Xi Ursae Majoris...

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RMSteele
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Xi Ursae Majoris...

Post by RMSteele »

...is a double star in the hind feet of the Great Bear. Discovered by William Herschel, it was the first double to have its orbit properly calculated in the early 19th century. The period is about 60 years and the separation varies between just less than 1 arcsec to around 3 arcsec. The system is actually multiple and both visual components are spectroscopic binaries; and there is a fifth Brown Dwarf component some distance from the main pairing.
I had a look at Xi this evening at 22.05 UT, using my 90mm f10 achromatic. The seeing was around Pickering 5 with fair transparency. Using an 8mm TV plossl (x114) I could see straightaway it was double, although not cleanly separated at this magnification. Using a x2 TV barlow with the plossl (x227) the pair were clearly defined and distinctly separated, with a sliver of darkness between them. I estimate the separation as 2 arcsec in PA 170 by eye. There is no real colour to the pair - both are "off white" (although some see them as yellowish), a neat pair of seed pearls on velvet. Burnham's gives the visual magnitudes as 4.3 and 4.8 but subjectively the brightness difference seems less than that, as is often the case with close pairs of similar stars.
Kind Regards, Bob
Last edited by RMSteele on Tue May 08, 2018 6:27 pm, edited 2 times in total.
brian livesey
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Re: Xi Ursae Majoris...

Post by brian livesey »

Your interesting observation Bob shows that when it comes to magnification, we can far exceed the theoretical limit of X50 per inch of aperture when it comes to observing point sources, i.e. stars.
The limit applies to extended objects, where any surface details above X50 per inch of aperture would be magnified but without showing any extra details, only an increasingly grainier view.
W. Herschel's rule was not to go for the highest magnification on extended objects, but the one that shows the most detail. As for splitting very close doubles, Herschel would resort to fantastic magnifications.
brian
RMSteele
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Joined: Thu Jun 02, 2016 11:32 am
Location: New Farnley, Leeds lat 53.8N long 1.6W
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Re: Xi Ursae Majoris...

Post by RMSteele »

Yes indeed Brian, I'm thinking of getting a x3 barlow for boosting the magnification to x340 for fine nights with double stars. It would require more attention following with manual slow motions and for focussing of course, but no reason why not.
Kind Regards, Bob
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