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CTIO IR Imager Commissioned (1Dec94)
(from CTIO, NOAO Newsletter No. 40, 1 December 1994)

We have had commissioning runs with the new CTIO IR Imager (christened
CIRIM to distinguish it from its predecessor) on the CTIO 1.5-m and 4-m
telescopes, and it has been used for several science programs (scheduled
programs) already. There are a few minor problems that need further work,
but it is generally working quite well.

Below we describe the performance as currently measured, as an aid to
people contemplating proposing to use it, and as reassurance to those who
have proposed or have time with it. The instrument is based on a 256 X 256
Rockwell NICMOS III HgCdTe detector, run using the NOAO WILDFIRE
electronics plus the CTIO motor controller. The optics consist of a 1.5:1
focal reducer and three cold filter wheels and a cold Lyot stop wheel. The
different Lyot stops allow operation with different telescopes and
secondaries; those that we offer currently are listed below:

                          Telescope Scales

Telescope        F/ratio         Pixel Scale     FOV
  4-m            f/7.5           0.40             26"
  4-m            f/30            0.10            102"
  1.5-m          f/7.5           1.16            297"
  1.5-m          f/13.5          0.645           166"
  1.5-m          f/30            0.30             75"

We are considering offering the instrument on a smaller telescope (most
likely to be the 1-m), but this has to await both engineering tests with the
instrument on the 1-m and a decision on how extensive the instrument
complement on the 1-m telescope should be.

It is possible to flip between f/7.5 and f/13.5 on the 1.5-m in about 20
minutes. It is possible to flip between f/7.5 and f/30 on the 4-m provided
that the f/30 secondary has been installed; this must be requested (and
justified) in the original proposal. Changes to and from the f/30 top end on
the 1.5-m are a major operation, and can only be done if the telescope
schedule was set up accordingly. This means you must request the f/30
secondary at the time you request telescope time.

                                Filters

CIRIM contains three stacked filter wheels which can hold up to 30 filters
total. Currently, we have installed the following filters:

Broadband: I'(1.0 um), J, H, K[SUB S], K. K[SUB S] is UMass
    "K-short" and not UHawaii "K-prime."

Narrowband 1%: 1.083 u, 1.094 u, 1.237 u,
    1.257 u, 1.281 u, 1.644, 2.122 u, 2.166 u, 2.420 u.

Narrowband other: H[SUB 2]O (1.99 u 3%), continuum
    (2.20 u 4%), CO (2.36 u 3%), H[SUB 2] continuum (2.38 u 2%).

There is also a dark position and a small lens for diagnostic viewing of the
Lyot stop. Other filters may be present for test purposes, or may be added if
we acquire other astrophysically useful filters. The detector has minimal
sensitivity beyond 2.5 um, and the optics do not transmit longer wavelengths
in any case, so 3 um filters are not available.

The filter wheels are located in the converging beam, behind the Lyot stop,
which means that telescope focus varies from filter to filter. Since the optics
also re-image the focal plane onto the detector, the different optical
thicknesses of the filters have the effect of changing the effective geometry
slightly, so that the plate scales also vary from filter to filter, by amounts
of up to 1%. This effect will be important for programs where structure of
complex objects needs to be registered (for example, color gradients in
galaxies). Programs where the exact scale is important should measure the
relative scales in the individual filters (for example using a globular
cluster).  We may make an attempt in future to provide a set of JHK or
JHK[SUB S] filters that are exactly confocal.

                             Performance

Measured performance at the CTIO 1.5-m at f/7.5 is as follows:

               Counts, mag 15                    Background
Filter      ADU/         e^-/sec      ADU/       e^-/sec/      mag/
             sec                     sec/pix      pix         arcsec^2
I'          113          1020         15          135          17.6
J           135          1220         50          450          16.5
H           136          1220         300        2700          14.5
K[SUB S]     86.4         778         500        4500          13.5
K            94.2         848        1000        9000          12.8

The detector gain is about 9 e^-/ADU. Full well at nominal bias (900 mv) is
over 39,000 ADU, but non-linearity is significant. We recommend keeping
count levels below 15,000 ADU and correcting for the residual non-linearity
(2% or less). Read noise is about 37 e^- (i.e. a bit more than 4 ADU).


The numbers above can be scaled to the 4-m or other f/ratios. These
correspond to a total system efficiency, including telescope and atmosphere,
of about 34% at K. In practice, in about 10 minutes you should be able to
detect sources as faint as K[SUB S] = 17.5 at f/7.5; performance at f/13.5
will be a factor of two better because of the smaller pixels, and performance
at f/30 may be another factor of two better still under good seeing
conditions (subarcsecond).

Note that since both airglow and temperature affect background, those
numbers should be considered approximate. The tabulated values were
obtained at an ambient temperature of 8^oC, which is around the mid-point
of typical Tololo temperatures.

Ghost images appear to be negligible. Repeatability of photometry over the
chip and repeatability of flat fields both appear to be quite good (better than
2% at least), which is consistent with the observed stability of the Lyot stop
(well under 1% motion over the sky).

The WILDFIRE software provides the observer with control of the motor
and telescope functions from the instrument control window, and scripts can
be written (in tcl) which allow sequences of commands, such as grids, beam
switching or sequences of filters. Users who have written similar scripts for
KPNO instruments will find these familiar.

                                 Manual

A version of the manual (in LaTeX) will eventually be placed in the CTIO
public ftp directory. In the meantime, a draft version can be obtained from
one of the undersigned.

R. Elston, J. Elias, B. Gregory

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