WIYN / Bench Upgrade: VPH Optomechanical Issues
The primary limitation is to be able to keep the large grating-camera back-distance with the echelle (at 11 degree camera-collimator angle), while enabling the very large (> 90 deg) camera-collimator angles required to use low-density VPH gratings in the optical. In the current setup there is a maximum achievable camera-collimator angle of 68 deg (Question for Di: How dependent is this on grating-camera distance, and how much could this distance be decreased?)
For low-density gratings, such as v740a, the solution is to use a fold flat, placed in the current grating turret, and place the VPH grating on a new, second turret. The two turrets are connected with a spanning bar, just as the current turret is connected to the camera. The camera is then connected with a second spanning bar to the second turret.
Assuming the spectrograph geometry remains fixed, a 68 degree camera-collimator angle implies a minimum alpha (incident) angle of 56 degrees = (180-68)/2. Hence the new VPH grating turret should be able to accomdate gratings used at up to 56 degrees. Assuming a projected width of roughly 200mm mean that the second grating turret will need to accommodate gratings up to 360mm in width (375mm for a 210mm width.
Current flat dimensions:
Require flat dimensions: 210mm (spatial) x XXX mm (spectral)
Flat coating: If we must use a fold-flat in the final design we must make all efforts to get the best, durable coating on this surface, e.g., the LLNL coating.
Other outstanding issues:
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