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VLT, Chile -- 11/2000
Meech Research
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Instrumentation Development
The first stage of this program was the development of a unique 8192 x
8192 CCD mosaic camera to be used to carry out a program of distant
comet recovery. The goal of the comet recovery program is to detect
periodic comets prior to the onset of sublimation (i.e. between 4-10 AU)
in order to study the physical and chemical composition of cometary
nuclei which includes both the characterization of the bare nucleus and
modelling of the development of activity. The instrument will also be
used to conduct a survey in order to characterize the poorly defined
population of Earth-crossing Aten asteroids which can be potential
space hazards (Dr. D. Tholen). I am involved in the instrumentation
project by obtaining a large fraction of the funding for the project.
From these funds, through the support of the NSF Instrumetation
program, I have hired a Postdoctoral Research Fellow, Dr. Mark Metzer,
who has been largely responsible for the design and construction of the
camera with Dr. Gerry Luppino.
The camera is the largest CCD mosaic in existence, allowing for the
first time very faint imaging (to mag 24-26) over large areas. The 8192
x 8192 mosaic has been constructed from a 2x4 array of 2K x 4K three
edge-buttable devices from Loral. The device is capable of detecting
point sources in a 1 hour exposure with seeing of 0.8 arcsec to a
limiting magnitude of 26.2 in the B filter at S/N=5 on the UH 2.2m
telescope in dark skies. For further discussion of the technical
specifications, please see Luppino et al. (1994; SPIE
Proc. 2198). The device was completed in mid-March 1995. The
CCD area is 155 cm2 with a plate scale of 0.232 arcsec per pix on the
CFHT (giving a FOV of half a degree).
Scientific Results
In collaboration with Postdoctoral Fellow, Dr. Olivier Hainaut, we
have now had 3 observing runs (2 of which have been clear) for distant
comet recovery (7/95 - UH 2.2m, 9/95 - CFHT, 2/96 - UH 2.2m), for which
we have data for about 10 comet recoveries. Data reduction is in
progress, however we did obtain a recovery image of comet
Honda-Mrkos-Pajdusakova. We expect that this instrument will make
significant advances in our understanding of the initiating of activity
in comets as we will for the first time be able to systematically
recover comets at heliocentric distances prior to the onset of
activity. Figure 1 shows the magnitude of a
recovered comet as a function of its heliocentric distance, r.
The dark shading corresponds to the region where most of the recoveries
were performed; the light shading also includes the few extreme
recoveries (adapted from Kresak, 1973 (Bull. Astron. Inst.
Czech. 24, 264) and additional data). The vertical shading
indicates the magnitude limits for a traditional widefield Schmidt
telescope equipped with photographic plates and for the 8192 CCD on the
UH2.2m; the darkest shading at the top corresponds to the limiting
magnitude of the CCD mosaic on the CFHT.
Because of past instrumental limitation, most of the comet recoveries
were obtained in the region where the water sublimation was already
possible (as indicated by the vertical lines); only a few comets were
recovered while fainter than the plate limit, and they were the result
of extensive, time-consuming special efforts (e.g. the recovery of
1P/Halley with the Hale 200 telescope), and also with a prototype of
the 8192 CCD while installed on the CTIO Schmidt. The two diagonal
lines correspond to the magnitude of two extreme cometary nuclei:
1P/Halley's and a typical small cometary nucleus. We expect to see most
of the cometary observations to fall between these two lines as long as
their nuclei are not active. The regions where water and CO2
sublimation are most likely to start have been indicated. The dots
correspond to our first priority objects for recovery on the CFHT,
where for comets where the nucleus size was unknown, a 1P/Halley-like
size was assumed.
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