And then there’s always Sherburne Wesley Burnham to contend with. If you spend any time at all chasing down obscure double stars, you’ll run into his handiwork soon enough. He had a particular penchant for the difficult it seems.
Probably that’s because most of the easy pickings had already been gleaned from the heavens by the early observers of double stars — names likes Sir William Herschel and Sir John Herschel, father and son; Sir James South; Johann Heinrich von Mädler; and another father/son combination, Friedrich Georg Wilhem von Struve and Otto Wilhelm von Struve. But it didn’t hurt the cause in the least that Sherburne Wesley was also armed with some of the more exotic creations of Alvan Clark & Sons, including a particularly effective six inch f/15 refractor, with which he discovered over 1500 double stars — many, many of which are difficult even with larger instruments. A photo of that refractor can be found on the unnumbered page following p. xiv of this book.
As that dauntless surveyor of the skies over Scotland and dedicated historian of the 19th century refractor, Neil English, said somewhere, S.W. Burnham was “made of sterner stuff.”
We’re going to take a close look at two of Mr. Burnham’s discoveries, each of which shares the same enticing and exacting qualities so common to most of the stars that bear his Beta (β) prefix, and we’ll take them in ascending order of difficulty. Five inches of aperture is about the bare minimum needed for the first one, and I’m certain six is the minimum required for the second — unless of course you live in an exotic climate blessed with even more exotic seeing, such as the 7000 ft. slopes of Mauna Kea.
We’ll start with the easiest of the two, the innocently beguiling 6 Serpentis/Bu 32, and then we’ll move northwest a very short distance to see if we can succeed with it’s much more tempermental and higher-numbered relative, Bu 943.
First, you can see where in the sky we’re headed if you look at this chart of the Serpens Caput area from an earlier post. Allow your eyes to be drawn to the area around M5 in that wide view, and then return again to it in the zoomed-in version below:
Bu 32 (β 32) (6 Serpentis) HIP: 75119 SAO: 120955
RA: 15h 21.0m Dec: +00° 43′
Magnitudes: 5.5, 8.8
Position Angle: 22° (WDS 2008)
Distance: 244 Light Years
Spectral Classification: K2
Now if you look at the numbers in the data line above, that separation — 3.4 seconds of arc — really doesn’t look the least bit intimidating. Heck, that should roll over and surrender in a 60mm refractor. Right. But wrong, really — because the 3.3 magnitudes of difference between primary and secondary is just the right amount to make things diabolically difficult.
But I was prepared to do battle, and wasn’t about to go into this adventure at a disadvantage. Knowing that Burnham had pried this pair out of the sky with his six inch Clark, I knew I should at least start with my six inch f/10, even though it couldn’t boast of a an Alvin Clark lens. So I made do with a well-corrected Jaeger’s achromatic lens that favors the red side of the spectrum.
And I got the dim little devil, too!
Haas found Bu 32 a bit of a challenge as well, using slightly less than five inches of aperture, and described it rather effectively: “125mm, 200x: Difficult but striking binary. It’s a bright citrus-orange star almost touched by a tiny, shadowy vapor that takes great effort to see.”
Now 5 Serpentis was actually discovered to be a double in 1872, but I have no idea who deserves the credit for that. It was observed twice by Burnham, the first time when he discovered it independently with his six inch Clark in 1879. He shows measurements that year of a separation of 2.37″ and a position angle of 13.6 degrees. He returned to it in 1889 and recorded a slightly wider separation of 2.44″ and a significantly different PA of 17.9 degrees. S. W.’s compilation of data from the observations of others can be found on p. 142 of the book mentioned above (the one with the photo of the six inch refractor). That data shows the separation steadily decreasing as the PA changes slowly, from 3.11″ and 14.8 degrees in 1875 to 2.63″ and 14.5 degrees in 1898. But in looking at that data, I was struck by the fact that the measurements are all over the place, which Burnham noticed also, and commented on in his notes: “Relative change is not entirely certain, but it is a physical system, as the components have a common proper motion.”
Now we’re going to execute a relatively short move to the northwest to reach our second star, Bu 943. Using the inset at the right, you’ll see that it sits about half a degree to the west of a line drawn from 6 SER to 5 SER. At a combined magnitude of 6.5, you’ll have no problem finding it in your finder.
Bu 943 (β 943)
HIP: 74895 SAO: 120938
RA: 15h 18.4m Dec: +00° 56′
Magnitudes: 6.7, 10.9
Position Angle: 92° (WDS 1991)
Distance: 580 Light Years
Spectral Classification: K1
This little devil is beguiling also, but innocent it definitely is not. I spent the better part of two hours attempting to make friends with its fidgeting 10.9 magnitude secondary, but every time I thought I was about to get a welcoming handshake, it retreated into the glow of its primarial parent. Part of that was because the seeing was totally uncooperative, and part of it was a late Spring murk that is long overdue to move on for the season. Still, I suspect it of devious secondarial inclinations — but maybe it’s just shy.
