VIRUS Collimators



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The Visual Integral-Field Replicable Unit Spectrograph (VIRUS) instrument is a baseline array of 150 identical fiber fed optical spectrographs designed to support observations for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). 130 VIRUS collimator subassemblies have been assembled in a production line and are now complete. Here we review the design choices and assembly practices used to produce a suite of identical low-cost spectrographs in a timely fashion using primarily unskilled labor.

The VIRUS spectrograph pairs will be mounted inside two climate-controlled enclosures located on either side of the telescope.

Section-view drawing of a pair of VIRUS spectrographs.

One complete collimator with all optics installed. Additional completed collimators (with their protective covers installed) may be seen in the background.

Installation fixture used to install the folding flat into the head plate. Two pairs of mirrors are installed into two head plates in one setup. The fold mirrors are precisely positioned with respect to the head plates using a Faro coordinate measuring machine (CMM) arm having a measurement accuracy of ~25 microns and are held in place with metal shims while RTV is applied around the edges of the mirror.

Collimator mirrors are installed in the instrument using an Invar mounting puck that is bonded to the back of the mirror. The puck is precisely located with respect to the edges of the mirror using an alignment fixture.

VPH gratings are optically aligned in their cell during installation using a laser (left). Internal baffles of the grating housings are installed using a fixture (right). The grating cell is later installed on top of the housing.

Collimator subassemblies were pre-assembled and stored before the final collimator assembly. Shown here are the base plates (left) and side, top, and bottom plates (right). Each of these components has multiple alignment and attachment features to facilitate the final assembly. Collimator mounting plates with attached Invar metering rods can be seen on the top rack of the carts.

In the end, the final mating of collimator subassemblies into a complete collimator unit was accomplished relatively quickly, with the final thirty pairs of collimators being assembled in just over two weeks in December 2013. The record assembly time of fifteen minutes to complete one pair of collimators is shared by the team of Nagasawa and Li.

Once the final collimator assembly is complete no further optical alignment is required at this stage. The completed collimators are fitted with a plastic cover that remains with the collimators once installed on the telescope, protected from dust with plastic wrap, and finally packaged and shipped to UT-Austin for integration with the VIRUS cameras and final optical alignment.

Astronomy Group
4242 TAMU
College Station, TX 77843-4242
Ph: (979) 845 7717
Texas A&M University - College of Science