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Sub-Arc Second Successes: Bu 63 (β 63) and Lambda (λ) Cygni

Excerpted from Stelladoppie (http://stelledoppie.goaction.it/index2.php?menu=39&iddoppia=91210).

Lambda Cygni separation and PA from 2000 to 2030 based on orbital data. Click for a better view.

Like many other parts of our planet, it’s not often the skies above me are graced with sub-arc second seeing.   Over the course of a year, I can count the number of times that happens on one hand. Even more to the point, some of those one-handed opportunities only last for an hour or so — which means I’m always watching for subtle signs of advantageous alterations in atmospheric conditions. When those happen, if I don’t grab the ring while it’s being dangled in front of my telescope lens, I find myself resorting to a very un-astronomical like vocabulary when the seeing returns to its normal wretched state.

Many moons ago, Neil English pointed me toward Lambda (λ) Cygni, a fourth magnitude star that lives on the Swan’s eastern wing. There’s more than one component listed for Lambda (λ) Cyg in the WDS, but the one Neil alerted me to is located just under an arc second from the primary.   The Stelladoppie chart at the right, which incorporates data from the WDS Ephemerides table, shows the AB pair with a separation of .0915” as of January 1st, 2014, with an imperceptible widening to .0916” by January 1st, 2015. Add the 1.53 magnitudes of difference between the two stars (4.73 and 6.26) and the result is a difficult pair to part.

Lambda Cygni   (OΣ 413)  (S 765)         HIP: 102589   SAO: 70505
RA: 20h 47.4m   Dec: 36° 29’

Identifier   Magnitudes  Separation  Position Angle     WDS
MCA 63      Aa, Ab:   5.40,   5.80       0.052″         61.4°     2011
STT 413           AB:   4.73,   6.26       0.915″          0.6°     2014
S 765              AC:   4.76,   9.65     82.90″        106°     2012
FYM 126          AF:   4.76, 14.00     52.00″        119°     2012
Distance: 769 Light Years (Simbad)
Spectral Classification: “A” is B5, “C” is K2
Lambda (λ) and Delta (δ) Cygni are shown in light blue, click to enlarge the chart.

Lambda (λ) and Delta (δ) Cygni (mentioned further down in this post) are shown in midnight blue, click to enlarge the chart.

As is frequently the case in such cases, I failed on my first few attempts to crack the pair, but on the fourth (or fifth or sixth or seventh) try, I finally glimpsed the secondary hovering like a miniscule cracker crumb at the north edge of the primary.   I managed that success with a 9.25 inch Celestron Edge SCT, but later I parted the pair with my six inch f/10 refractor, and after that I matched Neil’s prowess with my five inch f/15 D&G refractor (he cracked the pair with his five inch f/12 Istar refractor).

Many, many, many moons later I found myself confronted with another difficult pair of almost attached stars while I was bush-whacking my way through Delphinus. Flamsteed graced the star with a “1”, as in 1 Delphini, but what made my hair stand on end and caused my curiosity quotient to quiver was its double star designation, Bu 63 (or β 63). I knew if the eagle-eyed Sherburne Wesley Burnham had been here, I was in for a visual tussle. And when I dug out the data, that’s precisely what I discovered:

Bu 63  (1 Delphini)       HIP: 101160   SAO: 106172
RA: 20h 30.30m   Dec: +10° 54’

Identifier   Magnitudes   Separation   Position Angle    WDS
Bu   63   AB:   6.20,   8.02        0.90″          350°    2009
Bu 297   AC:   6.20, 14.40      16.90″          340°    2006
Distance: 743 Light Years  (Simbad)
Spectral Classification: A1e
Bu 63 is located just under a degree southwest of Epsilon (ε) Delphini.  Click to enlarge the chart.

Bu 63 is located just under a degree southwest of Epsilon (ε) Delphini. Click to enlarge the chart.

Well aware I would be dancing with the difficult, I cranked up my courage and with a salute to the sky gods and a humble bow to the memory of the beloved S.W., I allowed myself to be lured to the edge of the proverbial stellar ledge, creeping forward carefully with my six inch f/10 on two separate nights of poor seeing. Each time I thought I could detect the secondary at the north edge of the primary, but the image refused to hold still for long enough to call it a solid sighting.

