Low-Resolution Spectrograph Multi-object Unit (LRS/MOS)

This page documents information which is specific to the LRS/MOS. Most importantly, since the MOS moves 13-arcsecond length slitlets in a horizontal direction across the focal plane of the HET, the wavelength range and zero point shift of each slitlet will vary from field to field.

Also it is important to note that the slitlets are a fixed 1.3 arcsec wide. Due to the unit's design, these widths cannot be changed. Thus the resolution of the LRS when using the MOS is slightly higher than the normal slit (1.5 or 2.0 arcsec) allows.

Creating a MOS setup

An astrometry-based MOS setup tool has been developed which allows MOS configuration files to be generated without the need for a pre-image from the LRS.

An iraf package called msetup (version 1.2.2) (tar file) is available to assist in creating a mos setup. It can be used to generate a MOS configuration file from an HET image of the field. This package contains a number of scripts including the makemos (version 5.02) to assist in changing non-HET astrometric images into HET friendly images.

NOTE: A common mistake made when creating mos setups is to place a target at the top or bottom end of a slitlet. If an error is made in the setup or obtaining a good guide star incurs a small shift, then a significant amount of light can be lost. Better safe than sorry!

Please confirm that the position angle that you choose for your field can be observed by the HET. Check with the Position Angle Calculator. Please make clear in your Phase II whether the field setup is for an east track or a west track if the calculator places constraints on your PA. You must then name your finding charts, target names, azimuth restrictions, and configuration files with "E" or "W". Failure to do so will cause confusion and possible rejection of the Phase II. For MOS fields that transit nearly due north or south, the east or west designation is unimportant because there is no flip of position angle. For more information about the azimuth restriction please see the Phase II target list information .

NOTE: For MOS fields you cannot just flip the position angle by 180 degrees as with a long slit.

Also note that the position angle and star positions should be given to AT LEAST one significant decimal place. The difference between a position angle of 180 and 179.8 over 400 pixels is 1.4 pixels or 0.65 arcseconds or half a slit width.

When this is complete, send the configuration files to the HET Resident Astronomer for observations of your fields.

Observer setup of the targets onto the slits (in both shift and rotation) works best when two-five clearly visible reference objects (not necessarily targets themselves) are supplied having xy coordinates in the same system as the slitlets. If several of the actual targets are clearly visible, that enables another setup mode. At a minimum the pixel xy position of at least one (preferably central) reference object and on MOS slitlet must be furnished.

Makemos.cl History

Last updated: Sun, 08 Jan 2012 02:06:37 -0600 caldwell


The Telescope

Technical Overview

Object Observability




HRS Instrument Summary

HRS - Details

HRS Configurations

HRS Throughput

HRS Wavelength Calibration


MRS Instrument Summary

MRS Configurations

MRS Throughput

MRS Wavelength Calibration


LRS Instrument Summary

LRS Configurations

Multi-object Unit

LRS Throughput

LRS Spectral Resolution

LRS Wavelength Calibration

Visitor Filter Information

LRS Position Angles

Blind Offsetting

Guide Stars Limits

LRS Update from Gary Hill

Program Preparation

Web Management System

Other HET Documentation