The COSMOS HST/ACS Observations Webpage

Anton M. Koekemoer, STScI
 

LATEST NEWS

COSMOS HST/ACS Data V2.0 -- 22 Feb 2010

I have now completed the final, recalibrated, reprocessed mosaic of all the COSMOS ACS/HST data that was obtained during Cycles 12 and 13 in programs 9822 and 10092. The total amount of time spent on the program was 592 orbits, including 4 orbits that were repeated due to guidestar failures (thus a total of 588 usable orbits). The 588 usable orbits consist of 9 orbits in F475W (a 3x3 grid at the centre), together with 579 orbits in F814W which make up the bulk of the mosaic. The 579 usable F814W orbits are spread across 575 contiguous tile positions (two of the tile positions had to be reobserved in a 2-orbit pattern each, due to the presence of very bright stars). Each of the 579 usable F814W orbits (which are spread over the 575 contiguous tile positions) was divided into 4x507s exposures, executed over a 4-point box dither pattern which provided the following benefits: (1) ensure that the gap between the chips was covered by at least 3 exposures in all cases; (2) provide sub-pixel sampling of the PSF across the ACS field of view; (3) ensure good CR rejection across 4 exposures; (4) provide sufficient overlap with adjacent pointings to enable a contiguous mosaic to be created.

For the v2.0 reprocessing, the raw exposures were first processed through CTE correction (Richard Massey), after which the current best ACS calibration reference files were applied in my pipelines (including dark current, flatfield, bias subtraction, quadrant offset correction, low-level background removal), and subsequently CR-cleaned, astrometrically aligned, and combined using MultiDrizzle (Koekemoer et al. 2002). All the final drizzled images are 0.030"/pixel, drizzled with pixfrac=0.8 and a gaussian kernel, and are in units of counts/second. They also have corresponding weight images, which are in units of inverse variance.

Please use these mosaics for your science. If you do so, please reference my paper describing full details of all the HST/ACS observations and reductions:

Koekemoer, A. M. et al. 2007, ApJS 172, 196


HST/ACS COSMOS Mosaic
Tangent Point R.A. =   150.1163213  (J2000)
Tangent Point Dec. =   +2.200973097  (J2000)
Number of x-pixels =     168,000
Number of y-pixels =     168,000
Pixel scale =     0.03" / pixel

DATA ACCESS: The combined mosaic and individual files can all be obtained here:

v2.0/acs_I_mosaic_30mas_full/ The full mosaic science and weight images (225 Gb each!)*
 
v2.0/acs_I_mosaic_30mas_sci/ The 225 Gb mosaic science image (split into 128 files)
v2.0/acs_I_mosaic_30mas_wht/ The 225 Gb mosaic weight image (split into 128 files)
 
v2.0/acs_I_overlap_30mas/ The mosaic as a set of 12x12 overlapping tiles (on the "CFHT" grid)
 
v2.0/acs_I_drz_30mas_unrot/ All the separate, unrotated, drizzled I-band pointings (for weak lensing)
v2.0/acs_g_drz_30mas_unrot/ All the separate, unrotated, drizzled g-band pointings

*Note: The mosaic is *VERY LARGE*, being 168,000 x 168,000 pixels (225 Gb in size!) and some systems have problems retrieving files >2Gb in a single step, so to help with this I've made the mosaic available as a set of 128 pieces above, which are all gzipped, and which can be easily joined back together into a single 225 Gb file (after uncompressing them) using the unix "cat" command once they have all been retrieved, eg:

"cat acs_I_mosaic_30mas_sci.fits.split* > acs_I_mosaic_30mas_sci.fits"

The following figure is about 1.4 degrees across and shows the full grid of 575 pointings across the entire mosaic. There are about 2 million galaxies detected on the mosaic, to a limiting magnitude AB(F814W) ~ 27.

Here's also a medium-sized picture of the mosaic: acs_v2.0_I_mosaic_600mas.jpg


For older news, click here:

Previous News


 

 

 

The COSMOS Field - Individual Pointings

Click each pointing to see the image and obtain more detailed information about the pointing centre, exposure time and orientation, as well as direct access to the combined, drizzled FITS files for that pointing (note however that these are the old datasets, and are superceded by the new v2.0 data above, so the following are presented mainly for historical reasons).