Several nights later I was roaming the skies with the 9.25 inch SCT in search of other targets, but Bu 63 was bouncing around in the back of my mind. After about an hour, I noticed fog was approaching from the southeast, so I thought I had better give Sherburne Wesley’s 63rd discovery another try before the fog obscured it, even though the seeing was a bit below average. I parked a 6mm AT Plössl (408x) in the scope’s diagonal, focused pain-stakingly-carefully, and very much to my astounded surprise, I could detect the secondary immediately, even though the image was hopping around badly. I had the impression the brighter sky background caused by the fog was actually providing some assistance.

The fog receded and stayed away for the next hour, granting me an unexpected wealth of time to study Bu 63.   The view in the short eye relief Plössl quickly annoyed my observing eye, so I replaced it with a 6mm TMB Planetary II with longer eye relief to appease my visual apparatus.  I also tried a 5mm UO Ortho (490x) and a 4mm TMB Planetary II (613x), but the seeing conditions refused to support the magnification levels.

So I settled in with the 6mm TMB PL II to watch an unsteady 408x show for about half an hour, and was pleasantly surprised by what I could see during interrupted intervals of frantic hopping. At times the image was almost un-focusable and at other times, for quark length fractions of a second, it was crystal clear – and provided I was paying rapt attention, it was quite easy to steal a glimpse of the secondary perched on the top edge of the diffraction ring.

During periods when the image was living somewhere between a focused and unfocused state, the secondary was just a bit more than a bright spot on the ring, but during the brief moments of crystal clear seeing, it appeared as a weak dot of white light just beyond the reach of the primary. During those moments the segment of the diffraction ring between the secondary and primary seemed to vanish from view, which was probably an illusion caused by the sudden sharpness of the image.  Given the erratic seeing conditions I had to wrestle with, I was amazed at the way the secondary revealed itself provided I sat still and waited patiently.

But still . . . . . . and yet . . . . . . . . . . and somehow . . . . . . . . . . . . I wanted something more substantial.   There was no doubt I was seeing the secondary, but what I really wanted was an image that would hold still for long enough to distinguish the secondary as an actual star, not as a nervous dot of specterous white light fading in and out of view. What I wanted was the equivalent of a sharply focused mental photograph.

** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **

Bu 63 haunted me for the next month.   Every time I was patrolling the skies with either my six inch f/10 or the 9.25 inch SCT, I put it under a high-powered eyepiece to see what would happen.   But the seeing was just never quite up to it, so I gave up and went on to other things.   Then one night in late August, I was checking the seeing by zeroing in on Delta (δ) Cygni, which always provides me with a good idea of what possibilities the night holds. It split easily, so I switched over to the east wing of Cygnus to look in on Lambda (λ) Cyg again, and once I found the right magnification . . . . . . . . . success was mine!

Pure unfiltered beauty! A sight that makes any visual observer’s pulse pulsate several pulses faster. The 9.65 magnitude “C” component can be see almost due east of the primary. (East & west reversed to match the SCT view, click on the sketch to make your heart race).

Pure unfiltered beauty! A sight that makes any visual observer’s pulse pulsate several pulses faster. The 9.65 magnitude “C” component can be seen almost due east of the primary. (East & west reversed to match the SCT view, click on the sketch to make your heart race).

And then a dancing image of Bu 63 and its twitching secondary rose like a Phoenix and draped itself over my view of Lambda (λ) Cyg.

Well, why not?  The two would make an interesting comparison.  Both have virtually the same separation between primary and secondary, but Bu 63 possesses a wider magnitude difference (1.82 for Bu 63, 1.53 for Delta Cyg) and fainter stellar magnitudes (6.20 and 8.02 for Bu 63, 4.73 and 6.26 for Delta Cyg).   I was getting badly ahead of myself, though.  Assuming Bu 63’s secondary would be there waiting for me was subject to one major danger:  we all know what happens when you assume.  So I switched quickly to pre-sume instead.

