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Has Anyone Split Theta Coronae Borealis?

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#1 JimOfOakCreek

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Posted 11 July 2021 - 04:35 PM

This star is part of a double star system. Its magitude +6.3 secondary component appears 0.8 arcseconds away. Is this double star splittable with an 8” SCT? The theoretical limit of resolution of an 8” SCT is 0.68”.

 

I’m going to give it a try first clear night.


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#2 VanJan

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Posted 11 July 2021 - 10:18 PM

With 20 cm reflector on 17 June 2021: During moments of steadier seeing, B star tentatively revealed itself. White, pale blue. 313X

 

I'd give this observation a fifty-fifty chance of being a false positive, however. undecided.gif

 

You've set yourself a difficult double to resolve with your telescope, I'm guessing. Good luck and steady skies! fingerscrossed.gif


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#3 John Fitzgerald

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Posted 11 July 2021 - 11:09 PM

It is likely very difficult,  being discovered fairly recently,  in 1971.  It was at 0.5 seconds separation then, and has widened to 0.8 seconds. Delta mag is greater than two, 4.27 and  6.29.


Edited by John Fitzgerald, 11 July 2021 - 11:11 PM.

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#4 R Botero

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Posted 12 July 2021 - 03:02 AM

I last split this one in June this year. It’s always been a very close one and for my optics, sits on or within the first DR of the primary. My last observation notes of 15 June were: “ Definitely sitting just outside DR. Defined in moments of better seeing. PA200.” My notes of September 2013 were not very different…” Suspect double? More difficult than Eta CrB - why? PA 200 with secondary within first diffraction ring.”
The above two under mediocre English seeing conditions with a 10” f/20 Mak at between 250-300X.
Roberto

Edited by R Botero, 12 July 2021 - 03:04 AM.

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#5 Adam Long

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Posted 12 July 2021 - 06:31 AM

Cou 610 was one of my favourite observations last summer, full report here: https://www.cloudyni...tes/?p=10285338

 

In the 10" dob I first saw the companion at 436x, but on a subsequent night I was just able to see it at 300x. With the 14" dob I've visited this a few times this year and have been able to make out the secondary ok at 427x, but haven't been able to replicate the compelling view I enjoyed last year.

 

I find the primary easy to recognise due to the colour - a cold white with a bluish tint that can appear greenish.


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#6 Astrojensen

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Posted 15 July 2021 - 02:33 AM

I had a night of absolutely superb seeing on July 13th-14th. I was able to easily see the companion of Theta Coronae Borealis in my APM 152/1200 ED at 255x (4.7mm ES82) and 510x (4.7mm ES82 + 2x barlow). I estimated the PA to 200°.  

 

Eta CrB was surprisingly much more difficult, but I could see a slight elongation at 255x and 510x. I estimated the PA to 300° and StelleDoppie confirmed both the separation and the PA. I had looked at Eta CrB many years ago in a C8 and found it easily split, so I was a bit surprised to find it so hard, double-checking my maps and where I was aimed, but I immediately told myself, that it had probably narrowed down a lot, due to orbital motion, which turned out to be correct.

 

The seeing was so superb, the image hardly moved at all at 255x and only trembled ever so slightly at 510x. I also had a most stunning view of Zeta Herculis, by far the finest I've ever had. The companion was just sitting there, plain as day, solid, unmoving. It was as easy, if not easier, than Izar in a fine 60mm. The night was warm, foggy and extremely humid. Everything was dripping wet. There was no wind whatsoever. 

 

 

Clear skies!

Thomas, Denmark


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#7 Uwe Pilz

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Posted 15 July 2021 - 05:16 AM

I tried this star yesterday wit my 5 inch refractor. It was clear from the beginning, that splitting (that means black between the stars) is impossible with this instrument. Nevertheless, I saw a slightly elongated Airy disc with the right p.a. I could not detect, however, at which side the brighter of the stars is. I estimated a p.a. between 200° and 210° (some tries). I used 250x mostly but switched to 360x too.

 

I tried gamma CrB after that, saw a even less elongated Airy disc and missed the p.a. by 30°. So I don't count it as succesful.


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#8 JimOfOakCreek

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Posted 15 July 2021 - 11:43 AM

I tried this star yesterday wit my 5 inch refractor. It was clear from the beginning, that splitting (that means black between the stars) is impossible with this instrument. Nevertheless, I saw a slightly elongated Airy disc with the right p.a. I could not detect, however, at which side the brighter of the stars is. I estimated a p.a. between 200° and 210° (some tries). I used 250x mostly but switched to 360x too.

 

I tried gamma CrB after that, saw a even less elongated Airy disc and missed the p.a. by 30°. So I don't count it as succesful.

Interesting report! I want to give it a go with my 8” SCT but haven’t had a clear night since my last post. This weekend is going to be clear so I will find out then if I can split it.



