Spectroscopy

We do NIR spectroscopy from 0.8 to 2.4 microns with one single grism plus the broad-band filters ZYJHK. Each band has a different internal focus setting for the same slit. In the case of a low resolution spectroscopy mode covering the whole range from 0.8 to 2.4 microns, one would need to use one fixed focus setting. We have measured by how much we are out of focus at each wavelength extreme, both in the spatial and the dispersion direction, when using one fixed focus setting.

The focus shift in the dispersion direction is measured by setting the internal focus to the one for the J-band, and measure the width of the argon arc-lines at the approximate center of the wavelength bands. The focus deterioration is very small from J towards the red, and the maximum at Z would correspond to a loss in resolution of $\sim$20%.

Defocusing effects in the spatial direction were measured using a pinhole (which is displaced, however, with respect to the slit, such that only K is in focus at the upper end of the focus mechanism). ZYJHKs and a few narrow-band filter measurements were included. Here we see a best FWHM for Ks (2.14 microns) of 1.4 pixels and a worst case for Z (0.88 microns) of FWHM = 4.2 pixels.

Acquiring the target on the slit for NOTCam spectroscopy has up to now been done in a relatively manual way compared to the acquisition scripts available for ALFOSC and FIES. The procedure for NOTCam has two extra complications compared to optical spectroscopy: 1) sky subtraction may be needed to see the target in the acquisition image and 2) flexure of the instrument requires the slit position to be determined at every pointing. A slit acquisition script (notcam.acquisition) for NOTCam spectroscopy has been made. Some improvements are already identified, and some more testing is needed before it can be released to the users.