KCWI has 3 deployable slicers, 5 blue gratings (low, medium, and high dispersion), and one blue filter. The articulating camera can be pointed to place the desired central wavelength on the CCD. There is a deployable Nod and Shuffle mask which blocks the upper and lower thirds of the detector in order to perform source/background nod-and-shuffle (NAS) observations. Gratings can be removed to allow direct imaging. There is also a polarizer, for which information will be forthcoming.
The Keck Cosmic Reionization Mapper (KCRM) is bringing KCWI to its full potential by adding a red spectrograph arm that can be used simultaneously with the blue spectrograph arm; they share the same field of view and use the same slicer. The red spectrograph will have 6 red gratings (1 low, 2 medium, and 3 high dispersion) and a red filter. Nod and Shuffle will be eventually be available for the red side but is not offered for 2023A shared risk observing. Full specifications for the gratings will be posted once we have measured them in the lab and updated on sky, but initial numbers are posted in Table 2.
Please note that the instrument does not have the full complement of grating at this time. Specifically:
BL and BM are available and perform very well
BH2 is available and performs very well
BH3 is available but the throughput is lower than for the other gratings. See Throughput.
BH1 is *NOT* available at this time.
It is anticipated that BH1 will be delivered with the installation of KCRM, the red side.
Table 1 summarizes the interplay between the various instrument configuration choices and the science objectives. The choice of slicer and grating is determined by your desired field of view (slicer), slice spatial sampling (slicer), spectral resolution (grating and slicer), and wavelength range (grating and NAS mask). The spectrograph is slit-width limited and therefore for a given grating the Large slicer gives the lowest resolution, Medium slicer double that, and Small slicer double that (4x Large slicer). Correspondingly, the Small slicer gives the highest spatial resolution (0.35 arcsec, roughly Nyquist sampling 0.7 arcsec seeing disk), Medium slicer medium spatial resolution (0.7 arcsec), and Large slicer low spatial resolution (1.4 arcsec). All slicers are 20 arcsec long, with Large 33 arcsec wide, Medium 16.5 arcsec wide, and Small 8.4 arcsec wide. Bandpass is determined by the grating, and is limited by the camera and CCD size, the brickwall pattern of the IFU, and the NAS mask if in (which reduces the bandpass by a little more than a factor of 3).
Table 1 : Impact of Instrument Configuration on Science Objective
Major Impact | Moderate Impact | Small Impact | No Impact |
|
Instrument Configuration |
|||
Science Objective |
IFU Slicer |
Grating |
NAS mask in/out |
Central wavelength |
Field of view |
Large 33” x 20” |
|
|
|
Spectral resolution |
Small: 4R0 |
BH R0~4500 |
|
Slight variation |
Spatial sampling |
Small 0.35” |
|
|
|
Bandpass (Instantaneous) |
|
BL ~ 2000Å |
NAS Out Dl |
Slight variation |
Low Surface Brightness Extended Emission Sensitivity & Sky Subtraction Accuracy |
Large slicer is best [more sky around object, faster sky measurement] Small slicer is worst |
If emission line then best sensitivity when line is resolved. |
NAS IN Recommended if extended emission <few % sky and/or significant fraction of FOV |
|
Efficiency |
Small slicer has slight vignetting |
BL has best efficiency, BM close, BH slightly lower but comparable except for BH3 |
Requires 4 x longer to obtain same Poisson S/N plus some overhead. |
Some variation (10-20% relative) |
Table 2 : KCWI Slicer and Grating Configurations
|
|
Slicer |
|||
|
|
Large |
Medium |
Small |
|
|
|
|| x ⊥ dispersion |
|||
|
Field of View |
33˝ x 20.4˝ |
16.5˝ x 20.4˝ |
8.4˝ x 20.4˝ |
|
|
Slice width |
1.35˝ |
0.69˝ |
0.35˝ |
|
Grating |
Parameter |
|
|
|
|
|
|
Bandpass/Dispersion |
|
|
|
BL |
R (central) |
0.563Å/pixel |
900 |
1800 |
3600 |
|
Δλ (total) |
3500-5600 Å |
|
|
|
|
Δλ (instantaneous) |
2000 Å |
|
|
|
|
Δλ (NAS) |
500 Å |
|
|
|
BM |
R (central) |
0.24Å/pixel |
2000 |
4000 |
8000 |
|
Δλ (total) |
3500-5500 Å |
|
|
|
|
Δλ (instantaneous) |
800-900 Å |
|
|
|
|
Δλ (NAS) |
200-220 Å |
|
|
|
BH3 |
R (central) |
0.129Å/pixel |
4500 |
9000 |
18,000 |
|
Δλ (total) |
4700-5600 Å |
|
|
|
|
Δλ (instantaneous) |
470-530 Å |
|
|
|
|
Δλ (NAS) |
120 Å |
|
|
|
BH2 |
R (central) |
0.111Å/pixel |
4500 |
9000 |
18,000 |
|
Δλ (total) |
4000-4800 Å |
|
|
|
|
Δλ (instantaneous) |
370-440 Å |
|
|
|
|
Δλ (NAS) |
100 Å |
|
|
|
BH1 is *NOT* available at this time, but below are BH1's anticipated specifications. |
|||||
BH1 |
R (central) |
0.09Å/pixel |
4500 |
9000 |
18,000 |
|
Δλ (total) |
3500-4100 Å |
|
|
|
|
Δλ (instantaneous) |
~400 Å |
|
|
|
|
Δλ (NAS) |
~100 Å |
|
|
|
Here are the minimum anticipated specifications for the Red Side's grating, to be updated upon confirmation in the lab and on sky. Reminder, total wavelength range possible is different than the instaneous range covered in one setting. |
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RL |
R (central) |
|
>500 |
>1000 |
>2000 |
|
Δλ (total) |
~5300-10800 Å |
|
|
|
RM1 |
R (central) |
|
>1400 |
>2800 |
>5600 |
|
Δλ (total) |
~5300-8200 Å |
|
|
|
RM2 |
R (central) |
|
>1400 |
>2800 |
>5600 |
|
Δλ (total) |
~7000-10800 Å |
|
|
|
RH1 |
R (central) |
|
>3250 |
>6500 |
>13,000 |
|
Δλ (total) |
~5300-6500 Å |
|
|
|
RH2 |
R (central) |
|
>3250 |
>6500 |
>13,000 |
|
Δλ (total) |
~6300-7800 Å |
|
|
|
RH3 |
R (central) |
|
>3250 |
>6500 |
>13,000 |
|
Δλ (total) |
~7700-9500 Å |
|
|
|
RH4 |
R (central) |
|
>3250 |
>6500 |
>13,000 |
|
Δλ (total) |
~9200-10800 Å |
|
|
|
The direct imaging plate scale at the detector is 0.147 "/pixel with an instrument demagnification of 7.21.