                                                                                      
               08-20 08-25 15-05 15-10 15-15 15-20 15-25                                                   
            09-11 09-16 09-21 16-01 16-06 16-11 16-16 16-21 23-01 23-06 23-11 23-16 23-21 30-01                                 
            09-12 09-17 09-22 16-02 16-07 16-12 16-17 16-22 23-02 23-07 23-12 23-17 23-22 30-02 30-07 30-12 30-17 30-22 37-02                  
            09-13 09-18 09-23 16-03 16-08 16-13 16-18 16-23 23-03 23-08 23-13 23-18 23-23 30-03 30-08 30-13 30-18 30-23 37-03 37-08 37-13 37-18 37-23 44-03   
            09-14 09-19 09-24 16-04 16-09 16-14 16-19 16-24 23-04 23-09 23-14 23-19 23-24 30-04 30-09 30-14 30-19 30-24 37-04 37-09 37-14 37-19 37-24 44-04   
            09-15 09-20 09-25 16-05 16-10 16-15 16-20 16-25 23-05 23-10 23-15 23-20 23-25 30-05 30-10 30-15 30-20 30-25 37-05 37-10 37-15 37-20 37-25 44-05   
            10-11 10-16 10-21 17-01 17-06 17-11 17-16 17-21 24-01 24-06 24-11 24-16 24-21 31-01 31-06 31-11 31-16 31-21 38-01 38-06 38-11 38-16 38-21 45-01   
         10-07 10-12 10-17 10-22 17-02 17-07 17-12 17-17 17-22 24-02 24-07 24-12 24-17 24-22 31-02 31-07 31-12 31-17 31-22 38-02 38-07 38-12 38-17 38-22 45-02   
         10-08 10-13 10-18 10-23 17-03 17-08 17-13 17-18 17-23 24-03 24-08 24-13 24-18 24-23 31-03 31-08 31-13 31-18 31-23 38-03 38-08 38-13 38-18 38-23 45-03   
         10-09 10-14 10-19 10-24 17-04 17-09 17-14 17-19 17-24 24-04 24-09 24-14 24-19 24-24 31-04 31-09 31-14 31-19 31-24 38-04 38-09 38-14 38-19 38-24 45-04   
         10-10 10-15 10-20 10-25 17-05 17-10 17-15 17-20 17-25 24-05 24-10 24-15 24-20 24-25 31-05 31-10 31-15 31-20 31-25 38-05 38-10 38-15 38-20 38-25 45-05   
         11-06 11-11 11-16 11-21 18-01 18-06 18-11 18-16 18-21 25-01 25-06 25-11 25-16 25-21 32-01 32-06 32-11 32-16 32-21 39-01 39-06 39-11 39-16 39-21      
         11-07 11-12 11-17 11-22 18-02 18-07 18-12 18-17 18-22 25-02 25-07 25-12 25-17 25-22 32-02 32-07 32-12 32-17 32-22 39-02 39-07 39-12 39-17 39-22      
      11-03 11-08 11-13 11-18 11-23 18-03 18-08 18-13 18-18 18-23 25-03 25-08 25-13 25-18 25-23 32-03 32-08 32-13 32-18 32-23 39-03 39-08 39-13 39-18 39-23      
      11-04 11-09 11-14 11-19 11-24 18-04 18-09 18-14 18-19 18-24 25-04 25-09 25-14 25-19 25-24 32-04 32-09 32-14 32-19 32-24 39-04 39-09 39-14 39-19 39-24      
      11-05 11-10 11-15 11-20 11-25 18-05 18-10 18-15 18-20 18-25 25-05 25-10 25-15 25-20 25-25 32-05 32-10 32-15 32-20 32-25 39-05 39-10 39-15 39-20 39-25      
      12-01 12-06 12-11 12-16 12-21 19-01 19-06 19-11 19-16 19-21 26-01 26-06 26-11 26-16 26-21 33-01 33-06 33-11 33-16 33-21 40-01 40-06 40-11 40-16         
   05-22 12-02 12-07 12-12 12-17 12-22 19-02 19-07 19-12 19-17 19-22 26-02 26-07 26-12 26-17 26-22 33-02 33-07 33-12 33-17 33-22 40-02 40-07 40-12 40-17         
   05-23 12-03 12-08 12-13 12-18 12-23 19-03 19-08 19-13 19-18 19-23 26-03 26-08 26-13 26-18 26-23 33-03 33-08 33-13 33-18 33-23 40-03 40-08 40-13 40-18         
   05-24 12-04 12-09 12-14 12-19 12-24 19-04 19-09 19-14 19-19 19-24 26-04 26-09 26-14 26-19 26-24 33-04 33-09 33-14 33-19 33-24 40-04 40-09 40-14 40-19         
   05-25 12-05 12-10 12-15 12-20 12-25 19-05 19-10 19-15 19-20 19-25 26-05 26-10 26-15 26-20 26-25 33-05 33-10 33-15 33-20 33-25 40-05 40-10 40-15 40-20         
   06-21 13-01 13-06 13-11 13-16 13-21 20-01 20-06 20-11 20-16 20-21 27-01 27-06 27-11 27-16 27-21 34-01 34-06 34-11 34-16 34-21 41-01 41-06 41-11 41-16         
   06-22 13-02 13-07 13-12 13-17 13-22 20-02 20-07 20-12 20-17 20-22 27-02 27-07 27-12 27-17 27-22 34-02 34-07 34-12 34-17 34-22 41-02 41-07 41-12            
                  13-23 20-03 20-08 20-13 20-18 20-23 27-03 27-08 27-13 27-18 27-23 34-03 34-08 34-13 34-18 34-23 41-03 41-08 41-13            
                                 20-24 27-04 27-09 27-14 27-19 27-24 34-04 34-09 34-14 34-19 34-24 41-04 41-09 41-14            
                                                   34-05 34-10 34-15 34-20 34-25 41-05 41-10 41-15            
                                                                  42-01 42-06               
                                                                                      

