|DEEP2 DR1 Photometry Primer|
The DEEP2 photometric data were taken with the CFH12K camera on
the 3.6-meter Canada-France-Hawai'i Telescope. For all details
regarding the processing of the DEEP2 photometric data,
please refer to
Coil et al. 2004. Here, we will simply
present the format of the photometric catalogs included in the DEEP2 DR1
with some brief discussion regarding how to access the data using IDL.
Once downloaded, the photometric data for a given DEEP2 pointing may be
read into an IDL structure (here named photo) using the
mrdfits.pro routine (contained in the IDLUTILS package, courtesy of
GODDARD IDL library) as follows: |
IDL> photo = mrdfits(file, 1, [hdr])
where file is a string specifying the FITS filename (including the path from whatever directory is current) for the desired photometric catalog (e.g. file = 'pcat22.fits.gz'), the second argument (1) instructs mrdfits to read the first FITS extension (or first HDU). The argument in [ ] is optional; if it is included, the FITS file header will be stored in an array of strings called hdr. Note that the mrdfits command is able to read gzip'ed FITS files without having them uncompressed.
IDL structures provide a convenient means for manipulating the information in the photometric database (or any database for that matter). The fields of the database can be addressed as "tags" in the structure photo. The DEEP2 DR1 photometric structures contain 16 tags for each object; there are ~55,000 objects in each photometric catalog. One can access the array of values for an individual tag in a structure using the IDL syntax photo.tag (e.g. photo.magr would be an array containing the R band magnitudes for every object in the catalog). To list all of the tags contained in a given structure, execute the following command:
IDL> help, photo, /str
which will list the tags and their values for the first object in the photo structure. Another option is to use the tag_names command like so:
IDL> print, tag_names(photo)
which will simply list the names of the tags.
There are 16 tags for each object in the photometric catalogs. In detail, the tags or fields are as follows:
Note that the aperture used for the B, R, I magnitudes is 3·RG unless that value is less than 1 arcsec. In such cases, the R magnitude given in the photometric catalog is measured in a 1 arcsec (5 pixel) aperture. B and I magnitudes are then constructed such that (B-R) and (R-I) give the colors as measured through a 1 arcsec aperture, to improve color stability. Also, note that all magnitudes have been corrected using the SFD_EBV dust corrections, and the B, R, I magnitudes are all AB magnitudes measured through the CFHT filter set, calibrateed using SDSS photometry (see Coil et al. 2004 for details). The response curves for the CFHT filters are available for download in ascii format -- the first column gives the wavelength and the last column gives the net (telescope + instrument, but no atmosphere) response: Bresponse.txt, Rresponse.txt, Iresponse.txt.
Each object in the photometric catalogs is assigned a value for the BADFLAG field based on the photometric bad-pixel-mask value at its central pixel. For B, R, and I individually, a pixel has a flag value from 0-5; these are combined together to obtain the BADFLAG value according to
BADFLAG = Bvalue + 6 · Rvalue + 36 · Ivalue
The B, R, and I values may be obtained from the combined BADFLAG value via integer arithmetic (e.g. Bvalue = BADFLAG mod 6). The codes used for B, R, I separately are
0: good pixel
1: no data available
2: excessively bright object centered here
3: saturated pixel at object location
4: identified as a suspect pixel in the dilation process by brightmask.pro
5: within 10-pixel buffer of a bad pixel (of type 1-4)
Each object in the photometric catalogs is assigned a PGAL value according to its probability of being a galaxy. PGAL values in the range 0-1 give the probability that a compact object is a galaxy (only compact sources will have PGAL values 0 - 1). A PGAL value > 1 indicates an extended source and definitely a galaxy.
For further details regarding magnitudes and PGAL or BADFLAG values, refer to Coil et al. 2004.
The FITS header of each photometric catalog contains a vast amount of information pertaining to the photometry. To read and view the header in IDL, you can use the following commands:
IDL> photo = mrdfits('pcat.32.fits.gz', 1, hdr)
IDL> hdr = headfits('pcat.32.fits.gz')
And to view:
IDL> hprint, hdr
The header includes details such as the seeing (in CFHT pixels [0.207"/pixel]) for each CFHT pointing.