wiki:H2RGs

H2RGs

Characteristics

Format

H2RGs are arrays of 2048 by 2048 pixels. However, the first and last four rows and the first and last four columns consist of light-insensitive reference pixels. Thus, the light-sensitive region is an array of 2040 by 2040 pixels.

Read Modes

We normally read the H2RGs using CDS with one pair of reads (N = 1) or Fowler sampling with four pairs of reads (N = 4). The number of pairs of reads N is given in the FITS headers by NREADS.

Data

The pixel values in the images from C2 and C3 are difference between the mean signal read at the end of the exposure and the mean signal read at the beginning of the exposure. They are not divided by the exposure time à la ESO.

Software Gain

The Teledyne software writes the images as BITPIX = -32 FITS files. This results oversampling of the read-noise which in turn leads to relatively poor compression, with bzip2-compressed sizes being about 65% of the original sizes. This is a problem since the Internet link to the OAN/SPM is quite slow.

Therefore, we divide the raw images by "software gain" factors of 2 and convert the images to BITPIX = 16 FITS files on the mountain prior to compression and transfer to the archives. This process reduces the bzip2-compressed sizes to about 23% of the original sizes. Since we are still sampling the images at better than 1/2 of the read noise, we lose no information (Price-Whelan & Hogg 2010).

If you have doubts as to whether an image has been divided by the software gain factor, look for a SOFTGAIN record in the header. If one is present, it specifies the software gain that has been applied.

One consequence of this is that the images effectively saturate at 32767 DN, but this is above the physical saturation of the detectors.

Last modified 5 years ago Last modified on Jan 4, 2013 1:34:56 AM