The raytrace at left above shows the optical layout of the 3-array survey cameras. Each NICMOS3 array views the same region of the sky via beamsplitting dichroics. The light passes through both dichroics to the Ks-band array. The first dichroic reflection feeds the light to the J-band array. All three optical paths share the same first element which lies behind a cryogenic field stop. Each optical path has six other lenses. This set of six lenses is identical for each band and is the same lens set that was used successfully in the prototype camera. This optical assembly transmits ~80% of the incident light. To the right above is an image of the completed camera optical assembly. The large shared lens lies at the top and the three separate color assemblies extend from the bottom. J-band is on the right and H-band is on the left. Ks-band is in the center.
The optical and mechanical design and fabrication of the 2MASS cameras was carried out by Telic Optics. The images below show the camera in various stages of disassembly at Telic. The left image shows the open cryostat looking from the direction of the telescope secondary mirror (i.e. light enters the system from this end). The aluminum ring is the top of the annular LN2 reservoir. The cold plate, to which the detectors and optical assembly are fixed, can be seen at the bottom of the cylinder. Extending up from the cold plate are four aluminum rods to which the optical assembly (seen on the table to the right) mounts. The copper pads attached to the cold plate are the mounting points for the detector arrays. The image at center shows the cryostat open from the detector side. The cold plate has been removed with the optical and array mounting structures in place and is sitting to the right of the dewar. To the left of the dewar are the pentium-based PC and Infrared Laboratories electronics which drive the arrays and sample and record the outputs. The image at right shows the camera mounted on the 2MASS survey telescope at Mt. Hopkins.
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