Instrument Cube Dichroic Fold Mirror
TAB  June 2018

A dichroic beamsplitter has been designed, fabricated, coated, and installed in the deployable fold mirror assembly for NIHTS, enabling simultaneous NIR spectroscopy and optical imaging with LMI.  Its design requirements included a 3 arcmin-square reflected field-of-view, a small wedge angle on the back surface to reduce tilted-plate astigmatism in the through-beam, very flat surfaces for a large width/thickness aspect ratio optic, and a unique coating that reflects at high efficiency from 0.9-2.5 microns while efficiently transmitting from 0.4-0.7 microns to cover the Sloan g’ and r’ bands.  In addition, there are very tight space constraints for such a mounted and translatable optic inside the DCT instrument cube.  The image quality requirement for the transmitted beam was approximately 1/2 pixel (15 micron pixels) RMS spot sizes to 1 arcmin radius, with 90% encircled energy within 1 pixel.  The modeled PSF ellipticity at median seeing (0.84 arcsec) was 2.5%.  

The above requirements translated into an elliptical 190mm x 134mm fused silica blank, 12.7mm thick (long-axis/thickness aspect ratio of 15), with a 0.147 +/- 0.004 deg (15 arcsec) wedge angle, and surface flatnesses < 1 fringe.  Three blanks were fabricated of Dynasil by Zygo Corp., meeting the wedge angle and flatness tolerances (the latter at < 0.5 fringe).

The coatings for these dichroic fold mirrors were challenging as the optics reflect in the NIR while transmitting in the optical bandpass, opposite to most configurations.  Multi-layer dielectric coatings were applied by Omega Corp. to the front surfaces, with resulting excellent performance of T(avg, 405-710 nm) = 90.1%, and R(avg, 900-2500 nm) = 96.8%.  A wide-band antireflection coating was applied to the back surface to minimize optical ghosting, with average reflectivities of 3.5% and 4.0% over these bandpasses, respectively.  

On-sky, the deployed dichroic fold mirror transmits an optical beam to LMI with an oval-shaped FOV of about 6.5 x 4 arcmin with a flat field illumination uniformity > 95%.  Introduction of the dichroic increases the optical path length of the through beam relative to the reflected NIR beam, requiring a 380 micron offset in M2 piston and 4.3 mm offset in the dichroic translation position to make both instruments parfocal.  Image quality of both the reflected and transmitted beams is good.  Optical ghosting from a single internal reflection, offset by 120 arcsec from the stellar image, has been observed as predicted by Zemax, but only yet for stars whose signal fully saturates the full well of pixels of the LMI e2v CCD231.  The area of this ghost is approximately 125 arcsec^2, and given the optical reflectivity of the two surfaces, should be brighter than the background of 50% lunar phase sky only when Mv </~ 9.5.