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The gravitational microlensing experiment currently underway
on the Mt. Stromlo 50-inch telescope is another excellent example of
how an old telescope can be outfitted with a modern detector system
to produce superb results. This experiment makes use of a novel an
sophisticated CCD imaging design whereby by 4 2048x2048 detectors are
butted together to form a single 4096x4096 pixel detector system.
In fact, a dichroic beam splitter placed in the optical path allows
the incoming beam to be split into red and blue channels with each
channel have a 4096x4096 detector. Since a single 2048x2048 detector
is 16 bits deep in its imaging plane, then the readout from 4 million
pixels is 8 Mbytes. The Mt. Stromlo system reads out 8 of these
devices every few minutes and thus generates a data rate on the order
of 10 Gigabytes per night of observing. Still, workstation and
computing power are able to handle this load. Figure 4 (1.1 Megs) shows an
example image from one of the LMC fields that is searched for gravitational
microlensing amplification of stellar brightness. Overall, the MACHO
project is a brilliant confirmation that small telescopes are still
quite useful and can perform novel imaging studies.
The Short Focal Length Approach
In the late 80's, prior to the proliferation of million-pixel
CCDs, the only practical way to achieve a wider imaging field was through
the reduction of the telescope image scale. This reduction requires a
modification of the telescope secondary mirror or some form of additional
reducing optics. At the Michigan-Dartmouth-Mit Observatory, located on the
Southwest Ridge of Kitt Peak, a focal reducing camera constructed by
Greg Aldering, can be used on the 1.3-m telescope to reduce the image
scale by a factor of 4.67. The cost of this camera, which contains
5 elements (lenses) was approximately $12k
and it is more fully described in the December 1991 issue of the Publications
of the Astronomical Society of the Pacific. Figure 5 (124 Kbytes) shows an image of
NGC 253, taken with this camera and a 320x512 RCA CCD which has 30 micron
size pixels. The image scale is 3.05 arcseconds per pixel. The rectangle
in the lower right shows the field size that would be obtained without
reimaging optics.
This figure also shows the downside of focal reducing
cameras. The many new optical elements in the light path, required to
reduce the image scale, also produce ghost images when there are very
bright sources in the field. These spurious images are so marked in
Figure 5. Figure 6 (151 Kbytes) shows
another image, of the high surface brightness galaxy M101, obtained with this reimaging camera. In this case, the detector
was a TI4849 CCD with 22 micron pixels resulting in an image scale of 2.32
arcseconds per pixel. The exposure time for this image was 30 seconds
(through a Blue filter)
which was sufficient to capture much of detailed structure in M101 (the black
dots in the centers of some bright stars indicate saturation of the CCD).
As a final example, Figure 7 (202 Kbytes)
shows a 30-second image of M33 taken with this camera
where again much of the structure of M33 can be seen and the dust lanes
are evident. Overall then, a
modest focal reducer can produce a large enough field-of-view on 1-meter
class telescopes with Cassegrain foci to effectively CCD image those nearby
galaxies which have been adopted as morphological standards, otherwise
known as the Hubble Sequence. In this way, images taken with this
camera through different filters can show how the morphology of a galaxy
is highly wavelength dependent
Page 5
The Electronic Universe Project
e-mail: nuts@moo.uoregon.edu