 

 

Previous News

[OLD --- COSMOS HST/ACS V1.2 & V1.3 (2005 - 2006)]

The older v1.2 & v1.3 COSMOS data is still accessible here as follows, but this is all now superceded.

DATA ACCESS -- the V1.2 Cycle 12 + 13 drizzled FITS images can all be obtained here, along with their associated weight files (expressed in terms of inverse variance):

http://www.stsci.edu/~koekemoe/cosmos/current/data/v1.2/
drz-30mas-unrot/ (single pointings, unrotated, 30mas/pix, counts/s/pix)
drz-50mas-unrot/ (single pointings, unrotated, 50mas/pix, counts/s/pix)
drz-30mas-overlap-cps/ (overlapping sections, North up, 30mas/pix, counts/s/pix)
drz-50mas-overlap-cps/ (overlapping sections, North up, 50mas/pix, counts/s/pix)
drz-50mas-overlap-nJy/ (overlapping sections, North up, 50mas/pix, nanoJy/pix)

Most of the COSMOS team would likely use the images drizzled with North up, which have been drizzled onto exactly the same frame as the Subaru images, except that the pixel scale is either 0.03"/pixel or 0.05"/pixel instead of 0.15"/pixel, thus the Subaru pixel scale can be recovered by binning the images by a factor of 5 (for the 0.03" images) or a factor of 3 (for the 0.05" images).

For those doing lensing work, it is more desirable to have the images in the unrotated frame, ie with the [sci,1] chip at the bottom and the [sci,2] chip at the top. These images have been drizzled to a pixel scale of 30mas/pixel, using a pixfrac of 0.8 and a Gaussian kernel.

The data are also available as a mosaic, currently at 0.05"/pixel; a version drizzled to 0.03"/pixel will also be made available although right now the most common use of the 0.03"/pixel data seems to be for lensing work which requires data to be unrotated and not mosaiced. The 0.05"/pixel mosaic is 100,800 pixels on a side; the pixels were mapped onto this mosaic by applying all transformations in a single step (including rotations, translations, and mapping onto the tangent plane) using MultiDrizzle (Koekemoer et al. 2002). The mosaic is oriented with North to the top and the central coordinate for the tangent plane projection is near the centre of the mosaic (at pixel 50040, 48600), which is the standard COSMOS reference point:

