The JCMT Newsletter Number 11 (SPIFI)
 South Pole Imaging Fabry-Perot
Interferometer (SPIFI)
SPIFI is a direct detection, imaging Fabry-Perot interferometer
designed for use in the submillimeter band (200 to 650 microns),
especially the 350 and 450 micron windows available to the JCMT.
SPIFI's detector is a 5 x 5 element monolithic silicon bolometer
array cooled to 60 mK in an adiabatic demagnetization
refrigerator. SPIFI uses free standing metal mesh Fabry-Perot
interferometers to deliver spectroscopic images at velocity
resolutions up to 30 km/s over the entire array. The velocity
resolution is continuously adjustable from 300 to 30 km/s in a
few minutes time at the telescope. Higher velocity resolutions
(better than 15 km/s) are possible for the inner 9 pixels.
The Winston cones coupling radiation to SPIFI's bolometers have
6.1" (~ lambda/D at 450 microns) circular entrance apertures and
are arranged on a 7.0' square grid, so that SPIFI images a 35" x
35" field of view at the diffraction limit of the JCMT telescope.
At present, SPIFI has 9 pixels operating, we plan to have the
additional 16 pixels installed in September 1998. SPIFI has
demonstrated background limited sensitivities in the lab that are
equivalent to single side band receiver temperatures of ~ 400 K
(5 pixels) and < 800 K (4 pixels) at 370 microns. This receiver
temperature is not a function of the velocity resolution
employed, and is only a soft function of the line wavelength from
300 to 500 microns. We estimate the best attainable receiver
temperatures are ~ 100 K (SSB), and are working towards this goal
with reasonable hope of success. At present, SPIFI can tune to
any frequency in the 350 micron window, and continuously scan 13
spectral resolution elements at any given wavelength. It is also
possible to set up in the 450 micron window. In the near future,
we expect to be able to easily switch between the two windows
while the instrument is cold. Important astrophysical lines in
the 350 micron window include the 371 micron [CI] fine structure
line, and the rotational transitions of CO (7-6) (372 micron),
HCN (10-9 & 9-8) (338 & 376 microns), and HCO+(10-9 & 9-8) (336 &
374 microns).
Tau(225 GHz) Tau(809 GHz) Tsys TA*(rms)
(nepers) (nepers) (K) (K)
0.045 0.70 2060 0.013
0.030 0.40 1500 0.010
0.060 1.00 2800 0.018
Estimated 809 GHZ sensitivities of SPIFI per pixel on the JCMT
in 1 hour of
integration time, scanning 6 spectral resolution elements. For
these estimates we have assumed the receptive efficiency of the
telescope eta(tel) is 65%.
These sensitivities are calculated based on current lab measurements at
a resolving power of 6000 (50 km/s). To scale to other resolving
powers, notice that since Tsys is independent of the resolving power,
TA* is proportional to R^1/2. For example, at a resolving power of
2,000 (150 km/s) TA* will be sqrt(2000/6000) = 0.58 times smaller (better)
than the values in the table.
Note that our sensitivity is not yet optimized. On JCMT it is possible to
reach sensitivities 2 or 3 times better than those above, and we are
working towards this goal. Current best estimates will be posted on our
Web page at the Cornell Astronomy Department Site:
http://astrosun.tn.cornell.edu/research/projects/spifi.html
SPIFI was developed at Cornell University under a NASA grant and
is a collaborative venture between individuals at Cornell, Boston
University and the SETI institute. It is our hope to achieve
first light with SPIFI on the JCMT in early 1999, and our intent
to make SPIFI available on loan to the JCMT for the foreseeable
future. Our group welcomes scientific collaborations with other
JCMT users. Please contact Prof. G. J. Stacey at Cornell
University
stacey@astrosun.tn.cornell.edu
to arrange collaborative efforts.
Modification Author: Graeme Watt (gdw)
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