Fabrication and testing of germanium grisms for LMIRcam

We diamond fly cut 2 sets of germanium grisms for the LMIRcam 3-5 micron Fizeau imager for the combined focus of the Large Binocular Telescope (LBT). The grisms mount in a filter wheel near a pupil to enable moderate resolution (R~300) spectroscopy. Both sets have a measured blaze angle of 2.9°. The first set has a groove period of 40 lines/mm and will be used in first order with peak efficiency at 3.6 μm. The second set has 32 lines/mm. It can operate in first order with an efficiency peak near 4.4 μm and in second order with a peak near 2.3 μm. First results from testing the grisms in the instrument on the sky with the LBT are presented.