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.

A possible design option is to combine SHOC and SPRAT into one instrument. The following describes some testing of this concept on the Liverpool Telescope (LT) version of SPRAT.

Wavelength Range

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. 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.

Subsetting a region 512 pixels long (numbers 298-810) from the SPRAT detector with the 50mm lens and taking a Xenon arc we found the following 2nd order fit:

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

          Line    Wavelength  Calculated Discrepancy    RMS if
                              Wavelength                omitted
  1      57.717    4500.980    4498.411      -2.569     3.411
  2      74.249    4624.280    4622.838      -1.441     3.500
  3      80.635    4671.230    4670.978      -0.252     3.534
  4      88.689    4734.150    4731.747      -2.403     3.443
  5     128.755    5027.280    5035.034       7.754     2.445
  6     232.592    5823.890    5828.632       4.742     3.090
  7     340.583    6668.920    6665.565      -3.354     3.335
  8     348.130    6728.010    6724.497      -3.513     3.319
  9     398.201    7119.600    7116.941      -2.659     3.424
 10     457.434    7584.680    7584.477      -0.204     3.534
 11     464.812    7642.020    7642.967       0.947     3.519
 12     495.944    7887.400    7890.354       2.954     3.319
RMS error:      3.384 Angstroms

This can be tweaked slightly (e.g. to go from 4000 to 7950 Angstroms) by titling the camera slightly. A range of 3950 Angstroms is available, which basically meets the design requirement to cover one full octave.

Optical Quality

Comparisons of the extracted arcs using either lens show essentially identical optical quality:

The 50mm lens version looks slightly softer in images, but remember the pixel sampling is half what it would be using the SHOK camera, so its not really a fair comparison.