Quick Guide to KCWI Configurations

KCWI has 3 deployable slicers, 5 blue gratings (low, medium, and high dispersion), one blue filter, and 7 red gratings. 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 Red Side Is HERE

The Keck Cosmic Reionization Mapper (KCRM) is bringing KCWI to its full potential by adding a red spectrograph arm that is used simultaneously with the blue spectrograph arm; they share the same field of view and use the same slicer. The red spectrograph has 7 red gratings (1 low, 2 medium, and 4 high dispersion). Nod and Shuffle will be eventually be available for the red side but is not offered for 2023B observing. Full specifications for the gratings as measured in the lab are posted in Table 2, and need to be updated with values measured on-sky.

Important note on available gratings

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

BH1 is *NOW* available. Specs forthcoming.

BH2 is available and performs very well

BH3 is available but the throughput is lower than for the other gratings. See Throughput.

All 7 red gratings are *NOW* available. Specs forthcoming.

Summary of Configuration Choices:

1. IFU Slicer (Large, Medium, Small)
2. Blue Grating (BL, BM, BH1, BH2, BH3, or direct imaging) + Blue Filter
3. Red Grating (RL, RM1, RM2, RH1, RH2, RH3, RH4, or direct imaging)
4. Blue NAS mask out/in; Red NAS mask not offered yet
5. Central wavelength

Summary of Science Considerations

1. Field of view
2. Spectral resolution
3. Spatial sampling (resolution, slice width)
4. Bandpass
5. Low surface brightness sensitivity
6. Sky subtraction accuracy
7. Efficiency (BH3 is lower than desired).

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

Table 2 : KCWI Slicer and Grating Configurations

The direct imaging plate scale at the detector is 0.147 "/pixel with an instrument demagnification of 7.21.