R.A. = 10:00:28.6 (J2000)
Dec. = +02:12:21.0 (J2000)

Be warned that this mosaic is *VERY LARGE* (81 Gb!) and may take several days to download. In addition, some filesystems have problems retrieving files > 2Gb in a single step, so to help with this I've also made the mosaic available as a set of 41 pieces, each about 2Gb in size (and gzipped), which can be easily joined back together using the unix "cat" command once they have all been retrieved. The single mosaic as well as the split pieces are all available here:

Single FITS file: mos_v1.2cyc12+13_drz.fits (a single 81 Gb file)
Split FITS files: mos_v1.2_cyc12+13_split/ (41 gzipped files, 1 Gb each)

Some mosaic pictures and other related binned-up files are also available here:

Small (500x500 pixels, 10"/pix):mos_500pix.gif (90 kb)
Medium (5000x5000 pixels, 1"/pix):mos_5000pix.gif (8 Mb)
Large (16000x16000 pixels, 0.25"/pix):mos_16000pix.fits (1.1 Gb)
Larger (20160x20160 pixels, 0.25"/pix):mos_20160pix.fits (1.6 Gb)
For January 2006 AAS meeting:mosaic/aas/


The target coordinates for all the COSMOS pointings in the HST PhaseII file are available here:

cosmos-pointings.lis


July 2005 - COSMOS Complete: All HST Cycle 12 + 13

All 588 orbits of the COSMOS project have now been obtained and are available. Note that unfortunately two of the more recent pointings failed as a result of guidestar problems, leaving a small gap near the top of the mosaic. We are requesting that these be repeated ASAP, preferably with different guidestars.


June 2005 - Full Cycle 12+13 V1.2 Mosaic

The full Cycle 12+13 mosaic is now available, which I've constructed based on all the Cycle 12+13 V1.2 data (588 orbits in I-band obtained during Cycle 12). This mosaic uses tiles that are registered to an accuracy of ~0.2 ACS pixels (10 milliarcseconds), and also has greatly improved cosmic ray rejection along the overlap regions between tiles, so it can be treated properly as a contiguous image. There are still low-level background variations (on the order of 5% of the original sky values), mostly due to scattered light effects. Other remaining low-level effects include faint satellite and asteroid trails, and ghosts from bright stars. These are in the process of being removed for the next version of the data. The mosaic is at the full ACS resolution (50 mas/pix) and is 100,800 pixels on a side; previously only the binned-down versions were available. The pixels were mapped onto this mosaic by applying all transformations in a single step (including rotations, translations, and mapping onto the tangent plane) using MultiDrizzle (Koekemoer et al. 2002). The mosaic is oriented with North to the top and the central coordinate for the tangent plane projection is near the centre of the mosaic (at pixel 50040, 48600), which is the standard COSMOS reference point:

R.A. = 10:00:28.6 (J2000)
Dec. = +02:12:21.0 (J2000)


Thursday 31 March 2005 - Full Cycle 12 V1.2 Mosaic

The full Cycle 12 mosaic is now available, which I've constructed based on all the Cycle 12 V1.2 data (261 tiles in I-band obtained during Cycle 12). This mosaic uses tiles that are registered to an accuracy of ~0.2 ACS pixels (10 milliarcseconds), and also has greatly improved cosmic ray rejection along the overlap regions between tiles, so it can be treated properly as a contiguous image. There are still low-level background variations (on the order of 5% of the original sky values), mostly due to scattered light effects. Other remaining low-level effects include faint satellite and asteroid trails, and ghosts from bright stars. These are in the process of being removed for the next version of the data. The mosaic is at the full ACS resolution (50 mas/pix) and is 80,000 pixels on a side; previously only the binned-down versions were available. The pixels were mapped onto this mosaic by applying all transformations in a single step (including rotations, translations, and mapping onto the tangent plane) using MultiDrizzle (Koekemoer et al. 2002). The mosaic is oriented with North to the top and the central coordinate for the tangent plane projection is at the centre of the mosaic (at pixel 40000, 40000).


January 2005 - ACS Cycle 13 V1.2 Data Release

A total of 177 pointings from the Cycle 13 data have so far been obtained. These have been processed through the COSMOS cluster pipeline at STScI to the same level of accuracy as the Cycle 12 data to date (see next item below), in terms of data calibration and astrometric alignment. Additional data will be processed to the same level of accuracy, as new images arrive.


