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Karen Jean Meech -- CCD Wide Field Comet Program

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.

Last modified: February 17, 2001
Karen Meech
Institute for Astronomy
2680 Woodlawn Drive
Honolulu, HI 96822