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 IIby

S. M. Faber and the DEIMOS TeamSupported 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.

• 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)

DEIMOS Masks and Detector

Arc Spectrum: 133 slitlets

8000 px 800 px

4 px FWHM

First-light Spectrum

Sky-subtracted Sub-regions

z= 0.19

S II under OH line

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

Vrot ~100 km/s z = 0.92

Vrot ~100 km/s z = 0.90

Kinematic Information

Kinematic Information

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

Kinematic Information

< 30 km/s

O II at z = 0.80

Kinematic Information

O III 5007/4959 at z = 0.62

v = 680 km/s

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.

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.

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 amplitude saturates at about 2.5 e–

• The image from CCD 6 appears negatively on CCD5

• The main effect is to create negative sky lines. The widths depend on line brightness unpredictably

• Action is TBD

• Possibly due to open wire on CCD5 A amplifier

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.

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

El 8/9 spacing X,Y lateral adjustment

screwsDetector tilt

Sample Images: Dome Lights

Line profile

Image: 0.5” pinholes

Detector center

Line Profiles: Top, Center, Bottom

Center

Bottom

Top

No coma

No coma

Far Corners vs. Center

Measured Image Sizes

• Estimated RMS image sizes, corrected for 0.5” pinhole

Actual Predicted 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

FCS ActuatorsY 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

• 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

Y Correction: First Results

0.75 1.00 1.620.31 0.75 1.250.50 1.25 1.19Position on detector

RMS = 1.0 pxRMS resid= 0.4 pxGoal = 0.6 px

Y motionsY

X

• 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

X Correction: First Results

2.43 2.25 2.881.62 2.38 2.001.25 2.13 1.95Position on detector

RMS = 2.1 pxRMS resid= 0.5 pxGoal = 1.0 px

X motionsY

X

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

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+05 1.00E+06 1.00E+07 1.00E+08

Volume (h -3 Mpc3)

Num

ber o

f Gal

axie

s

CFA+SSRS PSCZ

LCRSDEEP2

2dF

SDSS

z~0z~1

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

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

Courtesy A. Coil

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

0<z<?Only half with z >0.7

0.7 < z < 1.4Redshift Range

CFRS for the 21st century

LCRS at z~10-order Summary

IAB< 22.5 – 24IAB< 23.5 – 24.5Magnitude Limit

~2500 Å~2600 ÅWavelength Range

130,000+50,000+300065000+6500Survey Size750 galaxies120-140 galaxiesMultiplexing

200 200 25005000Resolution R=/

VLT/VIRMOSDEEP2Property

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< 23.5 from 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