DEIMOS has been in regular use at Keck since mid-2002 and is the
primary optical spectrograph now employed on the Keck II
telescope. This document for observers new to DEIMOS summarizes
the key differences between observing with LRIS and observing
You must be aware of several important considerations prior to
your observing run.
- Slitmask design software: DEIMOS observers use
the IRAF-based DSIMULATOR package provided by Drew Phillips
of UCO/Lick Observatory to select targets from an
observers's astrometric catalog and generate the files
necessary for milling DEIMOS masks. The output files are
submitted through a web interface at UCO/Lick.
- Slitmask submission deadlines: LRIS and
DEIMOS slitmask are milled at the summit, but to keep the
operation running smoothly we require that observers submit
slitmask design files to Lick a minimum of four weeks in
advance of the run unless valid scientific reasons (e.g., the
need to observe a target of opportunity such as a supernoa)
prevent this. When last-minute mask designs are submitted
they must reach UCO/Lick by 3:00AM Pacific Time in order to be
milled at Keck that day.
DEIMOS is performing well. Highlights of the instrument
- Throughput is higher than anticipated due to
conservative estimated that were used for the CCD
sensitivity. The total system efficiency peaks near 30%,
comparable to the LRIS performance. Please see DEIMOS
Spectroscopic Throughput Measurements by Ricardo
Schiavon for measurements of DEIMOS throughput.
- Image quality is excellent, with
the observed FWHM being limited by the delivered image
from the telescope at essentially all times. Image quality
of FWHM=0.5 arcsec is achievable under good seeing conditions.
- Grating selection includes 600, 830, 900, and
1200-line gratings, of which any two can be used for
observing on any given night. In addition, an imaging
mirror is always available. Please note that significant
ghosting is seen with the 830 and 900-line gratings.
- Flexure is present in both for the TV guider and
for the internal optics. An open-loop flexure compensation
system is used with the guider to reduce flexure-induced
image motion to 0.2 arcsec peak-to-peak (from 1.6 arcsec
uncorrected). A closed-loop internal flexure compensation
system (FCS) reduces internal flexure to < 0.25 pixels.
- Readout of the science mosaic is currently
about 40 seconds in imaging mode and 80 seconds in spectral
- Image size is 8k×8k pixels, or 150MB per
raw image in spectroscopic (8-detector) format, and
half that for the imaging (4-detector) format.
- Motor moves are generally completed quickly
and reliably. DEIMOS grating changes requires several
minutes, comparable to the time required for an LRIS grating
change. For that reason, observers trying to maximize
efficiency will want to use zeroth-order imaging for
The mask alignment procedure is very similar to the approach
used for LRIS mask alignment, employing several alignment
boxes 4 arcsec square. A variant of the xbox
slitmask alignment software developed for LRIS is employed
with DEIMOS. Rather than spending several minutes to switch
between the grating and the mirror to take your alignment
images, observers take images in zeroth order with the
Numerous options exist for transferring DEIMOS data from Keck to your
home institution. In approximate order of popularity, they are:
Please refer to the instructions on Writing Backups and FTPing Data for
- burning your data onto DVDs
(each DVD will hold about 30 DEIMOS images) using the
autobackup software at Keck;
- storing your data on your laptop's high-capacity
- transferring your data home via the Internet using ftp,
scp, rsync, etc.
The UCB DEEP team has developed a nearly complete IDL-based
data reduction package for DEIMOS multislit spectroscopy which
reduces the data to 2-D and 1-D extracted spectra, cosmic-ray
cleaned and sky subtracted, without human intervention
(usually). The pipeline works very well on data acquired with
the 1200-line grating employed in the DEEP2 survey, reasonable
well on the 600-line grating, and somewhat less well on the 830
and 900-line gratings. The pipeline properly subtracts
tilted sky lines resulting from tilted slits, and the sky
subtraction residuals are usually near the Poisson level.
This code is based on the magnificent SDSS spectral code of
D. Schlegel. DEIMOS observers are welcome to use this code,
and all DEIMOS observing accounts at Keck are configured to
run the pipeline. The code is well documented and the DEEP2
team has generously provided both an online primer
and a corresponding cookbook
for DEIMOS observers to use.
If you do use this code, beware that it creates standard
FITS files for all its output, mostly in compressed gz files
(which save a factor of 2.4 in disk space and are in fact
faster to read than uncompressed files). The problem is
that many existing reduction packages cannot read FITS
binary tables, nor can they read the compressed
files. Beware, or consider switching reduction platforms.
The FITS files are usually multi-HDU, and in IDL the
retrieved HDU's become structures. This would be a good
opportunity to learn about structures (supported in
Fortran-90 and C) if you are not yet familiar with them.
Those intending to use IRAF to reduce their DEIMOS data will
need to upgrade to version 2.12 (release in May 2002) in
order to work with the DEIMOS data. Familiarize yourself
with the tasks in the MSCRED package.
DEIMOS generates a healthy data volume (about 1 Gbyte/hr) and
and since no computers currently at Keck are fast enough to
process DEIMOS images with the pipeline software in "real time",
you will most likely end up doing your reductions at home.
Suggested requirements for running the pipeline are 1 Gbyte RAM,
preferably 2. Expect it to require several hours of processing
to fully reduce the data from 3 exposures on a single mask.