257 replies to this topic

[Voyager 3]

Sub arc second double stars are the most interesting types for me ... Good seeing and good aperture are the requirements here . I would love to read reports of your sub arc second doubles here . Sketches are most welcome . 

[Astrojensen]

A couple days ago, around midnight, March 4th/5th, I managed to elongate Alpha Comae in my APM 152 ED, at magnifications 255x (4.7mm ES82) and 510x (4.7mm ES82 + 2x barlow). Its current separation is around 0.45". The elongation was immediately noted at 255x and quite noticeable at 510x. The seeing was very good, but not exceptionally so. 

 

 

Clear skies!

Thomas, Denmark

[Voyager 3]

Alpha Comae Berenices ( Diadem from sky safari ) is a cool double ! Its orbit is few tenths of a degree from being a perfect edge on orbit . So at the closest seperation it's nearly unsplittable ! Thank you Thomas for bringing this double into my small knowledge of stars .

I do have a small doubt - how in world did you manage to split 0.45" double with a 6" frac ? I think the Dawes limit for a 8" is 0.58" ... Pls crct me if I'm wrong and what's the formula ? 

Edited by Voyager 3, 07 March 2021 - 06:16 AM.

[Astrojensen]

Alpha Comae Berenices ( Diadem from sky safari ) is a cool double ! Its orbit is few tenths of a degree from being a perfect edge on orbit . So at the closest seperation it's nearly unsplittable ! Thank you Thomas for bringing this double into my small knowledge of stars .

I do have a small doubt - how in world did you manage to split 0.45" double with a 6" frac ? I think the Rayleigh criterion for a 8" is 0.58" ... Pls crct me if I'm wrong and what's the formula ? 

I certainly didn't split it, just elongate it. Think egg-shaped, instead of round. 

 

 

Clear skies!

Thomas, Denmark

[Voyager 3]

I certainly didn't split it, just elongate it. Think egg-shaped, instead of round. 

 

 

Clear skies!

Thomas, Denmark

Is there a limit after which we can't even elongate it ( notched pair according to Knisely ) ?

Edited by Voyager 3, 07 March 2021 - 08:19 AM.

[Jon Isaacs]

I have split a number of sub-arc second doubles, I think all of them with Newtonians.  Two that come to mind because of their difficulty:

 

- STF 2215.  It's a magnitude 6.0-6.9 double listed at 0.44" by StelleDoppie.  I have split it a few times, always with the 13.1 inch F/5.5  The last time, I remember using the 4.8 mm Nagler with the Paracorr and a 2X Barlow for 875x. I catch it when it is high in western sky, 70-80 degrees and hope the seeing is cooperating. 

 

https://www.stelledo...?iddoppia=71332

 

- Zeta Bootes:  I was able to split zeta Bootes when it was ~0.5" with my 10 inch.  I managed it several times, one is etched in my memory, perfect seeing, 820x, I watched it time and time again as it drifted across the field, the pair separated and a single diffraction ring surrounding both stars.  I split it with both the 13.1 inch and the 16 inch when it was somewhat closer, somewhere around 0.4", it's was closing quite rapidly.

 

- One in Cygnus that is now an early morning double is Lambda Cygni,  0.91" at mag 4.6-6.3.  Not far from there is STT-410, another 0.9" at mag 6.7... 

 

When the seeing is good, I search SkySafari for close doubles in a well positioned constellation, I verify using Sky Tools 3 and StelleDoppie.

 

My records are not too good when it comes to searching for older observations so I mostly depend on memory.

 

Jon

[JuergenB]

A very long time ago, according to my notes of August 7, 1976, I have split the binary ADS 14238 = BU 64 in Delphinus, 8.7/9.0 mag, with my venerable C8 at 250x. In a book by W. Wepner about binary ephemeris, published in 1976, it was written that the separation should have been 0.54" at the time of observation, but I recalculated it today with the Excel spreadheet "Binaries_6th_Excel97" by Brian Workman, based on the SIXTH CATALOG OF ORBITS OF VISUAL BINARY STARS by  Hartkopf and Mason (2002), which indicated a separation of 0.63" in 1976. Not sure what it really was because of different orbit data. 

 

I found a huge mismatch between this source and Stelledoppie. The sixth catalog gives a revolution period of 403 years, whereas Stelledoppie states 2559.04 years. Thus, the present separation could be either 0.33" , PA 4.11°, or 0.64", PA 357.6°.

