ISLE: a general purpose near-infrared imager and medium-resolution spectrograph for the 1.88-m telescope at Okayama Astrophysical Observatory

Abstract

ABSTRACT ISLE is a near-infrared (1.0-2.5 µm ) imager and spectrograph for the Cassegrain focus (f/18) of the 1.88 mtelescope at Okayama Astrophysical Observatory. The detector is a HAWAII 1024 × 1024 HgCdTe Array, whichcovers 4.2 × 4.2 arcmin 2 “eld of view with a pixel scale of 0.25 arcsec/pixel. ISLE also provides medium (R=300- 4800) resolution long-slit (4 arcmin long ) spectroscopic capabilities using re ”ection gratings. A dedicated front-end electronics for the detector achi eved a readout noise of 8 el ectrons by the conventional Fowler sampling, anda operation scheme that combined with a number of discarded readout greatly suppressed the reset anomaly.The measured limiting magnitudes were J =18.6 and K =17.7 (imaging of point sources at S/N=10 for 10 min.exposure).Keywords: HAWAII-1 array, NIR-imager, NIR-spectrogr aph, Reset Anomaly, O dd-even column eect. 1. INTRODUCTION ISLE has been developed as a upgrade instrument of OASIS, 1 which was the “rst common-use near infraredinstrument in Japan. OASIS was a successful instrument, produced several tens of scienti“c papers, signi“cantlycontributed to produce science basis. It was decommissioned in 2003 by a malfunction of the OASISs NICMOS3detector. We have started the ISLE project to renew the instrument just after the accident.We designed ISLE to improve the spatial resolution than OASIS. The detector was upgraded to the HAWAIIarray and the optics inside the cryostat were replaced to match the excellent seeing - expected to be 0.55 arcsecat 2.2 µ m - at Okayama Astrophysical Observatory. The pixel scale of ISLE was set to be 0.25 arcsec to samplethe best seeing in the infrared, which yields a “eld of view of 4.2 × 4.2 arcmin