Monday 22 November 2004 - ACS Cycle 12 V1.2 Data Release

The V1.2 images for the full 270 orbits from Cycle 12 have been produced by completely re-drizzling using improved absolute astrometry, together with the latest improved geometric distortion corrections for the ACS camera. The images have been registered onto the fundamental astrometric frame for COSMOS, which makes use of the CFHT Megacam i-band image. The CFHT image (courtesy of Olivier LeFevre, Herve Aussel, Henry McCracken, et al.) is tied to the USNO-B1.0 system, which in turn has been tied to the VLA 1.4 GHz image (courtesy Eva Schinnerer and Chris Carilli). These V1.2 ACS images therefore supercede the V1.0 and V1.1 images, and should provide an absolute accuracy of order 0.05-0.1" or better.

All images have the same pixel scale = 1 ACS pixel = 0.05"/pixel. Images with North up are 5600x5600 pixels (thus 120 Mb each) while the unrotated images are smaller at 4600x4600 pixels (80 Mb each). The single-exposure images are drizzled onto the same scale and orientation as the combined images.


Thu 17 Jun 2004 - ACS Cycle 12 Data Release V1.1: Improved Astrometry

The full 270 orbits from Cycle 12 have now been registered onto the fundamental astrometric frame for COSMOS, which makes use of the CFHT Megacam i-band image. The CFHT image (courtesy of Olivier LeFevre, Herve Aussel, Henry McCracken, et al.) is tied to the USNO-B1.0 system, which in turn has been tied to the VLA 1.4 GHz image (courtesy Eva Schinnerer and Chris Carilli).

NOTE: The CFHT catalog that was used here was a preliminary catalog that Herve Aussel had shifted onto the VLA frame. In the meantime, Henry McCracken is using this CFHT catalog to completely regrid the CFHT image and then generate a new catalog from that. Once this new, final, master catalog has been generated, it will be used to further refine the astrometry of the HST images. However, the current V1.1 release should already be a substantial improvement over the V1.0 images.

The initial astrometry in the earlier V1.0 data release was based purely on the astrometric information from the HST pipeline, which in turn was based on an assumed knowledge about the guidestar positions in GSC1. However, the GSC1 positions are typically off by ~1-2", and in some cases in the COSMOS data up to 3-4", therefore the absolute astrometry in the original V1.0 data release was also inaccurate to this level. These guidestar positional uncertainties also introduced slight errors in the image orientation, typically to the level of 0.02 degrees or less.

The V1.1 ACS images, which have been registered onto the VLA/USNO-B1.0/CFHT frame, now have absolute astrometry as well as relative alignment between each ACS tile to better than ~0.1", and have also corrected the orientation uncertainties. At levels below ~0.1", small differences are still apparent (of the order 1 ACS pixel = 50 mas or less). These can be addressed by incorporating improvements to the ACS distortion model that have been developed at STScI in recent months, and these improvements will be included in a subsequent COSMOS data release. However, the astrometric changes between V1.1 and any subsequent release will likely be below the level of 0.1" and will be primarily aimed at improving the registration between adjacent ACS tiles to an accuracy of 2-5 milliarcsec.

Therefore, the present V1.1 release can be considered accurate to an absolute astrometric level of approximately 0.1", which is an improvement of 10-50x over the original V1.0 astrometry.

 

Tue 18 May 2004 - Total now 270 orbits: CYCLE 12 COMPLETED!!

The final 7 pointings from Cycle 12 have been obtained and processed:
31-19, 31-20, 31-21, 31-22, 31-23, 31-24, 31-25


Fri 14 May 2004 - Total now 263 orbits

An additional 11 pointings have been obtained and processed:
30-15, 30-25, 31-01, 31-02, 31-03, 31-04, 31-06, 31-08, 31-16, 31-17, 31-18


Thu 6 May 2004 - Total now 252 orbits

An additional 10 pointings have been obtained and processed:
30-20, 31-05, 31-07, 31-09, 31-10, 31-11, 31-12, 31-13, 31-14, 31-15


