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 IIbyS. M. Faber and the DEIMOS Team
Supported by CARA, UCO/Lick Observatory, and the National Science Foundation
DEIMOS was conceived to be maximally efficient for faint-object spectroscopy of objects densely packed on sky
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
4 px FWHM
S II under OHline
S II under O2 band
z = 0.28
S II at z = 0.075
6 e– peak cts
Vrot ~100 km/s
z = 0.90
Vrot ~100 km/s
z = 0.92
O II at z = 1.29 vrot ~ 100 km/s
O II at z = 0.80
< 30 km/s
O III 5007/4959 at z = 0.62
v = 680 km/s
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.
Unsmoothed 1-d spectrum with background sky (red) offset and rescaled.
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.
A small group of galaxies with velocity dispersion 250 km/s at z 1. Note the clean residuals of sky lines.
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!
Radial comatic tails, max 15 px
Accounts for about half of effect
El 8/9 spacing
X,Y lateral adjustment screws
Image: 0.5” pinholes
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
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.
X actuator: on detector
Y actuator: on tent mirror
X motion: 40 px Correctable range: 26 px
Y motion: 7 px 13-23 px
MUST FIX X MOTION!
X motion: 8 px
Y motion: 18-23 px (depends on grating or mirror)
0.75 1.00 1.62
RMS = 1.0 px
0.31 0.75 1.25
RMS resid= 0.4 px
0.50 1.25 1.19
Goal = 0.6 px
Position on detector
2.43 2.25 2.88
RMS = 2.1 px
1.62 2.38 2.00
RMS resid= 0.5 px
1.25 2.13 1.95
Goal = 1.0 px
Position on detector
Phobos and Deimos were the horses that pulled the chariot of Aries, the god of war.
MORAL: be careful naming your instrument; names have a way of coming true
Redshift distributions in early masks are consistent with expectations
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
200 200 2500
IAB< 23.5 – 24.5
IAB< 22.5 – 24
0.7 < z < 1.4
Only half with z >0.7
LCRS at z~1
CFRS for the 21st century
HAS VIRMOS chosen quantity over quality?