50mmLensTest

SPRAT on LT works from 4000 - 8000 Angstroms. This "one-octave" range is the most one can get before order overlap in a non cross dispersed spectrograph.

On the afternoon of 2017 12 17 the 85mm f/1.8 Nikon camera lens on the LT SPRAT spectrograph was swapped for a 50mm f/1.4 lens. Both lenses are used wide open.

This was to test what spectral range we could get on a smaller SHOC type EMCCD detector which has 1024 x 13 micron pixels as opposed to the current SPRAT iDus detector which has 1024 x 26 micron pixels.

The length of the SHOK detector would therefore correspond to 512 SPRAT pixels. This means we have to use a wider angle lens to capture the same wavelength range spectrum on a smaller physical length (half the size). The current 85mm lens actually on the SPRAT CCD has an unused portion on the left hand side, and it was calculated that the 50mm lens should "just" work.

Note this test is independent of the final collimator focal length chosen for the instrument, as the spectral range on the detector is just a function of the grating lines per mm, the camera focal length and the detector length.

Using 512 pixels (numbers from the SPRAT detector with the 50mm lens and taking a Xenon arc we found the following

Mean dispersion = 7.70 angstroms/channel Start wavelength = 4073.64 angstroms End wavelength = 8018.33 angstroms Central wavelength = 6009.04 angstroms

If desired this could be tweaked slightly (e.g. to go from 4000 to 7950 Angstroms) by titling the camera slightly. So we can get the full wavelength range.

So in summary using the Nikon 50mm lens and SHOK type EMCCD camera works optically from the point of view of the spectrograph wavelength range.