 

Thus, it turns to be out that it will be a very interesting object for the upcoming observing season. I will also dig deeper into the literature in order to understand the differences between the orbital elements.

 

Juergen

[Astrojensen]

Is there a limit after which we can't even elongate it ( notched pair according to Knisely ) ?

"Notched" is a much more obvious detection than elongation, according to David Knisely's illustration. 

 

My personal limit for detecting elongation is somewhat better than 0.5x Dawes limit for equal or slightly unequal doubles. It strongly depends on the brightness of the stars and very quickly gets much worse (or impossible), when the pair gets increasingly uneven. The brighter the stars, the more you can magnify and the easier you can detect a slight elongation. The 152mm APM collects much more light than the other, smaller, scopes I've used the technique on previously, so I suspect I can actually get down to perhaps 0.3x Dawes under ideal circumstances. I've suspected some extremely tight pairs in the 0.2" - 0.3" range, but haven't been able to confirm them on better nights. On a night of good seeing, the merged airy disks of a 0.5" pair of fairly bright stars, say 6m0 or brighter, is very obviously not round in my APM 152 ED. Single stars are round like buttons, with a single, sharp diffraction ring. 

 

Here's a quick illustration, made in Autocad, that shows what I mean. The separation is in fractions of Dawes limit. As you can see, a 0.5x elongation is fairly obvious, if you can see the airy disk(s) sharply. The 0.3x is much more difficult, especially if there are no equally bright stars to compare with. The tiny notch in the elongated airy disk is NOT visible in real life. 

 

 

Many years ago, I used a method described in Paul Coteau's book on double stars, to make artificial double stars, by letting two bright lamps shine on a small, polished steel ball. I used this method to practice observing extremely small elongations in perfect indoor seeing. It's quite another matter to do it in reality outside, but the practice was invaluable. 

 

Paul Coteau described the technique in some detail, including how to use it to estimate the separation. I once made a chart, which the varying separations down to 0. It was quite useful, occasionally. Just for fun, I made a new one just now:

 

 

As you can see, elongations below 0.3x are getting inhumanly difficult and are not possible under real-world conditions, but 0.5x is quite obvious.  

 

 

Clear skies!

Thomas, Denmark

[Spikey131]

Sky Safari has Omega Leonis separated by 0.9 arc seconds.

 

I was able to split it tonight with a 12.5” f/7 dob and 6mm Ethos at 370x.  Seeing was average.

[fred1871]

A very long time ago, according to my notes of August 7, 1976, I have split the binary ADS 14238 = BU 64 in Delphinus, 8.7/9.0 mag, with my venerable C8 at 250x. In a book by W. Wepner about binary ephemeris, published in 1976, it was written that the separation should have been 0.54" at the time of observation, but I recalculated it today with the Excel spreadheet "Binaries_6th_Excel97" by Brian Workman, based on the SIXTH CATALOG OF ORBITS OF VISUAL BINARY STARS by  Hartkopf and Mason (2002), which indicated a separation of 0.63" in 1976. Not sure what it really was because of different orbit data. 

 

I found a huge mismatch between this source and Stelledoppie. The sixth catalog gives a revolution period of 403 years, whereas Stelledoppie states 2559.04 years. Thus, the present separation could be either 0.33" , PA 4.11°, or 0.64", PA 357.6°.

 

Thus, it turns to be out that it will be a very interesting object for the upcoming observing season. I will also dig deeper into the literature in order to understand the differences between the orbital elements.

 

Juergen

Juergen, you've found two sets of orbital elements that are greatly different. I looked up a recent version of the 6th Orbit Catalogue (which is updateded with new and revised orbits over time), and it had the 2007 orbit calculation of 2500+ years.

 

Looking at the resulting orbit diagram, which records the measures over time, it's evident that only a small part of the orbit has been measured thus far - a gently curving arc followed by a more significantly curved short arc in the period of speckle measures (the last near-40 years). The earliest speckle measures (1983) are around 0.63" separation. There's not much increase by 2014 at 0.68". The PA changes about 10 degrees over that period of 31 years.

 

The ephemeris you get from the different orbit calculations are unsurprisingly very different, as one orbit period is six times the years of the other. The longer period looks more plausible at the moment, but the next 50-100 years will, according to what the orbital path looks like, reduce the range of possible parameters - in other words will give us a better idea of the approximate form of the true orbit. This one is not going to change rapidly. Faster change was in a past period, some time after discovery, when the separation was near minimum.

 

Here's the 6th Orbit Catalogue orbit diagram.

