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OSIRIS Visits CTIO (1Dec92)
(from CTIO, NOAO Newsletter No. 32, 1 December 1992)

OSIRIS is not, in this case, the Egyptian god of the dead coming to
check up on staff members who are slow to publish, but rather the
acronym standing for "Ohio State IR Imaging Spectrometer." CTIO has
reached an agreement with the Ohio State University (OSU) infrared
group whereby this instrument will be made available during the second
semester of 1993 for use by visiting astronomers. The agreement,
similar to that originally covering the Rutgers Fabry-Perot, ensures
that the instrument will be supported by OSU staff; it provides a quick
and economical means of providing additional infrared capabilities at
CTIO.

In the rest of the article, we provide a brief summary of the
instrument's characteristics, followed by our evaluation of the
circumstances under which we recommend that visitors apply to use it.
The intention is that the instrument will be available during the
entire semester. However, because of the burden on OSU personnel
implied by support of the instrument, we intend to schedule its use
only when reasonably large blocks of time can be arranged---that is,
there will not be any isolated two-night runs.

OSIRIS Characteristics
Briefly, OSIRIS at CTIO will work at the f/30 foci of both the 4-m and
1.5-m telescopes. It has two imaging configurations and several
spectroscopic configurations. The detector used is a 256 x 256 Rockwell
HgCdTe array (NICMOS III array). Switching between the different
configurations can be done in less than a minute.  A description of the
instrument may also be found in the proceedings of the ESO Large
Telescope Conference (Atwood et al. 1992, in press). The construction
of OSIRIS was partially supported by the NSF under a grant to OSU.

Imaging Configurations
Two different plate scales are available through use of two different
cameras in the system. The plate scales provided are:

                           Scale (Arcsec/Pixel)
              Telescope     4-m           1.5-m
              Coarse       0.38           0.95
              Fine         0.15           0.37

The following filters are currently expected to be available:

Broadband JHK and K', also a "long K" for planetary work.
Narrowband (1%): 1.083, 2.090, 2.124, 2.140, 2.162, 2.190, 2.270,
   2.305 um
Narrowband (3%): 1.995, 2.205, 2.360 um (standard H2O and CO filters).

Potential users are reminded that the Rockwell arrays are not sensitive
longward of 2.5 um.

Spectroscopic Configurations
Several spectroscopic configurations will be available. Two use a
grating to provide the dispersion, plus a blocking filter for order
separation. In these configurations, the resolution is set by the
choice of camera optics. The low resolution configuration gives R ~
550, while the higher resolution configuration gives R ~ 1550. (In both
cases, resolution is defined as two pixels). The scales along the slit
are the same as for imaging, but there is anamorphic demagnification
along the dispersion direction, resulting in slit widths (on the 4-m
telescope; 1.5-m values are three times greater) matched to the
resolution of 1.2 arcsec for R = 500 and 0.45 arcsec for R = 1550. In
all cases the usable slit extends the full height of the array. At R =
550, an atmospheric "window" is covered with a single grating setting;
at R = 1550 three or four settings (depending on the amount of overlap
desired) are required.

In addition, a grism cross-disperser can be used instead of a blocking
filter; this provides coverage in multiple orders at the price of a
shorter usable slit. This configuration is only useful at R = 550.  If
coverage only from 1.18 to 2.45 um is desired, the usable slit length is
roughly 30 pixels. If an additional order is to be included, which
would extend down below 1.0 um, the usable slit length is reduced to
approximately 10 pixels. This last configuration is likely to
complicate sky subtraction because of the very short slit, and
prospective users may find it easier to get the short wavelength
coverage separately but with a longer slit.

When to Use OSIRIS
By second semester, CTIO instrumentation is expected to include (1) the
SBRC array IR Imager, (2) the SBRC array IRS, and (3) the CTIO HgCdTe
Imager (see accompanying article for details).

OSIRIS's imaging capabilities do not differ significantly from those of
the new CTIO instrument, except for the ability to easily switch plate
scales at the telescope. We would therefore recommend use of OSIRIS
mainly for those proposals where switching quickly between the two
plate scales is important to the science. Since it is also the case
that the CTIO Imager will achieve 0.4 arcsec pixels only using the f/8
secondary on the 4-m, which will not be available during the first
months of the semester, proposals which require this scale should also
request OSIRIS.

For spectroscopic work, OSIRIS offers substantial improvements in
performance over the CTIO IRS except for work beyond 2.5 um or where a
resolution greater than R = 1550 is needed (the IRS offers R = 2000 and
R = 3000 configurations). We therefore recommend that it be requested
for all proposals except those involving long wavelength observations
or requiring high resolution. In addition, prospective users planning
to observe at R = 2000 with the IRS should consider that the greater
wavelength coverage of OSIRIS for a given grating setting may well
offset the higher signal to noise required (1.3 times higher at R =
1550) to detect a weak feature against a strong continuum.

How to Apply for Time
We will provide an update on instrument performance in the next
Newsletter (March 1993), based on the results of further lab and
telescope tests of the instrument; interested users may also contact
any of the undersigned.  Proposals should be written as for any CTIO
facility instrument. Scheduling of the instrument does not require that
you specify the configuration to be used, but it is important to do so
from the point of view of evaluating technical feasibility and amounts
of time required.  Proposals will be handled in the same way as
proposals for facility instruments, except that technical feasibility
review will be done in consultation with OSU staff.

Bob Williams, Jay Elias (jelias@noao.edu),
Brooke Gregory (bgregory@noao.edu),
Richard Elston (relston@noao.edu),
Jay Frogel, Darren DePoy (OSU)

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