Xi Bootis is my fifth and final double star observation from the night of the 1st May (see my previous post for a discussion of this). Here is my image:-
This was a 5s exposure at ISO 1600 on a Nikon D90 at the prime focus of a 140mm Maksutov Cassegrain (an Orion OMC-140). The picture can be better seen by 'clicking' on it. The image is being viewed at full scale (in other words it has not been binned up). Xi Boo A and B are marked and these two stars are visual magnitude 4.70 and 6.97 respectively (actually A is a BY Draconis variable and ranges in brightness from 4.52 to 4.67). The image scale is 0.5859 arcseconds per pixel and this is a 1200x800 pixel crop of the original.
My method for orientating the image has been described in a previous post. Using this I have made some measurements of the separation and position angle (PA) of Xi Bootis (for a discussion of PA and how it is measured please again refer to my previous post). Three measurements of the separation gave me 5.6, 5.8 and 5.0 arcseconds which gives an average of 5.5 +/- 0.4 arcseconds. Again three measurements of PA gave 288.4, 294.0 and 290.6 degrees which averages to 291 +/- 3 degrees.
This is probably the most interesting of the five double stars I photographed that night because this pairing is a proper binary system with an orbital period of 151.5 years. I have in my possession an old copy of the Webb Society's double star catalogue (The Webb Society Observer's Handbook, Vol. I Double Stars, London 1975) and this star is listed on pages 89 and 109 (STF 1888). The average separation and PA between 1964 and 1974 was 7.1 arcseconds and 342 degrees. So in the 50 odd years since those observations were made the system has changed quite a bit (as you would expect). In 2019 the separation and PA were 5.2 arcseconds and 298 degrees. My measurement of the separation is in good agreement but I think the PA has decreased even further since that date.
All text and images © Duncan Hale-Sutton 2024
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