I decided to make my approach to Bu 943 with an eight inch Celestron Edge SCT, which actually may have been more aperture than was suitable for the seeing conditions. A six inch refractor might have been a better choice, but in the back of my mind I had a hunch I might not see another clear night before the moon returned the following week for another circuit of the skies — so I wanted to make absolutely sure I could split this pair.
Now eight inches is a lot of aperture — at least for a humble Star Splitter like me who sticks to the 60mm to six inch range 99% of the time — which means you can frequently get by just fine with less magnification than you need in the lesser apertures. So I thought I would try my luck with a 24mm Brandon, which is a mere 83x. And that’s when my terrible travails took off.
There’s bad seeing, and then there’s BAD SEEING. The chart Greg and I use to convey seeing conditions just doesn’t work for what I experienced. When the seeing on that chart is a two (II), there are nights when I can still manage a view of a primary’s difficult companion. The image may be hopping and bouncing and leaping all over the eyepiece, but sometimes my eye can catch it in between leaps just long enough for my mind to grab a quick snapshot of it.
But then there are other nights where that erratic behavior becomes frantic — what I call high frequency hopping. And when that happens — and that’s precisely what happened on this particular night — all you can do is hope the Sky Gods will take pity on you and allow the atmosphere to calm down for a tenth of a second. I don’t know what causes that high frequency hopping, but it is very, very annoying, and for some reason it’s been fairly common for the past eight months.
So to return to the search, 83x didn’t work in the eight inch. It should under better conditions. 100x, in the form of a 20mm TV Plössl, didn’t either — and I looked long and hard and patiently for a good twenty minutes. Even though I doubted it would work given the lack of atmospheric cooperation, I tried a 15mm TV Plössl (133x) and then I gambled everything away with all 200 of the x’s provided by an old 10mm Celestron Plössl. But no luck. I had a two speed focuser attached to the SCT, and I gingerly tweaked the fine focus knob about five thousand times in either direction, hoping against hope for a glimpse of secondarial success at just the right moment — but again, no luck — none whatever. Nothing but incessant frantic high frequency persistent palpitations.
And then a brilliant idea struck me right square in the center of my Star Splitter hat — Go back to 6 Serpentis, get a precise focus on it, and then come back to Bu 943 and try again! Which is what I did, and when I looked at 6 Serpentis —- mind you, with eight inches of aperture ——- and remember, it fell right into my lap with no struggle whatever using a six inch f/10 refractor nine nights earlier —————- I could see why I was having problems. With all that high frequency hopping going on, its secondary was engaged in a less than charming disappearing dance also. But after a ten minute battle armed with an 18mm Radian (111x), I finally found a focus that would preserve that secondary for about a tenth of a second. So back to Bu 943 for another round.
Now that may have helped a bit — although not a whole heck of a whole lot — but it did seem to provide a slight Edge (no pun intended) I didn’t have before. So I looked and looked; and looked and looked; and looked and looked ——– really, I swear, I sat there and looked and stared and averted and searched for over thirty minutes. Several times I had very quick glimpses of a phantom-like wisp of light at the correct position. I’m reasonably sure that was it, because it looked like the same phantom every time …………………….. but I sure would have liked a longer glimpse of it.
Given the two hours of effort I put into it, I should have felt like an accomplished achiever. But I didn’t. I felt like I had been thrashed by an eight hundred pound gorilla.
After I packed every thing up that night, I just happened to take a look at the data on Bu 943 in Sky Safari — it shows the two stars are separated by 2.6 seconds of arc, using a date of 1991, which matches the date of the last WDS observation. I have no idea where the information for that measurement comes from since there’s no orbit computed for Bu 943 in the WDS, but considering how difficult it was to see the secondary, it could well be correct, even allowing for the sinister nature of the seeing.
On the other hand, I should have suspected something from looking at Burnham’s data (p.142 of the same book referred to above). He discovered this demon double star in 1879 — with an 18 1/2 inch Clark refractor. And that was four years after he had first looked at 6 Serpentis/Bu 32 with his six inch Clark. There he was in 1875, within half a degree — that’s thirty arc seconds! — of what was to become known as Bu 943, and he didn’t see it with the six inch. I can almost hear S.W.’s fiendish laughter echoing down through the corridors of the past 130 plus years.
So maybe I didn’t do so badly after all. Regardless, and whatever, I’ll be back when conditions are better. I seem to have inherited a sizeable dose of S.W’s particular penchant for the difficult.
Here’s to less difficult skies! 😎