Two Infinities

Mon, 01 Jun 2020

Double Stars and Seeing
For the last few trips to our Magdalena home, I've been trying to take seeing measurements as part of evaluation of possible sites for an observatory. A friend who is also a professional astronomer suggested checking whether is was possible to split double stars whose angular separation was near the seeing limit.

One pair, in Coma Berenices, is STT (Otto Struve) 266. From the Washington Double Star catalog the stars are nearly equally bright at magnitudes of 7.97 and 8.42 and are separated by 2.00 arcseconds as of 2018.

It was windy and seeing had been predicted to be below average by the Canadian Government website. This was the best exposure; most exposures smeared, all in the same direction. 2.1 arcsecond FWHM computed by Ginga on the 9th magnitude star close by:

But FWHM computed this way varied across exposures, exposure time and target star.

Turning to STT 266 itself, clear separation was observed in exposures without the shake smear. In one, Ginga's cut plugin shows a valley between two peak intensities:

In the second, centroids are guessed by eye and Ginga's ruler utility measured the distance between them. With focal length of telescope at 1200 mm and pixels 2.9 μm each pixel subtends about .49 arcsecond, so a measurement just over 4 px is expected:

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