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