LRIS Data Format
LRIS raw data files for both LRIS blue (LRISb) and red (LRISr) sides are in multi-extension FITS format. This consists of a primary header, which is the first header of the FITS file, followed normally by four extensions, called header data units (HDUs), one per CCD output amplifier. Each extension HDU contains the two standard components of any FITS file: (a) an ASCII text header holding a list of keywords that describe the data unit of that HDU, and (b) the binary data following it. For a description of the keywords in the primary header unit, visit http://www2.keck.hawaii.edu/koa/public/keywords/lrisKeywords.php.
LRISr uses a mosaic of two LBNL 2K x 4K fully depleted, high resistivity CCD detectors, and LRISb has a mosaic of two 2K x 4K Marconi CCDs, with a pixel scale of 0.135 "/pixel for both sides.
The image pixels are in 16-bit integers. A full-frame, unbinned raw image contains 38,041,920 bytes for the blue side, and 36,774,720 bytes for the red side.
The following table shows the number of prescan and postscan pixels per readout amplifier for a full-frame, un-windowed and un-binned image produced by each camera. Note that NAXIS1 and NAXIS2 are the number of pixels contained in each HDU and include the prescan and postscan pixels.
| Camera | Number of amplifiers |
NAXIS1 | NAXIS2 | PRELINE | PRECOL |
POSTLINE | POSTPIX |
|---|---|---|---|---|---|---|---|
| X | Y |
||||||
| Column | Line (Row) |
||||||
| LRISb | 4 |
1155 |
4096 |
0 |
51 |
0 |
80 |
| LRISr | 4 |
1111 |
4116 |
0 |
7 |
20 |
80 |
| Old LRISr | 2 |
2250 |
2048 |
0 |
(PREPIX) 21 |
0 |
80 |
The image orientation of the blue and red sides is flipped with respect to each other across the X axis (NAXIS1). See http://www2.keck.hawaii.edu/inst/lris/detectors.html for descriptions of the orientation and detailed characteristics of the detectors on both cameras.
If desired, the multi-extension FITS files can be converted into simple, single-HDU format using scripts provided at the LRIS post-observing page.
Some examples of LRIS images on the blue side are shown here or here, which also display the prescan and postscan regions.
For data obtained with LRISr before June 2009, when the red side detector was upgraded to a mosaic of two CCDs, the data files are stored in standard simple FITS format consisting of a single primary HDU without any extensions. The image pixels are in 16-bit integers and are 2048 x 2048 pixels long for a full-frame, un-windowed image. Old LRISr employed a back-illuminated Tektronics 2K x 2K CCD with a spatial scale of 0.213 "/pixel.
Characteristics for the old LRISr detector are given here.
