the deep imaging multi object spectrograph for keck ii by s m faber and the deimos team
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The Deep Imaging Multi-Object Spectrograph for Keck II by S. M. Faber and the DEIMOS Team. Supported by CARA, UCO/Lick Observatory, and the National Science Foundation. Structural Overview. During Assembly. Final Assembly: Santa Cruz. At the Nasmyth Focus at Keck.

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the deep imaging multi object spectrograph for keck ii by s m faber and the deimos team

The Deep Imaging Multi-Object Spectrograph for Keck IIbyS. M. Faber and the DEIMOS Team

Supported by CARA, UCO/Lick Observatory, and the National Science Foundation

goals vs performance
Goals vs. Performance

DEIMOS was conceived to be maximally efficient for faint-object spectroscopy of objects densely packed on sky

  • Minimize the effect of sky background
    • “Get between” OH lines: =1.25 A, R = 6000 , x4 speed gain
    • Accurate flat-fielding: 0.2% rms (photon-limited for 10-hr exposures)
    • Stable image position (fringing): 0.6px rms(goal)
  • High observing efficiency
    • Long slit length on sky: 16.7’, >130 slitlets
    • Broad spectral coverage: 2000 resolution elements
    • High throughput: 28% peak (with atm & tel; 50% DEIMOS alone)
    • Low readout noise: 2.3 e–
    • Fast readout time: 50 sec
    • Rapid slitmask alignment: 5 min(goal)
  • Excellent image quality (3800 A to 10,500 A)
    • Hoped for: 0.6-0.8 px (1-d rms with 15 pixels)
    • Actual: 0.8-1.2 px => ~2.0-2.8 px FWHM
detector performance

Detector Performance

The detector is a mosaic of 8 2K x 4K CCDs from MIT/Lincoln Laboratories. The CCDs are high-resistivity, red-sensitive devices that are 45  thick, with a peak QE of 85% and enhanced QE of 23% at 10,000 A.


DEIMOS Masksand Detector

  • Slit masks are curved to match the focal plane and imaged onto an array of 2k  4k CCDs
  • Readout time for full array (150 MB!) is 50 seconds (8 amplifier mode)
kinematic information

Vrot ~100 km/s

z = 0.90

Vrot ~100 km/s

z = 0.92

Kinematic Information
sky subtraction is key
Sky Subtraction is Key

Left:Raw data from an unaligned DEIMOS slitmask, with serendip (detail). Some slitlets are tilted to allow rotation curve measurements; this poses unique challenges for automated sky subtraction.

Below:test analysis of one tilted slitlet. From top: raw data, b-spline model of the night sky lines, and rescaled residual. We already can achieve sky subtraction at close to the Poisson limit in cases like this.

typical extracted 1 d spectrum
Typical Extracted 1-d Spectrum

Unsmoothed 1-d spectrum with background sky (red) offset and rescaled.

poisson limited sky subtraction
Poisson-Limited Sky Subtraction

Plot shows residual of flux from b-spline sky model in region of sky emission lines, in units of local RMS.

Smooth curve is gaussian, width 1.

Work in progress todo non-local sky subtraction using narrower, sky-only slitlets, for the shortest slitlets where local sky subtraction is impossible.

the ucb automated data pipeline
The UCB Automated Data Pipeline

A small group of galaxies with velocity dispersion   250 km/s at z 1. Note the clean residuals of sky lines.

ccd crosstalk
CCD Crosstalk
  • The image from CCD 6 appears negatively on CCD5
  • The amplitude saturates at about 2.5 e–
  • The main effect is to create negative sky lines. The widths depend on line brightness unpredictably
  • Possibly due to open wire on CCD5 A amplifier
  • Action is TBD
optical performance

Optical Performance

The camera was designed by Harland Epps. It has exceedingly wide field of view (11.4° radius), three steep aspherics, three large CaF2 elements, a passive thermal plate-scale compensator, and three fluid-coupled multiplets.

