Efficient high-precision CCD-field lens alignment and integration process of mass-produced fast astronomical spectrograph cameras with VIRUS as an example

Abstract

Spectrographs in high-multiplex fiber integral-field spectroscopy tend to have fast large field-of-view camera systems in order to meet the desire of packing as many spatial elements and wide a spectral bandpass as possible, thereby maximizing the use of detector real estate. This, very often, leads to a camera design with a field flattening lens very close to the detector active area with tight tolerances in the relative alignment between those two, on the order of a few hundredths of a degree and millimeter. This requires dedicated optical metrology process, particularly in cases where a large number of spectrographs, on the order of 100 or more, need to be accurately and reliably aligned and integrated. We have developed such a metrology process for the 156 camera systems in the VIRUS instrument. We detail the working principle of this metrology process, the implementation for the VIRUS camera systems, and how it can be applicable to multi-lens camera system alignment and integration.