Instrument Design

The GMT produces a focal surface with a scale of about 1 arcsecond per millimeter. In order to collect images with a multi-arc-minute field of view through different wavelength bandpass filters and sample the best seeing, corrected by Ground Layer Adaptive Optics and with reasonable pixel sampling, the focal surface must be reimaged and demagnified to a smaller scale on a flat detector. Moreover, the images delivered by the reimager optics must be outstanding in order to avoid degrading the exquisite image quality produced by GLAO. ComCam is that reimager. It's basic imaging characteristics are:

• Image quality optimized for λλ370-950 nm with good imaging at shorter and longer wavelengths within the CCD's bandpass

• < 10% degradation of the 10th percentile best image quality that GLAO will deliver

• f/2.16, yielding a 256 μm/arcsec plate scale

• Teledyne e2v CCD290-99 with 9216 x 9232 10μm pixels; 39.1 milliarcsec per pixel

• 6x6 arcminute field of view on the sky

• Capable of holding up to 10 broadband and 5 narrowband filters for deployment during the night

Secondary observing modes are under consideration, including narrowband imaging using a Fabry-Perot etalon, imaging polarimetry, slitless, low-resolution spectroscopy, and others.

Major instrument subassemblies sre shown mounted in a structure held fixed with respect to the GMT focal surface. The camera support weldment holds the camera, broadband filter wheels, shutter, and cryostat, and is moved axially to switch between broadband, narrowband, or specialty observing modes.

The instrument is held on a static hexapod support system inside the upper half of an Instrument Mounting Frame that occupies sector D of the GMT's Gregorian Instrument Rotator. When in use, the IMF with ComCam is deployed to the center of the GIR on the GMT's optical axis.

In addition to the truss-mounted instrument, the upper IMF includes a protective cover, an image quality verification tool (FakeStars), a system to reposition the instrument axially within the IMF, and a deployable corrector lens that is used with the facility wide field corrector and atmospheric dispersion compensator (C-ADC). The lower IMF contains the main control system electronics and auxiarly support equipment. Stairs connect the upper and lower IMF sections.

ComCam can be used with or without the C-ADC by moving the instrument axially about 200mm and inserting or retracting the Corrector Field Lens. 

Electronic controls are distributed throughout the IMF, with the primary control center housed in an electronics enclosure on the lower level of the IMF.