AAS 197, January 2001
Session 115. At the Observatory: UV and Sky Conditions
Display, Thursday, January 11, 2001, 9:30-4:00pm, Exhibit Hall

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[115.11] The APO Differential Image Motion seeing Monitor

A. Rest (Univ. of Washington), J. W. Briggs (Univ. of Chicago), G. A. Miknaitis, C. Stubbs (Univ. of Washington), N. C. Hastings, R. J. McMillan (Apache Point Observatory)

We describe two years' experience of building, testing and using two off-the-shelf Differential Image Motion Monitors (DIMMs) at the Apache Point Observatory (APO) for recording the seeing. Seeing is the result of image motion over a range of frequencies. In a single frame, all image motion with a frequency greater than the reciprocal of the exposure time is averaged out. Therefore the DIMMs sample only a truncated power spectrum, and it is critical to scale the seeing measurements by a function of the exposure time.

We have verified that the seeing at APO is proportional to the 0.6th power of the airmass, as theoretically predicted. Small deviations from this relation occur for some weather and wind conditions. However, we find that the relationship is robust enough to reasonably extrapolate a seeing measurement at one airmass to all other airmasses. Thus, it is sufficient to monitor the seeing of a single convenient star, regardless of its position on the sky.

Using both DIMMs, we find that the seeing at APO varies with location. Seeing measurements separated by only a few hundred meters can differ by up to two arcseconds. This variation is due mainly to differences in the surrounding landscape, weather and wind conditions. Thus, in order for the DIMM measurements to accurately reflect the telescope seeing, selecting an appropriate site is crucial.

If you would like more information about this abstract, please follow the link to http://www.astro.washington.edu/rest/dimm. This link was provided by the author. When you follow it, you will leave the Web site for this meeting; to return, you should use the Back comand on your browser.

The author(s) of this abstract have provided an email address for comments about the abstract: rest@astro.washington.edu

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