And it worked. I even got that sharp mental snapshot I had lusted after for over a month:

I really half expected to see “S.W. was here” carved on the side of the secondary, but maybe I just wasn’t using enough magnification. (East & west reversed again, click on the sketch to get the sharp mental snapshot effect).

I really half expected to see “S.W. was here” carved on the side of the secondary, but maybe I just wasn’t using enough magnification. (East & west reversed again, click on the sketch to get the sharp mental snapshot effect).

Click to enlarge the image.

Click to enlarge the image.

Of course, S.W. Burnham was here, as you can surmise from the two identifiers he left imprinted on 1 Delphini, Bu 63 and Bu 297. In fact, according to his notes (which are shown at the right), he discovered the secondary of Bu 63 with his six inch f/15 Clark refractor – he was, after all, the owner of a pair of eyes which would have been the envy of any eagle. You can see his first estimate of PA and separation there, dated 1874.70, followed quickly by another PA and separation dated 1874.92 (source). Those measures were made for Burnham by Baron Dembowski (identified by Δ, the Greek symbol for capital “D”), who provided the measures for Burnham’s discoveries during the years he was using the six inch refractor.

To get back to the comparison between our two stars, there was no doubt whatever about the prognostication of the data: Bu 63 was every bit more difficult as the numbers said it would be. I could actually detect Lambda Cyg’s secondary as a bump on the primary at 136x (18mm Radian) in the 9.25 inch SCT, which wasn’t quite the case for Bu 63, although I did see some indication of an irregularity at the correct spot on the primary’s edge. I didn’t need to increase the magnification all that much to pry Lambda “B” loose – a jump to 175x with a 14mm Radian was enough.

But Bu 63 was a few orders of magnitude (no pun intended) different. Because of their fainter magnitudes, the apparent diameter of the Bu 63 pair is smaller than that of the Lambda Cyg pair – not by a huge amount, but definitely enough to raise the level of difficulty, and definitely very obvious when you view the two stars back to back. The upshot is Bu 63’s secondary appears closer to the primary than Lambda Cygni’s, which is an optical illusion caused by the smaller apparent diameters of the stars.   The comparison below, which gets darn close to replicating the relative sizes and brightness levels of our two tight-knit pairs, provides a good indication of how much visual difference there is between the two stars.

Even though Bu 63 is under higher magnification, the apparent diameters of its two components is noticeably smaller than those of Lambda Cygni. Be prepared to experience genuine eye strain.

Even though Bu 63 is under higher magnification, the apparent diameters of its two components is noticeably smaller than those of Lambda Cygni. Be prepared to experience genuine eye strain.  Click for a slightly better view.

So give this a try some night when both stars are well placed in the sky and the atmosphere over your head is in an agreeable mood.   I highly recommend starting with Delta Cygni first to test the seeing, and then going on to Lambda Cygni if conditions warrant.  If you don’t succeed with Delta Cygni, it may be time to go hunting for galaxies instead . . . . . or to go find a good book.

Clear Skies!   😎

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3 Responses

  1. Many congratulations John on that tough split!

  2. Hello John,

    Just read your report on Lambda Cygni and BU63!

    Great write up! I was on the edge of my seat all the way through this one!

    I’m sure glad you got that rare evening when you can go sub-arc second. Lambda is just doable in my 5 inch but I’m not complaining; that’s about the limit of what I think that telescope can do under the best conditions.

    I’m afraid you’ve put me to shame with Bu63. I’ve not managed that one yet but your experience has inspired me to try again with my 7 inch Maksutov. Maybe its reduced altitude here is to blame? Southwestern Delphinus is fairly low down where I am whilst Lambda Cygni is very high up.

    Who knows?

    Thanks again.

    With best wishes,

    Neil.

    • Many thanks, Peter and Neil. I was thrilled to the tips of my focus fingers about getting Lambda Cygni the first time, but Bu 63 was not only more thrilling, but a totally unexpected surprise.

      You never know, though, until you try. It just requires being out there when the seeing conditions decide to cooperate.

      Prost! to each of you. It’s always a pleasure to get comments from across the pond!

      John

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