Edited by fred1871, 08 March 2021 - 01:13 AM.

[R Botero]

Sky Safari has Omega Leonis separated by 0.9 arc seconds.

 

I was able to split it tonight with a 12.5” f/7 dob and 6mm Ethos at 370x.  Seeing was average.

I had excellent seeing last Friday, March 5th. I was out for a couple of hours looking at doubles in Cancer, Lynx, UMa, Hydra (only Epsilon) and Leo.  Amongst them,

 

I split Omega Leonis, my notes were:  "Almost kissing discs.  Both white.  No DRs in lower altitude.  PA110-120."  According to Stelle Doppie this one is at 5.7/7.3, PA114 and separation 0.92.

 

Earlier that evening I had seen elongation in STF3121 - IP Cancri.  My notes were:  "Split from time to time but mostly rod-like, elongated.  PA45."  According to SD this one is at 7.9/8, PA34 and separation 0.52.

 

And lastly, I think I imagined  some elongation in 12 UMa (Talitha Australis).  My notes were "Perhaps some elongation EW."

 

All with my 10" f/20 Mak at between 250-350X.

 

Roberto

Edited by R Botero, 08 March 2021 - 04:55 AM.

[Astrojensen]

And lastly, I think I imagined  some elongation in 12 UMa (Talitha Australis).  My notes were "Perhaps some elongation EW."

Now that one looks interesting. I need to check it out. 

 

And the elongation is right around 0.5x Dawes for a 250mm, so it should be quite doable in your Mak. If the seeing supports it, of course.

 

 

Clear skies!

Thomas, Denmark

[JuergenB]

Juergen, you've found two sets of orbital elements that are greatly different. I looked up a recent version of the 6th Orbit Catalogue (which is updateded with new and revised orbits over time), and it had the 2007 orbit calculation of 2500+ years.

 

Looking at the resulting orbit diagram, which records the measures over time, it's evident that only a small part of the orbit has been measured thus far - a gently curving arc followed by a more significantly curved short arc in the period of speckle measures (the last near-40 years). The earliest speckle measures (1983) are around 0.63" separation. There's not much increase by 2014 at 0.68". The PA changes about 10 degrees over that period of 31 years.

 

The ephemeris you get from the different orbit calculations are unsurprisingly very different, as one orbit period is six times the years of the other. The longer period looks more plausible at the moment, but the next 50-100 years will, according to what the orbital path looks like, reduce the range of possible parameters - in other words will give us a better idea of the approximate form of the true orbit. This one is not going to change rapidly. Faster change was in a past period, some time after discovery, when the separation was near minimum.

 

Here's the 6th Orbit Catalogue orbit diagram.

BU 64 wds20450+1244a.png

Fred, thank you very much for your feedback, it is very much of help. It seems that I still have an earlier version of the 6th Orbit Catalogue. I just checked the Excel file and it is written that the content has been compiled in January, 2003. I have to look for an updated version.

 

Best regards

Juergen

[fred1871]

Thomas, there's a problem with your diagram in #8 above, that shows the double circles gradually merging, as it goes from Dawes to zero. In the real world, that's not how the telescope images work, nor how they appear to the eye. Couteau points out in his book that the form of the image with increasingly close equal pairs does not follow such a pattern. Rather, the appearance is, when going from separated (1.0) to 0.5 of that - where separated approximates Rayleigh - in sequence, separated, tangential, figure-eight, flattened eight, narrow rod, rod, olive, slightly oval. The narrow rod appears at 0.80; the last eight (flattened) is at 0.85. That last can be read as visibly notched.

 

In the table he gives, his list of image elongation, which can be compared to the separate "elongation of 0.5 isophote", is interesting, because of their difference at various points of closeness. At 0.85 (Rayleigh) image elongation is 1.425, the 0.5 isophote elongation 2.0 - so it looks more extended than it is. But by 0.50 ®, image elongation at 1.25 is nearly the same as 0.5 isophote elongation at 1.26.

 

Couteau's descriptions of appearance are similar to those of Christopher Taylor in his article in Bob Argyle's Observing and Measuring Visual Double Stars book. Taylor's descriptions of the appearance of star disks relative to separation with his 12.5-inch reflector are listed with the separations at time of observation for various binaries with well known orbits. Taylor gets notching down to 0.75 R, but the pattern is similar - tangent, figure 8, elongated single image (rod), oval disk (olive), slightly oval disk, less oval disk, down to 0.4 R rather than 0.5 R.

