Science Operations

Instrument Calibration

Wavelength calibration for MODS1 and MODS2 are obtained using the MODS internal calibration unit. Flat-fielding is done with this unit as well or on the sky during twilight. The following pages describe the basic calibrations required for the standard MODS observing modes, as well as provide details on obtaining these calibrations and show examples.

This page describes the various types of calibrations needed to reduce MODS spectroscopic and imaging data. It links to line lists and plots for wavelength calibration and, for flux calibration, flux tables for the set of MODS primary spectrophotometric standards as well as a model LBT extinction curve.

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 ~modseng/modsScripts/modsCalibs_OT.

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

This page is under construction (12 August 2020).

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. The information below and in the chart will be conglomerated (TBD – 12 Aug 2020 -opk).

Biases & Overscan

MODS biases are stable and should need to be obtained only once per run for each of the readout modes used. The full-frame (8Kx3K) images contain 48 columns of prescan on each end which may be used to subtract the bias levels. The columns of prescan exist for both binned and unbinned data. However, for all of the sub-frame readout modes — the 3Kx3K and 1Kx1K regions of interest (ROIs) used for imaging and the 4Kx3K ROI used for prism spectroscopy — there is no prescan, and a set of biases is needed to reduce the data.

To speed up the read out, the detector is read out through four amplifiers through two chains, one for the even and the other for the odd columns. The different bias and gain values produce a pin-stripe pattern on the raw images. Bias subtraction and flat-fielding should remove this, although residual quadrant-to-quadrant level differences or column-to-column ripple may remain at a low level.

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.