HST/ACS Coma Cluster Treasury Survey - Data Release 1
This data release contains:- ACS Images in F475W and F814W bands processed with the custom pipeline at STScI developed by the project.
Pipeline processed images
Pipeline processed images are released for 25 fields, for 21 of these fields observations are complete, for a further four we release partial data as not all of our four dither positions were observed before the ACS failure.Description of the pipeline
The custom pipeline carries out the following processes on the data:- Basic CCD reduction (bias subtraction, gain correction, dark subtraction, flatfielding and identification of bad pixels, carried our with reference files (superbias and superdark) obtained contemporaneously with the science data.
- Measurement of and correction for bias level offsets between the four quadrants of the images.
- Spatial registration of the four different dither positions by determination of shifts and rotations between individual images, and iterative application of MultiDrizzle to align the images.
- Masking of cosmic ray events, initially using the cosmic ray rejection algorithm of MultiDrizzle, then by identifying and masking those pixels for which three of the four dither positions were affected by cosmic rays, and finally, for those frames which did not have four dither positions, and also for the central strip areas covered by only two dither positions, by application of the routine lacosmic by Pieter van Dokkum.
- Final combination of the images with MultiDrizzle, shrinking the input pixels by a factor 0.8 at the stage when the PSF is convolved by the input pixel scale, the final_pixfrac parameter in MultiDrizzle. The final output images were produced in the default unrotated frame of the ACS/WFC CCDs
- Registration of the images to the Sloan Digital Sky Survey reference frame using SCAMP, by matching objects in F814W with those in SDSS r band, and adjusting the WCS information (CRVALs, CD matrix) in the FITS headers of both F814W and F475W images to correct for offsets. Visits 10 & 59 also required a distortion correction for good agreement with SDSS (PV coefficients in header). The original CRVALs and CD matrix values are saved in the fits header with the prefix "X" (e.g. "XCRVAL1"). This stage does not involve resampling of data.
The units of the final science images are electrons/second. The weight images are 1/variance, where the units of variance are (electrons/second)^2. The variance takes into account background noise, readnoise and flatfielding, but does not take into account the poisson noise of objects.
Known issues with the data
There are some remaining issues with the data, which will be addressed for Data Release 2 (expected towards the end of 2008):- There is an offset in pixel space between the F814W and F475W images. This offset is fairly consistent at 0.3 pixels, and in the Y direction in pixel space (F475W being at positive Y compared with F814W), except for Visit 45, which has a much larger offset of order 1.5 pixels. This will be addressed in Data Release 2 by redrizzling using appropriate pixel shifts in the distortion-corrected reference frame. The Visit 45 anomaly is still being investigated.
- All visits are drizzled with a projection centre fixed in pixel space rather than to a common projection centre on the sky. This means that in the headers, CRPIX1 and CRPIX2 are fixed at (2112.5, 2150.0), and CRVAL1 and CRVAL2 vary between visits. The only exception to this is Visit 13, for which the first dither position failed, and for which CRPIX1 and CRPIX2 are (2055.18, 1121.78). We have not yet decided whether Data Release 2 will be drizzled to a common CRPIX or common CRVAL.
- In images containing bright targets there are negative cross-talk ghosts, which mirror real images recorded in the opposite quadrant of the same CCD. A more detailed description of these ghosts is given at The ACS anomalies and artifacts page at STScI.
Fields and Exposures
In the table following we list the Visits, target names, co-ordinates and details of the exposures in both F814W and F475W filters. Exposure details include exposure times, and the number of dither positions observed (the target for each field/exposure combination is 4). Further details are listed in the image section of this data release.| Visit | Fieldname | RA | Dec | Orient | Date | Exposure (s) | Dithers | Comment | |
|---|---|---|---|---|---|---|---|---|---|
| (J2000) | (Degrees) | F814W | F475W | ||||||
| 01 | Coma1_1 | 13:00:45.90 | 28:04:54.0 | 282.0 | 09/Jan/2007 | 1400 | 2677 | 4 | |
| 02 | Coma1_2 | 13:00:30.30 | 28:04:54.0 | 282.0 | 09/Jan/2007 | 1400 | 2677 | 4 | |
| 03 | Coma1_3 | 13:00:14.70 | 28:04:54.0 | 282.0 | 20/Jan/2007 | 700 | 1397 | 2 | |
| 08 | Coma2_1 | 13:00:42.30 | 28:01:43.0 | 282.0 | 12/Jan/2007 | 1400 | 2677 | 4 | |
| 09 | Coma2_2 | 13:00:26.70 | 28:01:43.0 | 282.0 | 13/Jan/2007 | 1400 | 2677 | 4 | |
