Design and Performance of a Versatile Penn State near IR Imager and Spectrograph

Abstract

A versatile near IR instrument called Penn State near IR Imager and Spectrograph (PIRIS) with a 256 x 256 PICNIC IR array has been developed at Penn State and saw its first light at the Mt. Wilson 100 inch in October 2001. The optical design consists of five optical subsystems including (1) the slit aperture wheel, (2) an achromat collimator optic, (3) a grism/filter and pupil assembly, (4) a pupil imaging optic, and (5) achromat camera optics. This instrument has imaging, spectroscopy and coronagraph modes. It is being updated to have an integral field 3-D imaging spectroscopy mode and a very high IR spectroscopy mode (R ~ 150,000) with an anamorphic silicon immersion grating in 2003. The instrument is designed to take full advantage of high Strehl ratio images delivered by high order adaptive optics systems. Its imaging mode has f/37 and f/51 optics to allow diffraction-limited imaging in H and K bands, respectively. Its spectroscopy mode has R = 20, 180, 400, 2000, and 5000. The lowest resolution is obtained with a non-deviation prism. The medium resolution spectroscopy mode is conducted with three commercial fused-silica grisms. They can be either used in long slit spectroscopy mode with a blocking filter or used as a cross-disperser for a high resolution silicon grism. High resolution spectroscopy is done with silicon grisms and cross-disperser grisms, which are designed to work on high orders (~ 80) to completely cover H and K bands for R = 5000 separately, or simultaneously cover H and K bands for R = 2000. Coronagraphy is done by inserting an apodizing mask, held in the slit aperture wheel, in the focal plane and a Lyot stop (pupil mask) at a reimaged pupil inside the dewar. Image contrast can be enhanced by using different combinations of the apodizing mask and pupil mask. Several of Gaussian pupil masks have also been installed in the pupil wheel for high contrast imaging. We have successfully detected two substellar companions during our first light at Mt. Wilson 100 inch telescope. We were also able to evaluate our cononagraphy and gaussion pupil mask modes, which demonstrate 10^-3 - 10^-4 contrast 1 arcsec region around a bright point source. A hybrid coronagraph mode, a combination of an apodizing focal plane mask with a Gaussian shaped pupil mask, has been tested and produces 10^-5 - 10^-6 deep contrast as close as 4 λ/D at 2.2 μm in the lab. Low resolution spectroscopy modes including a vision prism (R = 20) and three fused silicon grisms (R = 200 - 400) have been tested in the lab. The spectroscopy results are reported here.