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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

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.