The Deep Imaging Multi-Object Spectrograph for Keck II

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

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 , x 4 speed gain • Accurate flat-fielding: 0. 2% rms (photon-limited for 10 -hr exposures) • Stable image position (fringing): 0. 6 px 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)

Detector Performance The detector is a mosaic of 8 2 K x 4 K 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 Masks and Detector • Slit masks are curved to match the focal plane and imaged onto an array of 2 k 4 k CCDs • Readout time for full array (150 MB!) is 50 seconds (8 amplifier mode)

Arc Spectrum: 133 slitlets 4 px FWHM 8000 px 800 px

First-light Spectrum

Sky-subtracted Subregions S II under OH line z= 0. 19

Sky-subtracted Subregions S II under O 2 band z = 0. 28

Sky-subtracted Subregions S II at z = 0. 075 6 e– peak cts

Kinematic Information 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

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 to do 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 CCD 5 • 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 CCD 5 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 Ca. F 2 elements, a passive thermal plate-scale compensator, and three fluid-coupled multiplets.

Camera/Dewar Layout 14 in diam!

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

Three Optical Adjustments X, Y lateral adjustment screws El 8/9 spacing Detector tilt

Sample Images: Dome Lights Detector center Line profile Image: 0. 5” pinholes

Line Profiles: Top, Center, Bottom Top No coma Center Botto No coma m

Far Corners vs. Center

Measured Image Sizes • Estimated RMS image sizes, corrected for 0. 5” pinhole Actual Predicted 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 31. 7 2. 75 px 41. 2 1. 41 px 1. 93 px 21. 1 px 29. 0 • Extra source of broadening equivalent to 11. 3 (1 -d ) • Possibility: refractive index inhomogeneities? Ca. F ?

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 • Needed during single exposure spectrum X is along slit Y is along • 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 Cu. Ar 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

FCS Actuators Y actuator: on tent mirror X actuator: on detector

Flexure History • Initial image motion on first assembly: X motion: 40 px Correctable range: 26 px Y motion: 7 px 13 -23 px MUST FIX X MOTION! • 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 0. 75 Y 1. 62 0. 31 0. 75 1. 25 RMS resid= 0. 4 px 0. 50 Y motions 1. 00 1. 25 1. 19 Goal = 0. 6 px RMS = 1. 0 px X Position on detector • 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

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 2. 43 Y 2. 88 1. 62 2. 38 2. 00 RMS resid= 0. 5 px 1. 25 X motions 2. 25 2. 13 1. 95 Goal = 1. 0 px RMS = 2. 1 px X Position on detector • 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

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. v Phobos means “fear. ” v 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 DEEP 2 1 HS and Local Surveys SDSS 2 d. F LCRS CFA+SSRS PSCZ DEEP 2 z~0 z~1

Masks Tiled on a 42’x 28’ 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 DEEP 2 Spatial Sampling Courtesy A. Coil Targeted objects are included when our slitlet assignment algorithm is performed on a mock DEEP 2 survey created from an N-body simulation; missed objects are those not selected

Another Redshift Survey: The VLT/VIRMOS Pro • • • 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

Property DEEP 2 versus VLT/VIRMOS DEEP 2 VLT/VIRMOS Survey Size 65000+6500 130, 000+50, 000+3000 Multiplexing 120 -140 galaxies 750 galaxies Resolution R= / 5000 Wavelength Range ~2600 Å ~2500 Å Magnitude Limit IAB< 23. 5 – 24. 5 IAB< 22. 5 – 24 Redshift Range 0. 7 < z < 1. 4 0<z<? Only half with z >0. 7 0 -order Summary LCRS at z~1 CFRS for the 21 st century HAS VIRMOS chosen quantity over quality? 200 2500 • 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 x 2 overhead for “nod and shuffle”

Advantages of DEEP 2 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 d. N/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. 5 from target list, yielding denser sampling at z ~1

Schedule of the DEEP 2 Survey • DEIMOS has been reassembled and tested at Mauna Kea • Commissioning began June 2002 under clear skies and was extremely successful • DEEP 2 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