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lambda Cygni

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#1 Uwe Pilz

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Posted 17 August 2018 - 04:12 AM

Yesterday night I observed lambda Cygni, which is a triple star. The components A and B are quite close (0.9") and are near the limit of my 4 inch refractor. Splitting them got even harder because there is a magnitude difference of 1.6 mag between the stars. At least, they are rather bright (4m7 and 6m3). I saw that it is a double only by a slight bulge of the airy disc. The component C could be seen easily, of course.

The sketch is mirror reversed.

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#2 flt158

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Posted 17 August 2018 - 07:40 AM

Hello, Uwe. 

What a delightful sketch that is!

I have been a big fan of Lambda Cygni in recent years. 

Your sketch is now a reminder that I must observe it again. 

It has been 2 or 3 years since I did so with my William Optics 158 mm apochromatic refractor. 

I was successful in splitting A and B at 280X with a William Optics 4 mm eyepiece in the past.   

 

Clear skies from Aubrey. 


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#3 Magnus Ahrling

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Posted 19 August 2018 - 05:10 PM

Wow, well done Uwe. Wonderfull sketch. I like the little bulge you caught. What mag did you use?
I like Lambda, have seen it split at 320X with my C8 and at 400X with my MK66 but also no luck in less than near perfect seeing.

/Magnus

#4 Uwe Pilz

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Posted 19 August 2018 - 10:31 PM

I increased the mag to around 200x to see the details oft the airy disc.

#5 cildastun

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Posted 20 August 2018 - 02:04 AM

Lambda Cygni is a tricky little devil, isn't it? It splits quite nicely with my 180 Mak at x270 (Baader 10mm Ortho), but my smaller scopes struggle a little with it.

 

Chris



#6 dpastern

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Posted 20 August 2018 - 03:34 AM

I wasn't sure if this would be visible from Brisbane, Australia, but it is, albeit low down on the horizon and hidden by trees/buildings from my house.  Would like to have a crack at this myself one day.  Do you think a 6" dob with very good seeing can split this at say 200x?



#7 cildastun

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Posted 20 August 2018 - 05:44 AM

I wasn't sure if this would be visible from Brisbane, Australia, but it is, albeit low down on the horizon and hidden by trees/buildings from my house.  Would like to have a crack at this myself one day.  Do you think a 6" dob with very good seeing can split this at say 200x?

I've certainly split it a few times with my 127 Mak (true aperture 119mm), so a well-collimated 150mm dob should do, particularly if you are in a location with good seeing!

 

Chris


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#8 dpastern

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Posted 20 August 2018 - 06:34 AM

I've certainly split it a few times with my 127 Mak (true aperture 119mm), so a well-collimated 150mm dob should do, particularly if you are in a location with good seeing!

 

Chris

Many thanks - I was a bit concerned cos the separation of the pair is very close to the theoretical resolution limit of the 6" dob I believe.  



#9 cildastun

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Posted 20 August 2018 - 08:57 AM

Many thanks - I was a bit concerned cos the separation of the pair is very close to the theoretical resolution limit of the 6" dob I believe.  

Theoretical resolution limit  (Dawes) of a 150mm scope should be about 0.8 arcsec for nearly equal mag stars I believe.

 

Chris


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#10 dpastern

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Posted 20 August 2018 - 09:06 AM

Theoretical resolution limit  (Dawes) of a 150mm scope should be about 0.8 arcsec for nearly equal mag stars I believe.

 

Chris

yeah, that's what I read.  Pretty close to that 0.9 arcsec separation of this double!  I'll need perfect seeing to nail this binary imho.  I'm learning more and more as I observe that seeing conditions trump EVERY other part of the telescope/eyepiece combination.  



#11 cildastun

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Posted 20 August 2018 - 09:12 AM

lambdacyg150.jpg

yeah, that's what I read.  Pretty close to that 0.9 arcsec separation of this double!  I'll need perfect seeing to nail this binary imho.  I'm learning more and more as I observe that seeing conditions trump EVERY other part of the telescope/eyepiece combination.  

Should look roughly like this (Aberrator simulation).

Chris


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#12 dpastern

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Posted 20 August 2018 - 10:31 AM

attachicon.gif lambdacyg150.jpg

Should look roughly like this (Aberrator simulation).

Chris

cool, I have an idea of what to expect.  That's a really tight split.  Where did you get that sim from if I may ask?



#13 cildastun

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Posted 20 August 2018 - 10:45 AM

cool, I have an idea of what to expect.  That's a really tight split.  Where did you get that sim from if I may ask?

You can find Aberrator on the web (Freeware); you can set it up to simulate double stars, by changing scope parameters (aperture, obstruction, f no etc) and stars (mags, sep, PA). It seems to work quite well as a guide to what a pair will look like, although I find it works better with nearly equal pairs - with unequal pairs, the size and brightness of the secondary star do not look quite like what I see through the eyepiece, or image using an ASI224 camera. 

 

Chris


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#14 dpastern

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Posted 20 August 2018 - 12:11 PM

You can find Aberrator on the web (Freeware); you can set it up to simulate double stars, by changing scope parameters (aperture, obstruction, f no etc) and stars (mags, sep, PA). It seems to work quite well as a guide to what a pair will look like, although I find it works better with nearly equal pairs - with unequal pairs, the size and brightness of the secondary star do not look quite like what I see through the eyepiece, or image using an ASI224 camera. 

 

Chris

Excellent, will check it out tomorrow.  Thank you.



#15 Uwe Pilz

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Posted 20 August 2018 - 05:48 PM

> I was a bit concerned cos the separation of the pair is very close to the theoretical resolution limit of the 6" dob I believe.

You may detect doubles well beyond the Rayleigh limit. Look for irregulatities oft the airy disc. I detected (but not real split) doubles downto 0.ü arcsecs with my 4 inch refractor.
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#16 dpastern

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Posted 20 August 2018 - 09:42 PM

> I was a bit concerned cos the separation of the pair is very close to the theoretical resolution limit of the 6" dob I believe.

You may detect doubles well beyond the Rayleigh limit. Look for irregulatities oft the airy disc. I detected (but not real split) doubles downto 0.ü arcsecs with my 4 inch refractor.

good point.  I'm still very much a newb though with doubles and much to learn.




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