Max Planck Institute for Astronomy - Annual Report 2007

110 IV. Instrumental Developments and Projects
IV.3 Instruments for Calar Alto
Panic
A wide field imager for the near infrared was named by
the astronomical communities both at the MPIA and the
IAA as the most desirable new instrument for Calar Alto.
Such an instrument, albeit not the first of its kind, would
have many applications, ranging from solar system studies
to cosmology. Since the overbooking is normally
much higher for the 3.5 m telescope than for the 2.2 m
telescope, it was decided to build Panic, a Panoramic
Near Infrared Camera, for the 2.2 m telescope.
The currently available detectors have sizes of
2048 2048 pixel. It was decided to use an array of four
detectors to increase the available field of view. To cover
a field of view of 0.5 degrees 0.5 degrees results in an
image scale of 0.45 arcsec/pixel. Since these detectors
are buttable with a gap of only about 167 pixels, Panic
will have a very convenient foot-print.
The instrument will have pure lens optics with ten
lenses in eight groups. The image quality is defined as
usual, 80 percent encircled energy within two pixels over
the whole field of view and for all wavelengths. The spectral
range extends from 0.8 to 2.5 microns and thus includes
the optical z-band.
The layout is shown in Fig. IV.3.2. The instrument
first has to meet limits of length and weight, which requires
the optical path to be folded by three mirrors.
Second, the cryostat is extremely lightweight in order to
meet the limits set by the telescope. Cooling is achieved
with liquid nitrogen, the holding time is estimated to be
34 hours with a 30 liter filling. A sec-ond small liquid
nitrogen vessel is exclusively used to cool the detector.
The conical steel element attaches the whole instrument
to the telescope; its weight is only 15 kg, and its flexure
is only 10 microns
Fig. IV.3.1: The optical system of Panic. The total length is
about 180 cm and requires a folded design.
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All optical elements are mounted to an optical bench
in order to minimize flexure. This is very important since
the optics requires small mechanical tolerances, typically
on the order of 50 microns. Finite element analysis
has shown that the flexure of the optical bench exceeds
this value by factors of a few, but only in the most extreme
cases.
Four filter wheels, with six positions each, are foreseen
to accomodate 20 filters. The optical design also allows
the use of narrow band 1 percent filters.
These detectors allow also on-chip guiding in a small
sub-window. This will be necessary since the standard
guiding unit of the telescope would vignette a part of the
field of view and therefore cannot be used in combination
with Panic.
The read-out electronics will be the latest development
of the standard MPIA read-out electronics. The four detectors
will be read out simultaneously in 128 channels.
A fast read-out mode will also be included.
To aid observers, an observation tool will allow easy
input of all information to set-up the instrument and perform
the observation. A quick-look system with online
data reduction will allow the observer to judge the quality
of the data obtained, and the data will be archived
automatically.
Although optimal for surveys, the image scale is inadequate
for studies requiring a high spatial resolution.
An enhancemant was investigated to supply a second
pixel scale of 0.25 arcsec/pixel by exchanging parts of
the optics, accomplished by mounting the corresponding
lenses on a motorized wheel; however, with the required
additional optics and mechanics the instrument would
have exceeded the weight limit at the 2.2 m telescope.
Fortunately, optical calculations have shown that Panic
can also be used at the 3.5 m telescope, with a pixel scale
of 0.22 arcsec/pixel. The instrument can thus be used
for observations which need the large field of view at the
2.2 m telescope and for observations which require higher
spatial resolution at the 3.5 m telescope.
Josef Fried in collaboration with IAA, Granada
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