ZEMAX DESIGN

The optical layout of BIRCAM shown below consists of 4 protected gold coated mirrors (2 elliptical flats a primary and secondary mirror). The dewar window is made of CaF₂ and there is an IR grade SiO₂ widow directly over the array for protection. A filter is also shown in the design. The line between the dewar window and first fold mirror is a 1.000” hole in the cold shield which otherwise surrounds the optics.

The design was optimized in ZEMAX at 77K under vacuum for 0.95um, 1.00um, 1.2um, 1.6um and 2.3um. The field angles used were: on axis and each corner at ±3'. The spot diagram shown above at the right has squares representing a 3x3 pixel cluster (54um on a side). The design inherently suffers from astigmatism and the cold pupil mask at the secondary contributes the the pattern seen off axis.

The optical prescriptions are as follows:
PRIMARY: 3.00” (76.2mm) outer diameter 3.00” (76.2mm) focal length Protected Gold coated (Edmund optics NT32-825)
SECONDARY 12.0 mm outer diameter 73.97 mm radius of curvature bi-convex lens BK7 (eci protected gold coated) (Edmund optics NT32-871)
FLATS 1.0” x 1.5”x 0.375” Elliptical flats (eci protected gold coated)
FILTERS 1.00” diameter filters range in thickness from 0.125”-0.3”
CaF₂ WINDOW 3.00” diameter 0.250” thick
SiO₂(IR) window 3.00” diameter 0.250” thick

The graphic below (left) shows how scattered light from outside the system was studied. I modified the ZEMAX design so that the entrance window acted as the source emitting over a cone angle of 35 degrees over the field. The Offner relay does a good job of naturally baffling the system due to the fold mirrors and it was decided not to complicate the design by trying to add baffles.

I back ray-traced the system (above right) making the array the source and emitting over 35 degrees to see where light hitting the array comes from. The array sees light coming from inside the cold shield at either side of the primary which can't be improved since these surface are already part of the cold shield and at 77K. Other than that the only light hitting the detector comes directly from the telescope system as seen by the rays entering the window from the left. The cold stop at the secondary is an undersized aperture with a central obstruction. The result of which is that the array sees only light from the secondary mirror and does not see an annular ring around it that would contribute to the background.

Below are images taken with BIRCAM on 1-15-2008. On the left is an out of focus image (pupil image) of RT Aur in the K-band (3s). The other 3 images are J, H, K images (15s, 15s 10s left to right, respectively) of Hunt standard field AS09. The pupil image shows the central obstruction and arms of the cold stop. Note that because the stop is located slightly in front of the secondary mirror there is an elongation in the features from upper left to lower right. Also, there is a focus shift between the J, H and K filters and these images were refocused on the star of interest but there is still some focus shift across the field. The field of view of these images is ~10.5 arcminutes.