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.
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 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 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 following2nd 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 titlingthe 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 opticalquality: [[File:sprat-comp-1.png]]
Mean dispersion = 7[[File:sprat-comp-2.70 angstroms/channelStart wavelength = 4073.64 angstromsEnd wavelength = 8018.33 angstromsCentral wavelength = 6009.04 angstromspng]]
If desired this could be tweaked The 50mm lens version looks slightly (e.g. to go from 4000 to 7950 Angstroms) by titling softer in images, but remember the camerapixelslightly. So we can get sampling is half what it would be using the full wavelength rangeSHOK camera, so its not reallya fair comparison.
So in summary using the Nikon 50mm lens and SHOK type EMCCD camera works optically from the point of viewof the spectrograph wavelength range[[File:image-comp.png]]