lirisdr - An IRAF package for LIRIS data reduction
The problem of observing in the near infrared range using
ground-based telescopes:
The main trouble to solve is the high background coming from the Earth
atmosphere, there are different terms of contribution: the OH emission
lines and the airglow produced in the high layers of the atmosphere
dominating in the H band, the thermal radiation originated at the
telescope and its environment, increasing as moving to the red extreme
of the K band.
Initial steps:
The first step is to subtract the bias level. Each
pixel of common infrared detectors are usually read following the
scheme: reset-read-integrate-read. In general data are
stored after subtraction of the first read level. If this is not
the case one has to perform this step.
Currently, in the case of LIRIS data there is a pixel mapping
problem which has to be corrected before any other processing steps.
There is a C routine which performs this correction for a single file.
This routine can also be called from IRAF for a list of files, for
example:
lirisdr>
lcpixmap @inframes "c" out=outframes
Imaging mode
The most common observing mode is to follow a dithern pattern. The
reduction steps to be followed are:
- flat-field correction
- subtraction of sky image
- construct and subtract sky image
- correct vertical gradient
- correct geometrical distorsion is required
- combine sky subtracted images
An sky image can be obtained as the median value for each pixel from
the whole set of frames.
The program which perform the whole process is called ldedither.