The following tables list -- when possible -- the as-built physical, imaging,
and spectroscopic characteristics
of DEIMOS. In some instances (denoted by *) predicted values
are listed in lieu of actual measurements.
Table 1: Physical and Optical Characteristics
| Location |
Right Nasmyth platform, Keck II; removable from
focus |
| Layout |
Capacity for two cameras fed from common collimator;
only one camera presently built |
| Rotator |
732° rotation (-330° < ROTPPOSN < +402°) |
| User access |
Slit masks, all gratings and all filters |
| Guiding system |
One guider; 3.6×3.6 arcmin FOV;
slit-viewing and offset modes are both available
|
| Spectrograph flexure control |
Two-coordinate beam-steering with closed-loop feedback
|
| Flexure (uncorrected) |
Dispersion axis: ~20 px peak-to-peak image motion over
360° rotation
Spatial axis: ~6 px peak-to-peak image motion
over 360° rotation
Precise amount varies among the sliders
|
| Flexure (corrected) |
<0.5 px image in each axis |
| Slit masks |
11 slots in cassette, with one spot reserved for dedicated
focusing mask and 10 available for observer use;
cylindrical radius of curvature
matched to focal plane curvature |
| Slitlet options |
Slitlets of arbitrary size and orientation
|
| Dispersive elements |
2 slots for 6×8 in gratings |
| Imaging option |
Silvered flat mirror replaces grating |
| Filters1 |
6.25×6.25 in glass filters;
7 slots; BVRIZ, BAL12 (clear), GG400, GG455, GG495, OG550
|
| Collimator focal length |
86.50 in |
| Camera focal length |
15.00 in |
| Beam diameter |
6.33 in for 10.95 m primary;
5.79 in for 10.02 m primary |
| Monochromatic f/ratio |
f/2.358 for 11.00 m primary;
f/2.586 for 10.02 m primary |
| Polychromatic f/ratio |
f/1.29 |
| Scale at detector |
0.00759 arcsec/µm;
0.1185 arcsec per 15µm px |
| CCD detector array |
8192×8192 px (15 µm px);
2×4 mosaic of 2K×4K
CCDs |
| Angular field radius of camera |
11.40° |
| Camera glass transmission* |
97% for lambda > 5200 Å;
88% at 3900 Å;
93% at 4400 Å |
| Total camera throughput* |
85% at lambda > 5200 Å;
77% at 3900 Å
(including coatings and transmission) |
Table 2: Imaging Performance
| Field shape |
Rectangular with 2 lopped-off corners;
sliver out of long side |
| Outer dimensions of rectangular field |
16.7 arcmin×5.0 arcmin |
| Area lost to CCD mosaic gaps |
0.4 arcmin×5.0 arcmin |
| CCD pixel scale |
0.1185 arcsec per px (15µm px) |
| Wavelength range (imaging) |
4000 Å - 10,500 Å |
| Standard filters1
|
BVRIZ + 2 user filters |
| Throughput at 6000 Å2 |
40% (including telescope) |
| Image size |
sigma=1.1 px in center, 1.4px at edge |
| Expected count rate3 |
800 e- per sec in V band at 21.0 mag |
| Mode changes |
120-190 sec changeover between mirror
and grating, depending on slider |
Notes:
-
Slightly redshifted B band with blue edge at ~4000 Å.
-
End-to-end throughput including telescope and assuming
CCD QE=0.6 and all reflecting surfaces are silver coated.
No filter or atmosphere included.
-
V mag is light entering telescope. Rate is through
standard V filter.
Table 3: Spectroscopic Performance
| Total slit length |
16.7 arcmin |
| Usable slit length1 |
16.3 arcmin |
| Total number of slitlets |
up to 130 slitlets with 1.5 arcsec gaps |
| Tilted slits |
up to 30° |
| CCD detector scale |
0.1185 arcsec per px (15µm px) |
| Wavelength range (spectra) |
4100 Å -- 11,000 Å |
| Order blocking filters |
BAL12 (clear), GG400, GG455, GG495, OG550 |
| Slit widths |
User selectable by choice of mask;
long-slits will provide variety of fixed widths |
| Design slit widths |
1.0 arcsec nominal, 0.5 arcsec good seeing |
| Typical alignment accuracy |
0.1 arcsec in both spectral and spatial axes |
| Current gratings (all 6×8 in) |
600 l/mm (7500 Å blaze; aluminized);
830 l/mm (8640 Å blaze; gold-coated);
900 l/mm (5500 Å blaze; aluminzed);
1200 l/mm (7500 Å blaze; gold-coated); |
| Number of grating slots |
2 slots available |
| Corresponding dispersions |
0.65 Å/px,
0.47 Å/px,
0.44 Å/px,
and 0.33 Å/px |
| Spectral lengths (8000 px) |
5300 Å,
3840 Å,
3530 Å, and
2630 Å |
| FWHM resolutions |
3.5 Å,
2.5 Å,
2.1 Å,
and 1.1-1.6 Å
for 0.75 asec slit |
| Throughput at 6000 Å2 |
29% (including telescope) |
| Count rate at 6000 Å |
1.0 e- per sec at V = 21.0 |
| Predicted S/N on faint stellar objects in one hour (low resolution)3 |
5 for V = 24.0;
12 for V = 23.0;
21 for V = 22.0 |
| Achieved S/N on faint galaxies in one hour (high resolution)4 |
0.3-0.6 for R = 24.0;
0.6-1.2 for R = 23.0;
1.2-2.4 for R = 22.0;
2.2-4.6 for R = 21.0;
|
Notes:
- Slit length subtended by
detector pixels.
- End-to-end throughput
including telescope assuming CCD
has QE=0.6 and all reflecting surfaces are silver coated.
- S/N per pixel for 3600
s integration, 0.75 arcsec slit, on
point source with 0.7 arcsec seeing at 5500 Å
and 0.65 Å/px (600 l/mm grating). V is magnitude of light
going through slit. Assumed V sky brightness is 21.25
mag/arcsec².
- Achieved S/N per pixel
for 3600 s integration through 1.0 arcsec slit near
7000 Å with 1200G grating (dispersion of
0.32 Å) for typical faint galaxies under average
observing conditions. R is total magnitude of target,
not accounting for slit losses.