Science Operations

Instrument Calibration

This page describes the basic calibrations required for most MODS spectroscopic and imaging observations. Almost all MODS calibrations are done with the internal calibration unit, although to correct for illumination gradients from the calibration source, twilight sky flats can be useful. The wavelength calibration for MODS1 and MODS2 is done using the internal comparison lamps, and a constant shift, based on night sky lines, is applied to correct for residual flexure.

Calibration scripts can be generated from the MODS OT library. A set of scripts that have been generated from version 10.23 of the MODS OT library are on the obsN computers in /lbt/lbto/mods/modsScripts/modsCalibs/

Note that the calibration scripts for MODS1 and MODS2 differ. To achieve similar count rates requires slightly different settings for each instrument, primarily due to differences in the lamps.

Calibrations Summary (chart)

The Baseline Calibrations Summary Chart summarizes the types of calibrations needed for each observing mode. The names of the scripts are old (corresponding to those found for MODS1 in ~modseng/modsScripts/modsCalib) and need to be updated to reflect the names given to the updated versions* of these scripts in the ~modseng/modsScripts/modsCalibs_OT directory.

* The updated versions should be run – either the user should generate these for themselves from the MODS OT library or use the ones in modsCalibs_OT – since the settings have changed as the calibration lamps have aged or been replaced.

Calibrations Details (chart)

A discussion of the rationale for taking the various types of calibrations is given in the Calibrations Details page.

Biases & Overscan

MODS biases should be taken with the dome dark and telescope stationary. The full-frame (8Kx3K) images have 48 columns of prescan on both ends, and these are used to subtract the bias levels for each quadrant and for each readout chain; the 8Kx3K images can be reduced without separate 8Kx3K biases. On the contrary, for all of the sub-frame readout regions of interest (ROI): the 3Kx3K and 1Kx1K used for imaging and imaging acquisitions and the 4Kx3K ROI used for prism spectroscopy, there is no prescan or overscan, and biases are needed to reduce the data.

The bias levels vary somewhat, so it is recommended to take biases for the sub-frame ROIs on the same night as the data or flats they will be used to calibrate.

Scripts to obtain biases in all of the aforementioned ROIs are available in the MODS OT library.

Biases should be taken with the telescope stationary and the chamber dark.