 

Part of the difference in details may be due to Couteau using a refractor, Taylor a reflector, where the diffraction pattern is changed by the secondary mirror. Another difference is that Couteau based his details on using the Nice Observatory 20-inch, Taylor a 12.5-inch, with the larger aperture more affected by seeing conditions which may affect the transition from overlap to extended single disk. Treanor, in his 1946 paper, divided his evaluation of resolution capability at the 15-inch mark, with larger scopes less capable relative to aperture than smaller ones. One could wonder if notching may disappear sooner (fraction of Rayleigh) with the larger aperture? as well as earlier relative extinction of resolution limit.

 

I've not done a systematic study of my own observing results on doubles that were less than fully resolved with a variety of telescopes, might get to it one day from many years of notes; I can say that the Couteau/Taylor progression is a good fit for those of my observations I've reviewed in recent times.

 

Not to labour the point any longer, the main point is the changing appearance - quite different from simple disc merging in a linear fashion - which makes a diagram, based on the disks merely becoming more and more overlapped, not a good representation of what one sees through the telescope, especially at the tighter end.

[Astrojensen]

Thomas, there's a problem with your diagram in #8 above, that shows the double circles gradually merging, as it goes from Dawes to zero. In the real world, that's not how the telescope images work, nor how they appear to the eye. Couteau points out in his book that the form of the image with increasingly close equal pairs does not follow such a pattern. Rather, the appearance is, when going from separated (1.0) to 0.5 of that - where separated approximates Rayleigh - in sequence, separated, tangential, figure-eight, flattened eight, narrow rod, rod, olive, slightly oval. The narrow rod appears at 0.80; the last eight (flattened) is at 0.85. That last can be read as visibly notched.

 

In the table he gives, his list of image elongation, which can be compared to the separate "elongation of 0.5 isophote", is interesting, because of their difference at various points of closeness. At 0.85 (Rayleigh) image elongation is 1.425, the 0.5 isophote elongation 2.0 - so it looks more extended than it is. But by 0.50 ®, image elongation at 1.25 is nearly the same as 0.5 isophote elongation at 1.26.

 

Couteau's descriptions of appearance are similar to those of Christopher Taylor in his article in Bob Argyle's Observing and Measuring Visual Double Stars book. Taylor's descriptions of the appearance of star disks relative to separation with his 12.5-inch reflector are listed with the separations at time of observation for various binaries with well known orbits. Taylor gets notching down to 0.75 R, but the pattern is similar - tangent, figure 8, elongated single image (rod), oval disk (olive), slightly oval disk, less oval disk, down to 0.4 R rather than 0.5 R.

 

Part of the difference in details may be due to Couteau using a refractor, Taylor a reflector, where the diffraction pattern is changed by the secondary mirror. Another difference is that Couteau based his details on using the Nice Observatory 20-inch, Taylor a 12.5-inch, with the larger aperture more affected by seeing conditions which may affect the transition from overlap to extended single disk. Treanor, in his 1946 paper, divided his evaluation of resolution capability at the 15-inch mark, with larger scopes less capable relative to aperture than smaller ones. One could wonder if notching may disappear sooner (fraction of Rayleigh) with the larger aperture? as well as earlier relative extinction of resolution limit.

 

I've not done a systematic study of my own observing results on doubles that were less than fully resolved with a variety of telescopes, might get to it one day from many years of notes; I can say that the Couteau/Taylor progression is a good fit for those of my observations I've reviewed in recent times.

 

Not to labour the point any longer, the main point is the changing appearance - quite different from simple disc merging in a linear fashion - which makes a diagram, based on the disks merely becoming more and more overlapped, not a good representation of what one sees through the telescope, especially at the tighter end.

I did note in my post that the tiny notch would not be visible in real life. I should probably have spent a little more time on the illustration and smoothened the notch, to give a more realistic impression. I can't model the gradual limb darkening of the spurious disk(s), which of course has a huge impact on their appearances in the eyepiece.  

 

Other than that, the elongations I've observed actually agree quite well with the illustration. Elongations below 0.3x are basically impossible, while 0.5x starts to become readily noticeable, if the seeing is very good and the pair of even magnitudes. 0.7x is very obvious, especially if one has a nearby single star to compare with. I recently observed Zeta Cancri in my 63mm Zeiss and AB was a clearly visible little rod. At 1.2" it's right at 0.7x Dawes in a 63mm. 

 

 

Clear skies!

