AAS Meeting #193 - Austin, Texas, January 1999
Session 11. Observatories, Telescopes and Instruments
Display, Wednesday, January 6, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[11.05] Reducing Scattered Light in CCD Images at the CTIO 0.9m Telescope

D. Barnaby (Ctr for Astrophysical Research in Antarctica), B. J. Rauscher (Univ of Durham)

We have been studying luminous halos around spiral galaxies in very deep near-infrared K-band images (limiting surface brightness =24~mag~arcsec-2; \lambdac = 2.35 \mum). Following our successful detection of a halo around the southern spiral galaxy ESO 240-G11 (Rauscher et~al.~1998), we collected optical CCD images of candidate galaxies at the Cerro Tololo Interamerican Observatory (CTIO) 0.9~m telescope from 17-21 Sept 1998.

Because light scattered by the telescope can mask or mimic faint, extended emission, we took precautions to minimize stray and scattered light. To assess the amount of scattered light, we collected images of the telescope pupil using a pinhole camera, which we constructed by laying a sheet of aluminum foil, perforated with a submillimeter hole, 19 mm in front of the CCD. These images showed that the telescope did scatter light on the order of a few percent over a radius of about 2 arcminutes. Supressing this extraneous light was highly desirable, since halo surface brightness can be < 0.2% of the disk central surface brightness.

To reduce the scattered light, the staff of CTIO and we added 3 baffles. One baffle consisted of a 0.95~m diameter tube added as an extension to the telescope, which has a closed-tube design. The other 2 baffles were diaphragms mounted between the primary baffle (the ``chimney'') and the shutter. These baffles effectively eliminated the largest sources of scattered light, which appear to be the edge of the secondary baffle and the inner surface of the chimney (Figure 1, http:// astro.uchicago.edu/home/web/barnaby/sctrdlite98.html).

In this paper, we quantify the reduction in scattered light and examine the most useful location to add baffles to this telescope.

\noindent Acknowledgements. We thank the CTIO Telescope Operations staff for their help in constructing and installing the baffles. We also thank support astronomer Nick Suntzeff for suggesting the pinhole tests.\\

\noindent Rauscher, B. J. et~al.~1998, ApJ, 506, 116

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