CTIO and The Ohio State University are upgrading the Ohio State Infrared Imager and Spectrometer (OSIRIS) for a comeback tour at CTIO. We will put a new HgCdTe 1024 format array into the instrument and expect it to be available for visitor use sometime during semester 1999A. We will offer OSIRIS for use on both the 4-m (f/14 tip-tilt) and 1.5-m (f/13 focus only).
As of mid-July the project schedule remains approximate. The detector upgrade, delivery to CTIO, and instrument characterization on the telescope should occur over the southern summer. However, to err on the conservative side, IR proposals for semester 1999A should be written for use of CIRIM or the IRS. When OSIRIS is user-ready (hopefully by March-April), we will offer it as an alternative instrument for CIRIM and IRS proposals that would benefit from its capabilities. We will place information about OSIRIS on the IR Instruments Web page as it becomes available, along with updates to the schedule.
OSIRIS provides two imaging scales, which can be changed while the instrument is on the telescope. On the 4-m, this will maximize efficiency with the f/14 tip-tilt system under differing conditions. A 0.15" per pixel scale will take advantage of Cerro Tololo's best corrected seeing, while a 0.4"/pix scale will allow productive observations even when the site seeing is not at its best. OSIRIS will not completely fill the 1024 pixel format in imaging mode because it was not designed for an f/14 beam. The fully illuminated square field of view will be approximately 70" on a side at the fine scale on the 4-m---—an increase of 1.75 in area at a 25% finer pixel scale than currently available. The field of view at the coarser scale will be about 3' × 3', or about 3× the area coverage of CIRIM with the same sampling at f/8. The corresponding fields on the 1.5m will be nearly 4' × 4' at 0.4" per pixel and 10' at 1.1"/pix. Because the internal cold stop is fixed and optimized for the 4-m, the 1.5-m will be effectively stopped down to 1.4-m. We expect the increased field size to more than compensate this in total throughput for many programs.
OSIRIS will provide at least the following set of filters: JHK, H2, continuum, Brg, and CO. We will likely add more narrow band filters to this list, up to a maximum of 16.
In addition to 1-2.5 µm imaging, OSIRIS provides spectroscopic capability in the same dewar. There is a long slit mode covering an entire band (J, H, or K) at resolution 2900 in one grating setting. This mode uses the fine pixel scale; the slit width is approximately 3 pixels (0.45" on the 4-m). In addition, there is a cross dispersed mode that covers J, H, and K bands simultaneously at resolution 1200. The XD mode will use the coarse scale with 0.4" pixels and a 1.2" slit on the 4-m.
The throughput in imaging mode should be similar to CIRIM. In spectroscopic mode, OSIRIS will have lower sensitivity than the IRS since the original f/30 optics will now overfill the grating at f/14. However, for many programs the much greater wavelength coverage at higher spectral resolution than typical IRS configurations should provide compensating advantages.
We expect that OSIRIS will largely supplant CIRIM for imaging programs. The IRS will remain available for programs requiring longer wavelength coverage (> 3 µm) or higher spectral resolution than provided by OSIRIS. We will still offer CIRIM for use on the 1.5-m when OSIRIS is scheduled on the 4-m.
The return of OSIRIS is made possible by an agreement between CTIO and OSU, which includes a small amount of guaranteed telescope time. If you have questions regarding OSIRIS and CTIO, please contact one of us.
Bob Blum (rblum@noao.edu)
Ron Probst (rprobst@noao.edu)
Patrice Bouchet (pbouchet@noao.edu)