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

This page documents infomation which is specific to the LRS/MOS. Most importantly, since the MOS moves 13 arc-second long 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 arc-seconds wide. Due to the units design, these widths cannot be changed. Thus the resolution of the LRS when using the MOS is slightly higher than a normal slit (1.5 or 2.0 arcsec) allows.

Creating a MOS setup

An astrometry-based MOS setup tool has now 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) 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. This version fixes a few minor bugs from 1.2.1 and improves the stability of the program.

NOTE: A common mistake made when creating mos setups is to place a target at the edge (top or bottom) of the slitlet. If an error is made in the setup or in the creation of the mos setup then a significant amount of light can be lost. Better safe than sorry!

Please confirm that the position angle that you choose for you field can be observed by the HET. Check with the Position Angle Calculator. Please include in your Phase II if the setup is for an East field or a West field 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 are significantly north or south in declination the East or West designation is unimportant because there is no flip of position angle. For more about the azimuth restriction please see the Phase II target list information .

NOTE: For MOS fields you can not just flip the position angle by 180 degrees like with a long slit.

Also note that the position angle and star positions should be given to AT LEAST one significant figure. 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 on your fields.

When generating MOS configuration files, you must also note the pixel location of a visible object on one of the slits for alignment on the sky. If no object in a slit is easily visible, the pixel location of an object elsewhere in the field can be used, along with the pixel location of any MOS slit.

Makemos.cl History