| 10 | Coma2_3 | 13:00:11.10 | 28:01:43.0 | 282.0 | 13/Jan/2007 | 1400 | 2677 | 4 | |
| 12 | Coma2_5 | 12:59:39.90 | 28:01:43.0 | 282.0 | 12/Jan/2007 | 1050 | 2012 | 3 | |
| 13 | Coma2_6 | 12:59:24.30 | 28:01:43.0 | 282.0 | 13/Jan/2007 | 700 | 1372 | 2 | |
| 14 | Coma2_7 | 12:59:08.70 | 28:01:43.0 | 282.0 | 08/Jan/2007 | 1050 | 2037 | 3 | |
| 15 | Coma3_1 | 13:00:38.70 | 27:58:32.0 | 282.0 | 26/Jan/2007 | 1400 | 2677 | 4 | |
| 16 | Coma3_2 | 13:00:23.10 | 27:58:32.0 | 282.0 | 25/Jan/2007 | 1400 | 2677 | 4 | |
| 18 | Coma3_4 | 12:59:51.90 | 27:58:32.0 | 282.0 | 25/Jan/2007 | 1400 | 2677 | 4 | |
| 19 | Coma3_5 | 12:59:36.30 | 27:58:32.0 | 282.0 | 25/Jan/2007 | 1400 | 2677 | 4 | |
| 22 | Coma4_1 | 13:00:35.10 | 27:55:21.0 | 282.0 | 22/Jan/2007 | 1400 | 2560 | 4 | |
| 23 | Coma4_2 | 13:00:19.50 | 27:55:21.0 | 282.0 | 27/Jan/2007 | 1400 | 2677 | 4 | |
| 24 | Coma4_3 | 13:00:03.90 | 27:55:21.0 | 282.0 | 24/Jan/2007 | 1400 | 2560 | 4 | |
| 25 | Coma4_4 | 12:59:48.30 | 27:55:21.0 | 282.0 | 24/Jan/2007 | 1400 | 2560 | 4 | |
| 33 | Coma5_5 | 12:59:29.10 | 27:52:10.0 | 282.0 | 15/Jan/2007 | 1400 | 2677 | 4 | |
| 45 | Outskirts_3 | 12:58:21.58 | 27:27:40.7 | 318.0 | 21/Nov/2006 | 1400 | 2677 | 4 | |
| 46 | Outskirts_4 | 12:58:34.00 | 27:22:58.8 | 280.0 | 17/Jan/2007 | 1400 | 2677 | 4 | |
| 59 | Outskirts_17 | 12:58:31.15 | 27:11:58.5 | 270.05 | 16/Jan/2007 | 1400 | 2677 | 4 | |
| 63 | Outskirts_21 | 12:56:29.80 | 27:13:32.8 | 265.0 | 21/Jan/2007 | 1400 | 2512 | 4 | |
| 75 | Coma7_7 | 12:58:47.75 | 27:46:12.7 | 280.0 | 17/Jan/2007 | 1400 | 2677 | 4 | |
| 78 | Outskirts_36 | 12:57:10.80 | 27:24:18.0 | 314.52 | 27/Nov/2006 | 1400 | 2677 | 4 | |
| 90 | Outskirts_13 | 12:57:04.22 | 27:31:34.5 | 299.1 | 19/Jan/2007 | 1400 | 2677 | 4 | Repeat of failed visit 55 |
Preview/Download/Inspect images
For each combination of Visit and Passband, a science image and weight image are available. The science image is an inverse variance weighted image (background noise only). The units of the science images are electrons/second. The weight images are 1/variance, where the units of variance are (electrons/second)^2. The variance takes into account background noise, readnoise and flatfielding, but does not take into account the poisson noise of objects. A background rms map which is corrected for the correlated pixel noise can be calculated from the weight image using: rms=1/sqrt(weightframe]*0.77). Use the button "Preview/Download/Inspect images" on this page to access the data. After clicking the button the weight frames are accessible by clicking "objectview" in the column data lineage and then using the link under "+weight".About Astro-WISE and this release
Key aspects of the Coma Legacy Survey public release via Astro-WISE
- Inspect/preview/download. The existing Astro-WISE infrastructure allows interactive inspection of the data via web applets (Aladin) world-wide in addition to the preview and download options.
- Connection to future releases. The Astro-WISE data lineage connects the data in future data releases to data in previous released from which they are derived. For example the COMA LS team is performing surface photometry fitting on the ACS data in Astro-WISE (via galphot and galfit). The data lineage allows tracing back exactly how these fits were derived. (Note: the released images have been calibrated outside Astro-WISE at STScI before they were ingested into the Astro-WISE environment. For this reason the data lineage tracing how the released data was derived from the raw data is not stored in Astro-WISE.)
Astro-WISE in a nutshell
Astro-WISE is an environment to archive, calibrate and scientifically analyze images and derived products from any astronomical imager. The following novel aspects of Astro-WISE which are relevant for the Coma Legacy Survey:- Easy release of data. At obtaining a result Astro-WISE users can decide to make the data accessible to only the collaborators or also the public by raising a flag. Automatically, the result is then accessible via web and command-line interfaces. The web interface to this public release is an example. Furthermore, public data is also automatically published in the Virtual Observatory. For this release, search for "COMALS" in the European ESAVO registry and US NVO registry.
- Federated system. Local storage spaces, processing grids and databases are connected into a single information system. Collaborators can work on literally the same data from any location (no Astro-WISE installation required) via web services such as the Target processor.
- Full data lineage. Full data lineage of all processing steps performed in Astro-WISE. Users can obtain recursively process configurations, used input data,...etc of any image, catalog or other derived products created in Astro-WISE.
- Standardization. Data from all instruments is standardized so that all operations in Astro-WISE are the same for any data from any imager. The Coma Legacy Survey team uses data from various imagers besides ACS.