AAS 207th Meeting, 8-12 January 2006
Session 173 Instrumentation: Ground Based or Airbourne
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

## [173.09] The Performance of Diamond Machined, Freeform Optics in the IRMOS Spectrometer

R. G. Ohl (NASA/Goddard Space Flight Center), J. W. MacKenty, M. Robberto (Space Telescope Science Institute), R. R. Joyce (National Optical Astronomy Observatory), IRMOS Team

The Infrared Multi-Object Spectrometer (IRMOS) is completing commissioning on the Kitt Peak National Observatory 2.1 m and Mayall 4 m telescopes. IRMOS is a near-IR (0.8-2.5 micron) spectrometer with low- to mid-resolving power (R = \lambda / \Delta \lambda = 300-3000). On the 4 m, IRMOS produces simultaneous spectra of up to ~100 objects in its 2.8x2.0 arcmin field of view using a commercial micro-electro-mechanical systems (MEMS) multi-mirror array operating as a real-time programmable slit mask. The all-reflective design consists of two imaging subsystems. The focal reducer images the telescope focal plane onto the MEMS field stop. The spectrograph images the slits onto a large-format detector. The all-aluminum instrument operates at ~90 K.

We describe the point spread function (PSF) of the imaging mode as observed on the 4 m. PSF morphology is given in terms of irradiance and encircled energy profiles in different bands. We compare the line spread function to observed stellar line profiles. We place limits on vignetting of point sources near the edges of the field and stray light effects. In addition, we describe an instrument-to-telescope alignment diagnostic using defocused images.

The design includes four large, fast, off-axis, highly aspheric mirrors; the camera mirror has a non-rotationally symmetric prescription. These aspheres, including the freeform" mirror, enable a compact, but high-performance, spectrometer. They were diamond machined at low cost using bare aluminum 6061 blanks, pushing the limits of achievable surface error for this technology and material. Deterministic mirror fabrication also enabled straightforward instrument assembly. IRMOS performance at Kitt Peak demonstrates that freeform optical fabrication technology is ready for ground-based instrumentation in terms of both wavefront error and cost envelope.

This work is supported in part by the Director's Discretionary Research Fund at the Space Telescope Science Institute and the James Webb Space Telescope project at NASA/Goddard Space Flight Center.

Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.