Thomas, Denmark 

[C.Hay]

A couple of days ago I had a look at STT 28AB (currently 0.85"; 7m6/8m8) with a 180mm Dall-Kirkham reflector at 200x. Seeing was mediocre, but the companion popped out in good moments. With such pairs I prefer not to know the PA beforehand. I estimated it at 295° and was pleased to find post-observation that Stelledoppie gives 283°, which I view as sufficient confirmation.

 

The nice thing about STT 28AB is that at a declination of 81 degrees north, in the northern reaches of Cepheus and only 8 degrees away from Polaris, it is readily viewable year-round from here in Europe - and from the OP's India. Spectral class suggests there may be some yellow colour involved, but that would require more than my 180mm of aperture.

 

CS, Christopher

Edited by C.Hay, 11 March 2021 - 06:47 PM.

[Voyager 3]

The nice thing about STT 28AB is that at a declination of 81 degrees north, in the northern reaches of Cepheus and only 8 degrees away from Polaris, it is readily viewable year-round from here in Europe - and from the OP's India. 

 

Well that's going to be a rooftopper ! 

[fred1871]

Thomas, I don't know how the attached images (from Couteau's book) can be made to fit within the diagram series you presented. I've left the description with them, essentially here we're looking at different percentages of the Rayleigh Criterion as separations, with equal brightness stars.

[Astrojensen]

Thomas, I don't know how the attached images (from Couteau's book) can be made to fit within the diagram series you presented. I've left the description with them, essentially here we're looking at different percentages of the Rayleigh Criterion as separations, with equal brightness stars.Couteau subRayleigh Image.jpg

I don't know, but the elongations I've seen in real life actually match my diagram quite accurately, minus the notch in the dots in the diagram, just as Coteau points out. I did mention in my post, that the notches weren't visible in real life and it should just be used as a guide. Couteau mentions that they disappear right around 0.8x, though he uses Rayleigh, and I am basing it on Dawes, and that seems fairly accurate. I've seen quite a number of elongations around 0.7x and they show no notches, just an elongated rod, or, in the case of somewhat unequal doubles, an egg-shape. Zeta Cnc AB in a 63mm comes to mind. 

 

A lot depends on the brightness of the stars. Fainter stars have smaller spurious disks, but they very quickly get faint enough that the resolution of the eye worsen dramatically and makes them harder to split and the elongation more difficult to see. The "Goldilocks' zone" is very narrow, pun not intended. 

 

 

Clear skies!

Thomas, Denmark

[rugby]

During the nights 13 and 14 of March I was blessed with clear and stable air.  I tackled Struve 1126 a sub arc-second pair close following Procyon with data as follows: 6.5 -6.9; 0.8; 178.  I used a 152 mm ES 6-inch refractor sitting on a DM6 with Planet Berlebach tripod. The whole ensemble was quite chilled in -4 C air.

 

At 50x I could detect elongation. A 3mm Delite at 400x revealed a small notch.

 

Be sure to catch this one before it leaves the evening sky. The pair is easy to find and bright enough that apertures less than 6-inches might show a bar shape.

[payner]

Very good observation on a sub-arcsecond double. I'll be looking for this one. Thanks.

 

Randy

[Adam Long]

Thanks for the reminder Rugby - STF 1126 was the last pair I visited on the night of the 25th Jan. We had very good seeing here - unusual for winter and very helpful as even at culmination this pair is a little low for high mag views at 52 deg N. Not been anything like as good in the weeks since.

 

I was able to see a hairline split with my 10" dob and Meade 5.5mm UWA delivering 218x.

 

This pair is featured in Bob Argyle's recent Anthology.

[Spikey131]

Good seeing tonight the northeast, so I did some double observing under the first quarter moon.

 

Checked out STF 1126 and was able to see separation at 333x with a C8 and Ethos 6mm.  A nice challenge!

 

I was working through Burnham's double star list by constellation.  In Taurus tonight, I came across STF520, a 7.5/8.5 magnitude pair with 0.7" of separation a little west of Mars.  With the C8 and Nagler zoom at 400x I was able to split this snug pair.

[Uwe Pilz]

I often observe sub arc second stars in my 4 inch refractor. The key is that you use a magnification that you may see the Airy disc clearly. If your instrument is well collimated every kind of deviation form a circle is visible.

After some training I find 0"7 arcsec stars of equal brightness not too hard.

[Uwe Pilz]

Here an example from my observation the day before yesterday: O∑ 517 in Orion: 6m8, 7m0. 0"7. Seen wit 4 inch refractor at 200x. It was not easy.

Attached Thumbnails

•