SPECTROSCOPY
LHIRES III - Addition of Helical Focuser
Note: I no longer use this system because it was not rigid enough to keep the calibration stable as the telescope moves around the sky and it was too long to fit with my new ATK16 IC camera. I believe a properly engineered system could work though .
Because the LHIRES III uses an achromatic collimating lens, the focus has to be adjusted for the wavelength being measured. This is particularly true for the short wavelength end of the range. The collimator adjustment is acessed through a removeable panel in the spectrograph body.
As an alternative, a helical focuser was fabricated out of an old M42 SLR lens with the optics removed and fitted between the spectrograph and the camera. Although moving the camera affects the geometry of the spectrograph optics differently compared with adjusting the collimator, in practise this appears to have no significant impact on the resolution. (Ideally, as the focal length of the collimator changes with wavelength, both the collimator and the camera should be adjusted to maintain correct collimation)
A graph of focuser setting against wavelength can be found on the LHIRES III calibration page