images at first assembly
Images at First Assembly

Radial comatic tails, max 15 px

causes of radial coma
Causes of Radial Coma
  • Inherent in optical design: performance at room temperature differs from 0 C

 Accounts for about half of effect

  • Element 8/9 spacing too short
  • Detector too deep in dewar
  • Multiplet 4 slightly too thick
three optical adjustments

El 8/9 spacing

Detector tilt

X,Y lateral adjustment screws

Three Optical Adjustments
sample images dome lights
Sample Images: Dome Lights


Line profile

Image: 0.5” pinholes

line profiles top center bottom
Line Profiles: Top, Center, Bottom




No coma


measured image sizes
Measured Image Sizes
  • Estimated RMS image sizes, corrected for 0.5” pinhole


Center Corners Center Corners

1-d : 0.88 px 1.17 px 0.60 px 0.82 px

13.2  17.5  8.8  12.0 

FWHM: 2.07 px 2.75 px 1.41 px 1.93 px

31.7  41.2  21.1 px 29.0 

  • Extra source of broadening equivalent to 11.3 (1-d )
  • Possibility: refractive index inhomogeneities? CaF2?
image stability

Image Stability

The original passive specification for image motion was 6 px peak-peak under 360 rotation in X and Y. This goal has not been met, but the final image stability specifications seem to be within reach nevertheless.

image stability flexure
Image Stability/Flexure
  • Reasons for wanting stable images
    • Image quality X is along slit
      • Needed during single exposure Y is along spectrum
      • Affects both X and Y
      • Specification: < 1 px rms
    • Flat-fielding accuracy
      • Needed between afternoon calibrations and evening observations
      • Flat-fielding accuracy requirement: 0.2% rms
      • Affects Y only (along spectrum)
      • Specification: < 0.6 px rms (originally 0.25 px rms)
    • Use flat fields to delineate slitlet edges
      • Needed between afternoon calibrations and evening observations
      • Affects X only (across spectrum)
      • Specification: < 1 px rms
flexure compensation system
Flexure Compensation System
  • Closed feedback loop: both centroid sensing and correcting
  • Operates in both direct imaging and spectroscopy modes
  • Sensing system
    • Four optical fibers pipe CuAr light (or LED) into telescope focal plane at opposite ends of slitmask
    • Two separate sensing CCDs are mounted on detector backplane flanking the science mosaic
    • These FCS CCDs are read every 40 sec when shutter is open
    • Feedback is achieved only when shutter is open
  • Correcting system
    • Steers image in X and Y; no rotation
    • X actuator: motor in dewar moves detector along slit
    • Y actuator: piezo on tent mirror moves spectrum in 
fcs actuators

X actuator: on detector

Y actuator: on tent mirror

FCS Actuators
flexure history
Flexure History
  • Initial image motion on first assembly:

X motion: 40 px Correctable range: 26 px

Y motion: 7 px 13-23 px


  • Year-long campaign discovered moving elements in camera and grating system
  • Current image motion:

X motion: 8 px

Y motion: 18-23 px (depends on grating or mirror)

  • Lessening X increased Y to some degree
  • Tilting grating is needed in Y in addition to tent mirror
y correction first results



Y Correction: First Results
  • Performance with closed-loop correction
  • Total image motion through 360° rotation, in px; slider 3; USING ONLY ONE FIBER ON ONE FCS
  • Nature of motion: sag in Y, larger with X (i.e., a shear)
  • Probable cause: pitch of collimator
  • Expectation: final rms will be 0.4-0.5 px …. meets goal

0.75 1.00 1.62

RMS = 1.0 px

Y motions

0.31 0.75 1.25

RMS resid= 0.4 px

0.50 1.25 1.19

Goal = 0.6 px

Position on detector

x correction first results



X Correction: First Results
  • Performance with closed-loop correction
  • TOTAL image motion through 360° rotation, in px; slider 3; USING ONLY ONE FIBER ON ONE FCS
  • Nature of motion: shift in X, mainly bulk motion
  • Probable cause: flexure in the fiber mount
  • Expectation: final rms will be 0.6-0.7 px …. meets goal