#9 fred1871

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Posted 18 July 2021 - 08:52 AM

Theta CrB (COU 610) is an interesting one that has been widening since discovery in 1971. Couteau discovered it with the 20-inch refractor at Nice Observatory in France. His measures of it that year averaged 0.47". In 1990 the separation had increased to 0.75" and in 2008 to 0.81". At present I'd estimate, based on the list of measures, that it would be ~0.85".

 

The PA of the pair has changed very little, suggesting the possibility of seeing the orbit nearly edge-on. Photometry of the stars is where it gets more interesting. Couteau's estimates of the brightness of the companion, based on the difference between the stars, ranged from 5.5 to 7.5, over a 10 week period for the four micrometer measures. Three of the estimates were in the range 6.5 to 7.5. In his book on Observing Visual Double Stars, published a few years later, he gives in a catalogue of doubles for amateur observers the magnitude of the companion as 7, with no decimal, unlike other doubles.

This of course is different from the WDS magnitudes. 

 

If we look at the short article online by the Spectroscopy guru Jim Kaler we find a note that

In 1970, Theta CrB did just the opposite, and dimmed by some 0.7 magnitudes (to about 50 percent normal brightness), perhaps as a result of some kind of dust ejection. At the same time it went into a series of brightness oscillations. It is now very peaceful, as it has been for well over a decade.

 

1970 is just before Couteau discovered it as a double. So there could be changing brightness difference for the pair in this period, depending on when it was observed. Is the secondary star variable? - I've not yet found anything definite on this. The WDS simply has a note "variable" without any more data including whether the note applies to one or both stars.

 

Looking at the magnitudes measured by Tycho and Hipparcos, we find Tycho giving a Delta-m of 2.02 magnitudes (as per the current WDS), and Hipparcos giving 2.31 mags. That's for a waveband in the Visually preferred part of the spectrum (centred on 530nm and 505nm respectively). Whatever the best number turns out to be, it seems very unlikely it will be less than 2 magnitudes, and possibly somewhat more; and if variability happens that will affect the degree of difficulty of separation.

 

Given that the brightness difference is at least two magnitudes, it can be expected that Rayleigh rather than Dawes will be a guide to the aperture needed for resolution, especially for telescopes with secondary mirrors which enhance the amount of light in the first diffraction ring.

 

Rayleigh for various apertures to give a rather minimal visual effect of separation is

5-inch  1.09"

6-inch   0.91"

7-inch   0.78"

8-inch   0.68"

   Therefore we can see that for a separation about 0.85" at present a 5inch or 6-inch might show some elongation, but rather more aperture - as well as good seeing - will be necessary for a hope of separation.

 

As for comparisons with other binaries - Zeta Herculis at present is about 1.5" separation, with Delta-m of 2.45, therefore much easier. Eta CrB is from the Ephemeris (a Grade 1 orbit) at about 0.40", with Delta-m of only 0.3, which could allow slight elongation with a 6-inch. Gamma CrB is in the closest part of its orbit, at present near 0.05", therefore a 4-metre telescope with speckle interferometry is the telescope of choice. It's a 91 year binary, that back in the 1990s-beginning 2000s was splittable with amateur scopes despite a Delta-m of ~1.5 magnitudes, when around 0.6" to 0.7".

 

Some of the above material I used in a discussion about Theta CrB back in 2013, here on CN. The separation of the pair was not much less at that time, and an 8-inch scope was the smallest to enable a split at fairly high power at that time.

 

I'll add some more thoughts later. This is long enough already.


Edited by fred1871, 18 July 2021 - 09:10 AM.

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#10 Astrojensen

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Posted 18 July 2021 - 10:11 AM

I can confirm, again, that Theta CrB is detectable in a 6" refractor. I re-observed it again last night in mediocre seeing at 255x and despite the boiling image, I could readily detect the companion as a small "droplet" hanging from the main star. It would have been much more difficult, though, if I hadn't first observed it in excellent seeing. 

 

It will be VERY interesting to keep an eye on it regularly, to maybe spot signs of its variability. Plus it's well-placed throughout the season with the most stable seeing here, so I should probably keep an eye on it once every month or so. 

 

If it drops half a magnitude, it will become MUCH more difficult in a 6"!

 

 

Clear skies!

Thomas, Denmark


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#11 Spikey131

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Posted 18 July 2021 - 05:40 PM

I could not detect it the other night with the C8.  Seeing was not great, though, and I will try again.


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#12 Astrojensen

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Posted 19 July 2021 - 01:33 AM

I could not detect it the other night with the C8.  Seeing was not great, though, and I will try again.

In a C8, the companion will sit right smack on top of the bright first diffraction ring... This will make its detection much more difficult, unless the seeing is near perfect. 

