALSH, J.R.; PIRZKAL, N.; PASQUALI, A. 
"Modelling the Fringing of the ACS CCD Detectors" 
2002 HST Calibration Wksp. 90-93, 2003

YAN, H.; WINDHORST, R.A.; COHEN, S.H. "Searching for z~=6 
Objects with the Hubble Space Telescope Advanced Camera 
for Surveys: Preliminary Analysis of a Deep Parallel Field"
ApJ 585: L93-L96, 2003


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