Monday, December 7, 2015

OR: Sirius B 1/29/15

One of the first books I read last year when I began to observe seriously (pun intended), was Burnham’s Celestial Handbook. In it he describes Sirius and the history of the discovery of its companion, B, also known as “The Pup.” Between 1834 and 1844 F.W. Bessel found irregularities in Sirius’ motion through space, and calculated it must have a binary companion with a period of 50 years. But Sirius’ brightness overwhelms the view of its companion. Alvan G. Clark was the first to make a visual observation, with his 18.5” refractor – then the largest in the world – in January 1862. Burnham then describes his own rapt experiences observing magnitude 8.75 Sirius B using various instruments (“very definite at 12 inches, difficult at 9 inches, and detectible at 6 inches only because its exact position was known” p. 395). Burnham then goes off on one of his wonderful tangents to discuss the nature of Sirius B, which is a white dwarf, with “a mass nearly equal to that of the Sun” packed into a star only the size of the earth, whose “luminosity, however, is less than 1/400 that of the Sun” (p. 397). Given that I was becoming very fond of observing double stars, I added Sirius B to my “bucket list” of objects to observe.

This observation may not be the monumental achievement it once was. Sirius B is now on its way toward maximum separation. It is currently 10.32 arc seconds, with position angle 078.5. Maximum separation is in 2023 at 11.28 arc seconds, PA 062.7. Nevertheless, it still generates excitement. Within the last couple of months, several local amateur astronomers have posted on internet forums that they had a clear split, one with a 16” and another with an 11” aperture reflector. The Double Star Observing section on the Cloudy Nights Forums website has a pinned thread called “Da Pup is Busted,” where many have posted their recent successful sightings, including photographs and field sketches showing its position relative to nearby stars. Surely there are many more people who’ve made sightings but didn’t report them online. The most remarkable observation I’ve come across is that of Jay Reynolds Freeman, who split Sirius with a 6” f/8 refractor at Montebello Open Space Preserve in 2002, when the separation was a mere 5 arc seconds. The common requirement was very steady seeing conditions and generally high (250x or more) magnification. Aperture seems less of a determining factor, but the reports do follow Burnham’s aperture-to-visibility scale as above. Steady seeing helps reduce the size of the roiling mess that Sirius presents in the eyepiece, providing the opportunity to see the comparatively faint B star just in or outside of A’s diffraction pattern. Reading these reports gave me encouragement me that I might be able make the observation, but more importantly how I might.

I set my scope (12.5” f/7 reflector, 14.4% secondary central obstruction) in my Fremont, CA back yard – elevation 53’ – upon returning home from work, at set a fan behind the primary to help it cool and track temperature. After dark I began observing various double stars and noticed I could push magnification rather high, 277x or more, and I was beginning to see nice Airy Disks. I paused at 7:30pm to attend to my evening chores, returning to the scope at 9:00pm. I noticed that Sirius was hardly twinkling – a sign that the seeing had become superb – and thought to make a go of it.

I collimated my scope once more and put in a 9mm orthoscopic eyepiece (246x) which I had fitted with an occulting bar – a piece of black photographer’s tape across the lens inside the barrel – which I planned to use to block Sirius A’s light and which I carry around in by eyepiece bag for just such an occasion. Since Sirius B’s PA is now 078, a little north of due east of Sirius, I let Sirius drift west through the field with the occulting bar set to the west. I tried several times, but no luck! I didn’t see anything but the roiling mess. I then used a 2x barlow with the same eyepiece (492x), and still did not see it. I thought maybe this wasn’t the night. After all, there was a 73% waxing gibbous moon; SQM-L to the target was 17.52, and 18.22 at zenith. Transparency was probably average, and my scope was dripping dew.

Then I thought to use an intermediate magnification, and inserted a 5mm Nagler into the eyepiece into the focuser, 443x, without an occulting bar. Sirius’s diffraction was reduced versus the 9mm. I once again scanned the area east of the star. After three passes I noticed it: a faint yellow-orange disk, to the edge of Sirius A’s diffraction and just below my spider’s diffraction spike. I let Sirius pass beyond the field stop, and for an instant there was Sirius B, clearly separated, before it too passed beyond the field stop. I did it! I kept looking at it for another 15 minutes, hardly able to contain my joy.

One of the tips I had read in other observing reports is, if one is using a reflector, to rotate the tube assembly so the spider diffraction spike is not due east, as it would interfere with the observation. I have no means to rotate my tube, so I decided to re-observe in a couple hours, when the diurnal motion of the sky would have rotated the field for me. I observed a few more doubles in the meantime. Sigma Orionis was a special treat. I came back to Sirius at 10:30pm, and sure enough B was halfway between my two spider diffraction spikes, plain as day. I kept observing it for another hour; I wanted to hold on to the magical feeling.

And so here I am this morning, not reading my work emails but typing up this report, elated. It was great. I hope to do it again.

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