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NOTCam focus shift with wavelength

NOTCam focus shift with wavelength

Currently we do NIR spectroscopy from 0.8 to 2.4 microns with one grism plus the broad-band filters ZYJHK. Each band has a different internal focus setting for the same slit, going from 4150 for K to 5750 for Y and Z. This seems like large focus differences, implying difficulties for spectroscopy settings where one would cover the whole wavelength range in one go, with for instance very low resolution spectroscopy.

Therefore 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. See figure below.

The focus shift in the dispersion direction is measured by setting the internal focus to the one for the J-band (5650), using the 128 micron slit, and measure the width of the argon arc-lines at the approximate center of the wavelength bands (large dots) as well as sampling the band (small dots). This is shown in the upper panel. The arc lines fwhm broaden from 2.4 pix for J (in focus) to 3.1 pix for Z and 2.6 pix for K, at both extremes. The focus deterioration is very small from J towards the red, and the maximum at Z would correspond to a loss in resolution of ~ 20%.

Defocusing effects in the spatial direction were measured using the 50 micron pinhole (which is displaced, however, with respect to the 128 micron slit, such that only K is in focus at 5750, 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.