The Camera/Collimator Dewar
The FLAMINGOS dewar will be two dewars matted together. The larger dewar will be the collimator/camera dewar (on the right in the figure above). This section contains all the optics, the filter wheel, the Lyot wheel, the grism wheel, and the detector. It will be cooled by LN2. I have opted for an LN2 cooled dewar rather than using cryocoolers since LN2 offers simple portability, without having to move compressors and put plumbing fixtures on the telescope. The MOSAIC dewar (Pogge et al. [3]) designed by The Ohio State University, Department of Astronomy, was found to have sufficient internal volume to hold the FLAMINGOS optics and mechanism. Using the MOSAIC dewar design provides a fully designed and field tested cryogenic optical bench for FLAMINGOS and will thus lead to lower design costs and speed deployment. The assembly drawings of the collimator/camera dewar are presented in the figure above, showing details of the dewar shell, radiation shields, optical bench, optics, mechanisms and LN2 tanks.
The dewar was fabricated mostly from 6061 aluminum due to its
excellent dimensional stability. All welded components were first
welded, rough machined, heat treated to relieve stress and finally
fine machined. The cylindrical LN2 tank is welded to the work-surface
and provides a great deal of mechanical stiffness to the work-surface.
The entire work-surface and LN2 tank assembly is suspended from
a very stiff continuous G10 ring. The overall flexure of the end
of the dewar work surface relative to the mounting surface is
44mm from vertical to horizon pointing.
The collimator/camera dewar will have three identical detented,
pinion driven wheel mechanisms which can place elements into the
long collimated beam between the second lens group of the collimator
and first lens group of the camera. The first wheel will carry
6 filters (0.9-1.8 microns, 1.2-2.4 microns, J , H, Kshort and
a TBD filter). Behind this wheel will be the Lyot stop wheel which
will place 7 Lyot stops exactly at the reimaged pupil of the telescope
(for example: 4-m f/15, 4-m f/7, 2.1m f/7, 2.1m f/15, Gemini f/16
and a dark position). This wheel must have a thickness of about
±10mm to allow for the changes in pupil locations FLAMINGOS
will encounter on different telescopes. The final mechanism will
be the grism wheel which will carry 5 grisms or additional filters
(for example: 0.9-1.8 micron survey grism, R=1000; 1.2 to 2.4
micron survey grism R=1000; J/K band grism, R=2400; and an open
position). All three wheels will use cryogenic stepper motors
so that no feed-throughs or complex mechanisms will be needed.
The materials and sizes of the optical elements we are using in
FLAMINGOS are similar to those used in other near-IR instruments
at NOAO, IRTF, OSU, Omega-Prime and UKIRT and they should present
no particular mounting problems. A full tolerance analysis has
been preformed on the FLAMINGOS optical design and find it to
be quite tolerant of decenters, tilts and spacing errors. The
optics for each lens group will be mounted into a single barrel.
Within each barrel the lenses will be loaded onto machined surfaces
matched to the surface of the lens using a spring. The lenses
within a group will be aligned by placing shims between and around
the lens cells. The mountings of the lens groups to the worksurface
will be machined to align the lens groups. Since the lenses are
all made from optically transmitive materials they can be aligned
using a laser and we have devised a testing alignment scheme.
The flat worksurface which the optics will be attached to will
experience a 20mm bowing when the dewar is evacuated. While well
within the allowed tolerances of the optical design, we will pressurize
the LN2 vessel during warm optical alignment.
The Focal Plane Dewar
To use FLAMINGOS as a multi-object spectrometer, we will place a smaller LN2 cooled dewar between the telescope and the collimator/camera dewar. This dewar will contain the focal plane masks for FLAMINGOS. It will have two wheel mechanisms in it (figure above). First will be a Decker wheel to select the rough size of the region, on the sky, to pass into the spectrometer (the full imaging field, the multi-slit FOV or a single long slit). Behind this wheel is the focal plane wheel, carrying 11 cryogenic focal plane slit masks, several long slits and an open position. While 11 plates should be adequate for many observing runs this small multi-slit dewar can be thermally cycled in less than 6 hours allowing a new set of slit plates to be installed during the day. The focal plane wheel will be removable through a side port. This dewar will be sealed to the front surface of the camera/collimator dewar. Thus, the window of the camera/collimator dewar will form the vacuum seal between the camera/collimator and the multi-slit dewars. Since the focal plane dewar is a separate dewar it can be opened without disturbing the detector system and the large optics in the collimator/camera dewar which will have a much longer thermal cycling time.
This dewar will be similar to the camera/collimator dewar and will be fabricated of 6061 aluminum. It will have an LN2 tank suspended from a very stiff large diameter G10 ring. In the case of the focal plane dewar the LN2 tank will be toroidal in shape with the optical beam passing through its center. To speed the thermal cycling of this dewar, it will have a Kapton heater panel glued to the tank. This dewar will have a separate vacuum valve and an easily changed block of getter. Given its small volume and low mass I estimate we can completely thermally cycle the focal plane dewar in about 6 hours (2 hours warming, 2 hours pumping and 2 hours cooling). This makes changing the focal plane masks an activity that can be accomplished in a single day.