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CTIO HgCdTe Imager (1Dec92)
(from CTIO, NOAO Newsletter No. 32, 1 December 1992)

This article is intended to provide a brief summary of the
specifications for CTIO's second generation IR Imager, now under
construction. This instrument is based on a 256 x 256 Rockwell HgCdTe
array, and will accept a larger format (512 x 512) array with the
addition of a simple field-flattener, when such devices become
available, if and when CTIO has the funds to purchase one. Our current
schedule would have it becoming available roughly at the beginning of
second semester 1993.

The imager has optics which provide a fixed demagnification ratio of
1.5, of the focal plane onto the detector. The instrument will work at
f/ ratios of 7.5 or slower, with optimization for a particular
telescope and secondary provided by use of an internally selectable
cold stop. Thus, a range of pixel scales are available through
selection of telescope and secondary, but changes in scale at the
telescope are generally inconvenient.

The virtue of this approach is the extreme simplicity of the mechanical
design - an important factor considering the limited resources of the
CTIO instrumentation program - and the high efficiency of the optical
design (four elements including the dewar window).

A focal ratio converter has also been designed and ordered for use on
the Schmidt telescope. This converts the f/3.5 beam of the telescope to
f/7.5, which is the fastest beam with which the imager will function.

A list of approximate plate scales is attached for all telescopes on
which operation of the imager is expected to be scheduled:

                        Focal          Arcsec/        Field
       Telescope        Ratio           Pixel        (Arcmin)

       4-m              f/30             0.10           0.4
                        f/8              0.40           1.7

       1.5-m            f/30             0.27           1.1
		        f/13.5           0.60           2.5
		        f/7.5            1.08           4.6

       0.9-m            f/13.5           1.0            4.2

       Schmidt          f/3.5+conv       2.72          11.6

The imager will have all presently available 1-2.5 um IR filters, plus a
variety more presently on order. The final complement will include IJHK
and K' broadband filters, H2O and CO filters, plus 1% bandwidth filters
for most emission lines of astrophysical interest in the 1-2.5 um
region.

When to Use It
The Rockwell HgCdTe arrays are not sensitive beyond roughly 2.5 um, and
therefore the new imager will not be useful for programs requiring L
band photometry or using the 3 m narrowband filters. For all other
imaging programs it will clearly be the instrument of choice.

Because there continue to be programs requiring 3 um observations, the
present IR imager will continue to be supported, but it will be
reconfigured with broadband JHKK' and L, plus our narrowband 3 um
filters.

Note also that Ohio State's OSIRIS (see accompanying article) will be
available during second semester 1993, providing both an alternative
and a backup on the two largest CTIO telescopes.

We will provide an update on the status of all the imagers in the March
1993 Newsletter. Prospective users may also contact any of the
undersigned in February or early March for more details.

Jay Elias (jelias@noao.edu),
Brooke Gregory (bgregory@noao.edu),
Richard Elston (relston@noao.edu)

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