Quick Guide to KCWI Configurations

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 Red Side Is COMING

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.

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

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.

Summary of Configuration Choices:

1. IFU Slicer (Large, Medium, Small)
2. Blue Grating (BL, BM, BH2, BH3 [BH1 not available yet], or direct imaging)
3. NAS mask out/in
4. 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.