12.1. Overscan subtraction

The usual operating mode for ESI is dual-amp readout. Each amplifier has a (slightly) different baseline value and (slightly) different ``gain'' (e-/DN ratio). The baseline varies with row. In particular it drops in the same rows affected by the eight bad columns. The format in unbinned two-amp readout mode is: 24 columns of ``prepix'' values (12 for each amplifier), 2048 columns of active area, and 160 columns (80 for each amplifier). The appropriate overscan region must be used to correct each amplifier separately. There is an IRAF script, lrisbias, written by Greg Wirth that should work for ESI images.

12.2. Low-D Mode

In the low-D mode, ESI spectra are similar to any long-slit spectrograph, with two differences: the spectra are tilted by 6.5 degrees with respect to the CCD rows, and the night sky lines are more highly curved than in most spectrographs.

The quickest way to get a quick look at the low-D frames is to use IRAF to rotate the frames and then use the usual FIGDISP row plot tools (or alternatively IRAF itself).

In a window with IRAF running, cd to a scratch directory with the frames. To create a copy of an image (say ``esi0001.fits'') that is rotated such that a source spectrum runs along CCD rows, use the following command:

cl> rotate esi0001 rot0001 rotation=-6.5 interpolant=spline3

This leaves esi0001 unchanged; rot0001 is the rotated frame.