 

 

Clear skies!

Thomas, Denmark


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#13 Jon Isaacs

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Posted 20 July 2021 - 04:59 AM

In a C8, the companion will sit right smack on top of the bright first diffraction ring... This will make its detection much more difficult, unless the seeing is near perfect. 

 

 

Clear skies!

Thomas, Denmark

 

Last night I had some stable seeing in the early evening with Theta Crb nearly at the zenith.  I was using my 10 inch but I couldn't make a split.  If it is clear tonight I will try it in the 13.1 inch.

 

Jon


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#14 JimOfOakCreek

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Posted 20 July 2021 - 06:13 PM

I wasn’t able to split it with my 8” SCT. We have hazy skies due to forest fires out west. But I don’t think that was the problem. I need a Dob, a  big Dob. crazy.gif



#15 fred1871

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Posted 21 July 2021 - 02:55 AM

When a number of us back in 2013 discussed this double here on CN, the smallest scope I recall getting a split was an 8-inch Newtonian. The same observer with a 15-inch found it easier and clearer. Some years earlier in 1998, I had seen flickery images of the companion with a 7-inch apo refractor at high power. Because the change in separation is slow, the 1998 separation (based on speckle measures on large telescopes) was 0.78", 2013 was 0.83", now only slightly more (perhaps 0.85"). The rate of widening has slowed over time.

 

The most important factor is likely to be steady atmosphere, and I would not like to try with hazy skies from forest fires. So a big Dob might not do it until conditions improve. Jon Isaacs not seeing the companion with a 10-inch was a surprise, but some nights the conditions look better than in fact they are. We'll see if Jon gets a split with his 13-inch.



#16 Astrojensen

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Posted 21 July 2021 - 06:37 AM

In superb seeing, I could see it very clearly in a 6" f/8 ED at 510x, so a large scope isn't needed. Steady seeing, and high optical quality is what you need. 

 

This is what it looks like in the 6" ED at 510x:

 

gallery_55742_4772_3988.jpg

 

Image created with Aberrator. It's VERY close to the actual appearance in the eyepiece. I felt that it was somewhat easier, than it appears here. 

 

 

Clear skies!

Thomas, Denmark


Edited by Astrojensen, 21 July 2021 - 06:47 AM.

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#17 blakesphere

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Posted 21 July 2021 - 10:50 AM

I tried with a 101mm, a Celestron 8 with no luck.

 

Split with a 14-inch SCT.

 

Both pure white, equally bright, very close, north-south orientation.



#18 Adam Long

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Posted 21 July 2021 - 04:16 PM

We're a week into a heat wave here in Yorkshire, which has made a change from the previous month of cloud with clear skies and high humidity and temps. For various reasons I've not been able to get the scope out but managed a couple of hours last night from 10pm to midnight.

 

At moments COU 610 was very nicely resolved at 285x and 325x, with the secondary appearing a blue/grey compared to the cold white primary, but the seeing was not great and the view didn't hold up at higher mags. The delta mag is clearly significant, but the 14" seperates them well with a gap at least as wide as the spurious discs. Interesting to compare with Eta CrB where the seperation is obviously much less, giving a hairline split at the same mags.

 

And a nice surprise this morning when Couteau's book finally arrived, after spending a month shipping from Canada. Lots of good info and always nice to get an extra gift on your birthday!


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#19 John Fitzgerald

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Posted 21 July 2021 - 05:11 PM

I tried with a 101mm, a Celestron 8 with no luck.

 

Split with a 14-inch SCT.

 

Both pure white, equally bright, very close, north-south orientation.

You were not looking at Theta Crb then.....confused1.gif   Delta magnitude is 2.02, no where near equal.



#20 fred1871

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Posted 21 July 2021 - 10:27 PM

Blake - it can't be Theta CrB, as John points out. The difference in brightness is not less than 2 magnitudes. It could be more. See some of the notes above.



#21 Astrojensen

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Posted 22 July 2021 - 12:07 AM

Perhaps he was looking at Eta CrB, instead of Theta? His description would suggest it.

 

 

Clear skies!

Thomas, Denmark



#22 blakesphere

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Posted 22 July 2021 - 07:24 AM

Sounds like I have to have another go.

 

I'll try the C8 in the back yard.



#23 Uwe Pilz

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Posted 25 July 2021 - 01:16 PM

I observed this star again, at an night with excellent seeing. Now I could see the direction clearly in which the secondary is.

 

Again, I used my 5 inch refractor at 250x and 360x.

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#24 rugby

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Posted 26 July 2021 - 11:47 PM

My results from July26/21 lie somewhere between those of Uwe and Astrojensen. Elongation shows around 400x with the first ring degraded on the preceding edge. From 486x to 600x the elongation becomes obvious. I suspected a very small notch at this magnification.
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