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

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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

Structural Overview

During Assembly

Final Assembly: Santa Cruz

At the Nasmyth Focus at Keck

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

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)

4 px FWHM



Arc Spectrum: 133 slitlets

First-light Spectrum

S II under OHline

z= 0.19

Sky-subtracted Sub-regions

S II under O2 band

z = 0.28

Sky-subtracted Sub-regions

S II at z = 0.075

6 e– peak cts

Sky-subtracted Sub-regions

Vrot ~100 km/s

z = 0.90

Vrot ~100 km/s

z = 0.92

Kinematic Information

O II at z = 1.29 vrot ~ 100 km/s

Kinematic Information

O II at z = 0.80

 < 30 km/s

Kinematic Information

O III 5007/4959 at z = 0.62

v = 680 km/s

Kinematic Information

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

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

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

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

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

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.

14 in diam!

Camera/Dewar Layout

Images at First Assembly

Radial comatic tails, max 15 px

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

El 8/9 spacing

Detector tilt

X,Y lateral adjustment screws

Three Optical Adjustments

Sample Images: Dome Lights


Line profile

Image: 0.5” pinholes

Line Profiles: Top, Center, Bottom




No coma


Far Corners vs. Center

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

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

  • 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

  • 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 

Flexure Compensation CCDs

X actuator: on detector

Y actuator: on tent mirror

FCS Actuators

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

  • 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

  • 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

  • “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

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









Masks Tiled on a 42’x28’ CFHT Pointing

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

Redshift distributions in early masks are consistent with expectations

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

  • 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




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

  • 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

  • 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

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