The Mid-Infrared Array Camera and Bracewell Infrared Nulling Cryostat (MIRAC-BLINC) is an instrument developed for use with the adaptive optics system on the 6.5 m MMT. It has two main modes of operation: imaging at 8-25 microns, and nulling interferometry at 8-13 microns. The system has been developed by Bill Hoffmann and Phil Hinz, and is available most trimesters in collaboration with the PIs.

BLINC is a lquid nitrogen-cooled set of fore-optics for the MIRAC system.

The entrance window provides the reflection surface for the visible light to be sent to the AO wavefront sensor. The Cassegrain focus is reimaged by an off-axis ellipse within BLINC which is capable of being rotated to send the light to either the nulling interferometer or the imaging channel. The imaging channel forms an image of the secondary on a reflective cold stop which baffles out warm radiation from the telescope. The mirror at the cold stop is mounted on a rotating voice coil actuator. This allows chopping at 1-10 Hz within the cryostat.

The nulling interferometer splits the entrance pupil in half and overlaps these two beams on a 50% transmissive beamsplitter. Manual alignment of the beams can be achieved with feedthrough actuators. Pathlength changes are introduced by translating the beamsplitter mount using a stepper motor with 42 microns of motion per revolution. Small pathlength changes can be introduced by a PZT-mounted mirror in the right arm of the interferometer. Both outputs of the interferometer are sent to a NICMOS3 detector within BLINC to allow sensing of phase variations in the interferometer. One of the outputs has a short pass dichroic which sends 8-13 micron light to MIRAC.

A fourth version of MIRAC (MIRAC4) has recently been completed and saw first light in late October 2006. The camera features a 256×256 array with high quantum efficiency and low dark current compared to MIRAC3. In addition a pulse tube mecahnical cooler is being used that eliminates the need for liquid helium cooling.

The filters in MIRAC4 are as follows. Note that not all of these were available for the first light time in Oct/Nov. 2006. Some filters require additional blocking which may slightly affect their transmission profile.

1. 8.7 microns 15% width
2. 9.8 microns 10% width
3. 10.3 microns 10% width
4. 11.7 microns 10% width
5. 12.5 microns 10% widths
6. 20.6 microns (Q3)
7. 10.6 microns 50% width
8. 7.9 microns
9. CVF filter

1. 18.0 microns Qshort
2. 17.4 microns (Q0)
3. 17.8 microns (Q1)
4. 3.8 microns (L’)
5. Pupil Imager
6. Grism

The sensitivity of MIRAC can be estimated by a calculator written by Joe Hora available on his web site for MIRAC.

• The telescope is pointed over to 30 degrees elevation.
• The dome is closed, and 20 x 1 second exposures per filter are taken.
• Open the dome and repeat the exposures.