AAS 201st Meeting, January, 2003
Session 76. Astronomers and Their Tools
Poster, Wednesday, January 8, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[76.08] The Use of Astronomical Seeing Measurements

S.W. Teare (Electrical Engineering Dept., New Mexico Tech)

Very few observatories have access to a daily record of the astronomical seeing over an extended historical period. An exception to this is the Mount Wilson Observatory (MWO) whose astronomical seeing logs cover the period from shortly after the observatory was founded in 1904 through to the present day. These measurements provide a unique look into the changes to the seeing conditions at a major US observatory site. While the keeping of this record has been entrusted to many at the observatory, most often the telescope night assistants, these measurements have been taken diligently and from all accounts repeatably over the years.

The early workers at MWO developed an 8-point scale that was used to evaluate the seeing. This scale began as a measure of how large a telescope aperture would provide diffraction limited seeing during a given night. If a small telescope aperture was needed to see diffraction rings, then the seeing was poor and the seeing number would be small. Of course a larger number on the scale then denotes better seeing. This became known as the Mount Wilson Seeing Scale and a variation of it is still in common use at the observatory. This scale has not always had the strongest support in the astronomical community, but its use has resulted in a nearly continuous set of comparable data.

In this paper astronomical seeing data from MWO is presented and analyzed using several different approaches. It shows that there are very long period events that can be identified and also shows that the astronomical seeing, even at a very good site, is not guaranteed over the life of the observatory.

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