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COSMOS Observations: Hubble Space Telescope

The COSMOS HST Treasury project was allocated 640 orbits in Cycles 12 & 13 to image a square 2 square degree field with the Advanced Camera for Surveys (ACS). Per instructions from the Telescope Allocation Committee (TAC), 50 orbits were given over to an outside group to pursue high-redshift supernovae. The remaining 590 orbits have been used to observe the COSMOS field.

Single orbit exposures with ACS using the F814W filter:

  • imaging at 0.05 arcsec resolution
  • excellent sensitivity -- IAB = 27.2 mag (10 sigma)

The ACS imaging is the central cornerstone of the COSMOS survey to study galaxy and large scale structure evolution. ACS provides morphologies of over 2 million galaxies at < 100 pc resolution.

Each orbit has 4 dithered exposures in F814W for cosmic ray and bad pixel rejection and to cover the gap between the two ACS CCD arrays.

Over Cycle 12 and 13, the entire 2 degree field was mosaiced starting from the center to the perimeter.

Parallel data
Two HST instruments were used in parallel with the ACS observations: NICMOS and WFPC2. These observations also exploit the excellent spatial resolution of HST, although neither provides contiguous coverage of the entire field.
  • NICMOS camera 3 in F160W
    (covers ~ 7% of the COSMOS field to 24 mag AB )
  • WFPC2 in F300W (U band) and F450W (B band)
Data Products

590 adjacent HST/ACS fields, each consisting of 4 exposures were imaged in the F814W band during cycles 12 and 13. These were calibrated and combined at STScI into a single image for each field using the MultiDrizzle software, and version 1.3 is available from the COSMOS Archive at IRSA. Work on the HST/ACS data products is led by Anton Koekemoer at STScI and additional information on the observations can be found on the COSMOS HST/ACS website.

Models of the varying focal length of HST and the corresponding point spread function during the time of the COSMOS observations are available from http://www.astro.caltech.edu/~rjm/acs/

Version 2.0 of the data, currently in preparation, will also include pixel-by-pixel correction for trailing due to Charge Transfer Inefficiency.


Updated June 2008.