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Observing Doubles 101

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#1 tchandler

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Posted 26 May 2019 - 08:48 AM

While there is absolutely nothing at all wrong with looking at a star (or pair of stars) simply to admire their beauty, I feel that the time has come for me to better understand the physics behind it. 

 

I’ve been scouring the web and my reference books to better understand the role of such phenomena as the Airy disk and diffraction rings. These rings show up so beautifully in the splendid telescopes that I am enjoying (85/600, 120/900 and 152/1200 refractors, and a 280/1250 reflector).

 

I gather that the factors controlling whether or not we’ll be able to resolve a double star will depend on many things that will include, but may not be limited to:

 

1. “Star Effects” Physical aspects of the two stars: separation, relative brightness, colour)

2. “Earth Effects”: Seeing, transparency, star altitude

3. “Optics Effects”: Telescope aperture, collimation, magnification, f/#

4. “Observer Effects”: Experience, fatigue, comfort level (seated/standing/cold)

 

Specifically, I am interested in role of Star Effects and Optical Effects on double star observing.

 

How will aperture effect the size of the Airy disk and/or the location of the first diffraction ring?

How is this determined? 

 

If a secondary star appears to be located at the first diffraction ring of the primary star in one telescope, will changing the aperture “move” this diffraction ring such that the secondary star no long falls upon this ring and so is easier to observe? 

 

Thank you in advance!



#2 Cotts

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Posted 26 May 2019 - 11:13 AM

"Specifically, I am interested in role of Star Effects and Optical Effects on double star observing.

 

How will aperture effect the size of the Airy disk and/or the location of the first diffraction ring?

How is this determined?

If a secondary star appears to be located at the first diffraction ring of the primary star in one telescope, will changing the aperture “move” this diffraction ring such that the secondary star no long falls upon this ring and so is easier to observe?"

 

I would start here.  

 

To superficially answer the questions above,

 

Bigger aperture ---> reduced radius to the first minimum of the diffraction artifact as well as the first bright ring.  i.e. the Airy pattern gets smaller so closer pairs can be 'split'...  Modern analogy - pixellation in photographs.  With a small telescope, pixels are larger, reducing resolution.  And vice-versa.

 

Radius, in arc seconds to the first minimum (D is scope diameter in mm.)  138/D  Note that the central disc of light is often smaller than this radius and varies with magnitude of the star - fainter --> smaller. 

 

Radius in arc seconds to the first ring.   163/D   These two formulae are from TelescopeOptics.net.  The numbers vary infinitesimally as well with increasing central obstruction but this difference is too small (3rd significant digit...) to matter in real-life observing situations..

 

These parameters are reached by some pretty fancy University-level math.....   

 

Changing aperture does change these radii so, yes, changing scope diameter can 'reveal' a companion sitting on the first ring..

 

Dave


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#3 tchandler

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Posted 26 May 2019 - 12:02 PM

Brilliant reference Dave. Thanks. 

Right off the bat I learned where the word lens was derived. I’ll never again think about lentil soup the same way.




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