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COSMOS HST/ACS Data   (I-band)                                  Version 2.0
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The Cosmic Evolution Survey (COSMOS) is an HST Treasury Project to survey
a 2 square degree equatorial field, centered on RA=10:00:28.6 and
DEC=+02:12:21.0, using the ACS camera. The project also incorporates major
commitments from other observatories around the world, including the VLA
radio telescope, ESO's VLT in Chile, ESA's XMM X-ray satellite, and the
Japanese 8-meter Subaru telescope in Hawaii.

These data represent the third public release of the COSMOS observations
from the ACS instrument on HST. Included in this release are 81 image tiles
comprising the entire survey, observed using the F814W (I-band) filter.

The key characteristics of these data:

	Instrument:            HST Advanced Camera for Surveys (ACS)
	Dates of observations: Cycle 12-13 (Jul 2003 - Jun 2005)
	Observing time:        4 exposures / 1 orbit / pointing
	Filter:                F814W (I-band)
	Area:                  ~2 sq deg
	Resolution:            0.03"/pixel (drizzled data)

The v2.0 processing was led by Anton Koekemoer 
(http://www.stsci.edu/~koekemoer). Raw exposures were first processed
though pixel-level CTE correction (Massey et al. 2010). The current best
ACS calibration reference files were then applied (including dark current,
flatfield, bias subtraction, quadrant offset correction, low-level
background removal). Individual exposures were finally CR-cleaned,
astrometrically aligned, and coadded using MultiDrizzle (Koekemoer et al.
2002). All images are in counts per second and have corresponding weight
images, which are in units of inverse variance.

When using this version of the HST/ACS data for any paper, please cite the
following two papers, which contain further details about all the
observations and reductions:

  Koekemoer, A. M., Aussel, H., Calzetti, D., Capak, P., Giavalisco, M.,
    Kneib, J.-P., Leauthaud, A., Le Fèvre, O., McCracken, H. J.,
    Massey, R., Mobasher, B., Rhodes, J., Scoville, N., Shopbell, P.
    2007, ApJS 172, 196 

and:

  Massey, R., Stoughton, C., Leauthaud, A., Rhodes, J., Koekemoer, A.,  
    Ellis, R., Shaghoulian, E., 2010, MNRAS, 401, 371

More information on MultiDrizzle can be found in Koekemoer, A. M.,
Fruchter, A. S., Hook, R. H., Hack, W. 2002, HST Calibration Workshop
(Baltimore: STScI), p. 337 or at 

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

The data products available here include:

	(1) drizzled, flux calibrated, astrometrically correct images,
	    rotated such that north is up, east to the left, and tiled
	    to match the COSMOS ground-based observations
	(2) corresponding weight images (exposure maps)

The image files follow a standard naming scheme where, for example:

	acs_I_095751+0228_unrot_sci_20.fits

refers to an HST/ACS I-band (F814W) mosaic image at RA 09:57:51, Dec
+02:28, unrotated (thus in the detector frame), version 2.0. Both science
images ("_sci_" in file name) and weigh images ("_wht_" in file name) are
provided.

Please note: The COSMOS ACS FITS images contain floating point data but do
not use IEEE NaN values to denote blank pixels.  Rather each image has an
associated weight image mask (also a floating point FITS image) that should
be used to weight image operations.  In particular, mask values of zero
indicate areas of no data (including pixels masked off because of radiation
hits).

Further information may be obtained from the main COSMOS web page:

	http://cosmos.astro.caltech.edu/

and the COSMOS HST/ACS web page:

	http://www.stsci.edu/~koekemoe/cosmos/

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Anton Koekemoer  (http://www.stsci.edu/~koekemoer)
Richard Massey
Patrick Shopbell

October 31, 2011