2.43 2.25 2.88

RMS = 2.1 px

X motions

1.62 2.38 2.00

RMS resid= 0.5 px

1.25 2.13 1.95

Goal = 1.0 px

Position on detector

lessons learned
Lessons Learned
  • “Success-oriented” does not work at this scale
  • Expect that most mechanisms will NOT work as designed the first time. Hence…
  • Build prototypes and test extensively before putting into spectrograph
  • The major source of flexure is not the main structure but rather mechanisms attached to the structure; not easily analyzed using FEA; hence the need for prototypes
final lesson naming
Final Lesson: Naming

Phobos and Deimos were the horses that pulled the chariot of Aries, the god of war.

  • Phobos means “fear.”
  • Deimos means “the awe one feels on the battlefield when in the presence of something greater than oneself.”

MORAL: be careful naming your instrument; names have a way of coming true

comparison between deep2 1hs and local surveys
Comparison Between DEEP2 1HS and Local Surveys









colors pre select distant galaxies
Colors Pre-select Distant Galaxies
  • Plotted at left are the colors of galaxies with known redshifts in our fields; those at low redshift are plotted as blue, those at high redshift as red(diamonds are beyond the mag. limit of the survey).
  • A simple color-cut defined by three line segments would yield a sample >90% at z>0.75 and missing <3% of the high-z objects. Most of the failures are likely to be due to photometric errors.
test of photo z selection procedure
Test of Photo-z Selection Procedure

Redshift distributions in early masks are consistent with expectations

simulated deep2 spatial sampling
Simulated DEEP2 Spatial Sampling

Courtesy A. Coil

Targeted objects are included when our slitlet assignment algorithm is performed on a mock DEEP2 survey created from an N-body simulation; missed objects are those not selected

another redshift survey the vlt virmos project
Another Redshift Survey: The VLT/VIRMOS Project
  • 50,000 galaxies to IAB< 24 (1.2 sq. deg)
  • 105 galaxies with IAB< 22.5 (9 sq. deg)
  • 750 simultaneous slitlets (4 barreled instrument)
  • Resolution R~ 180-2520: short spectra, multiple spectra per row
  • 100+ nights on VLT-3: Observations start November 2002
deep2 versus vlt virmos




Survey Size




120-140 galaxies

750 galaxies

Resolution R=/


200 200 2500

Wavelength Range

~2600 Å

~2500 Å

Magnitude Limit

IAB< 23.5 – 24.5

IAB< 22.5 – 24

Redshift Range

0.7 < z < 1.4


Only half with z >0.7

0-order Summary

LCRS at z~1

CFRS for the 21st century


HAS VIRMOS chosen quantity over quality?

  • Only half their galaxies will be distant
  • Most of their galaxies have resolution 200, not 5000; no kinematic info; inferior velocities?
  • They cannot subtract sky accurately at R=200; will lose x2 overhead for “nod and shuffle”
advantages of deep2 over vlt virmos
Advantages of DEEP2 over VLT/VIRMOS
  • Higher resolution:
    • Provides more precise redshifts and allows secure z measurements from the [OII] doublet alone
    • Permits us to measure linewidths/rotation curves
    • Reduces contamination by night skylines
    • Necessary for many of our science goals: e.g. T-F type relations, studies of bias (e.g. via redshift-space distortions), measurement of thermal motions, determining velocity dispersions of clusters, the dN/dz test… None of these will be possible with low-resolution VLT/VIRMOS data.
  • Photometric cut for z>0.7 will eliminate ~50% of all galaxies with IAB< 23.5from target list, yielding denser sampling at z ~1
schedule of the deep2 survey
Schedule of the DEEP2 Survey
  • DEIMOS has been reassembled and tested at Mauna Kea
  • Commissioning began June 2002 under clear skies and was extremely successful
  • DEEP2 observing campaign began in July 2002. (so far we have had 4:9 science nights clear, and on 3:4 of these, the TV camera was broken!)
  • Observations complete late 2004 (we hope)
  • Analysis complete late 2006