Tue 4 May 2004 - Total now 242 orbits

An additional 15 pointings have been obtained and processed:
26-11, 26-13, 26-14, 26-15, 26-16, 26-17, 26-18, 26-19, 26-20, 26-21, 26-22, 26-23, 26-24, 30-05, 30-10


Sun 2 May 2004 - Total now 227 orbits

An additional 10 pointings have been obtained and processed:
24-10, 24-17, 26-02, 26-03, 26-04, 26-05, 26-06, 26-08, 26-09, 26-10


Thu 29 April 2004 - Total now 217 orbits

An additional 2 pointings have been obtained and processed:
24-01, 24-02


Wed 28 April 2004 - Total now 215 orbits

An additional 8 pointings have been obtained and processed:
25-25, 26-01, 32-01, 32-02, 32-09, 32-10, 32-19, 32-20


Fri 23 April 2004 - Total now 207 orbits

An additional 4 pointings have been obtained and processed:
25-15, 25-22, 25-23, 32-11


Tue 20 April 2004 - Total now 203 orbits

All additional pointings obtained before 20 April have now been processed, bringing the total number of orbits to 203 (9 of which are g-band images overlapping with their i-band counterparts). The new pointings obtained after 31 March are:

23-1023-1523-2023-2524-0324-0424-0524-0724-0824-0924-1124-12
24-1324-1424-1524-1924-2024-2224-2324-2424-2525-0125-0225-03
25-0425-0525-1025-1125-1625-2032-0332-0432-0532-0632-0732-12
32-1432-1532-1632-1732-1832-2132-2232-2332-2432-2533-0133-02
33-0333-0433-0533-0633-0733-0833-0933-1033-1133-1233-1333-14
33-1533-1733-1833-1933-2033-2333-2433-2539-0139-0239-0439-05

DATA ACCESS: Note that the newest data are accessible via ftp, but due to security issues here our computing division needs to know the IP address numbers of the specific machines that COSMOS team members will be using to ftp the data. So, could everyone please let me know these IP address numbers and I will ask our computer people to add your machines to the access lists. You'll be able to download the newest data only after this is done.


Thu 15 April 2004 - Single-Exposure Images Released

In order to help with examining interesting features on the images, eg faint, long, curved "arcs" (most of which may be due to slow-moving solar system objects), the single-exposure images are being made available, all drizzled onto the same scale and orientation as the final images. They are available here: drz_single/


Wed 31 March 2004 - Total now 131 orbits

An additional 3 pointings have been obtained and processed: 19-24, 19-25, 23-05


Tue 30 March 2004 - Total now 128 orbits

An additional 3 pointings have been obtained and processed: 19-20, 19-21, 19-22

An issue has been identified with the data taken in October and November 2003, (mostly near the centre of the field, with a few other pointings across the field) which seem to have a scattered light problem, evident as a large-scale imprint on the final output images. This is possibly due to the fact that those observations were obtained within 50 - 60 degrees of the sun. Efforts are underway to model and subtract this from the images. However, the effect on the scientific quality of object measurements (photometry, SExtractor results etc) is likely to be insignificant.


Thu 26 March 2004 - ACS Data V1.0 Release: 125 pointings

All pointings up to March 25, 2004 (125 orbits total, with 116 pointings in i-band, and an overlapping 9 pointings in g-band) have been reprocessed and improved significantly since last year's V0.5 release, as follows:

  • Flat fielding: The flatfield reference files have been improved significantly, so that the images are now flat to below about 1% of the sky level.
  • Cosmic ray rejection: The CR rejection has now finally been optimized for the COSMOS datasets (4-point dithers with 500s exposures), which has corrected some over-rejection in the bright cores of some objects that was apparent in the V0.5 data, as well as eliminating almost all the spurious residual CRs in the field. The only CRs remaining now are simply the result of 4 cosmic rays overlapping at the same place in all 4 exposures, but these are very infrequent. Some care should be taken along the edges and in the gap along the centre of the images, where the depth is only 3 exposures deep, so I recommend examining the weight files to quantify the exposure depth across the images.
  • Astrometric improvements: Some attempt has been made to improve the absolute astrometry of the images, although this work is still on-going, so an update will be posted here as soon as that is complete.