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I think I finally saw the airy discs!

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#1 Buqibu

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Posted 05 August 2021 - 01:51 PM

A few days ago I made a post asking for advice because I could not see the airy discs in my 4 inch achro. Some people said it might be the seeing, others said maybe I wasn't magnifying enough. Well last night I was getting some unusally clear views of the planets, so I decided to try and see the airy discs again, maybe the seeing would permit. I tried and....nothing. The same smudgy shape I had been describing. I decided as a last resort to remove my diagonal, maybe it was the cause. So I did. Put in the barlow plus Celestron Zoom, at 165x. After a few minutes adjusting my position (sore neck, sitting on the floor, you know the deal)...there they were. These 3 or 4 rings around Polaris. The star itself looked like a disc. I was very pleased, having solved this "issue" that had been bothering me for a while. Not that it matters that much, its a very simple thing, all telescopes do it, but maybe that was the thing that bothered me. Anyway, back on went the diagonal, the optical imperfections didnt outweigh its conviniencešŸ˜. Thanks for the help! Just one question, the rings seemed to be only appearing in the lower part of the star, was this due to the seeing or maybe astigmatism or smth like that? Thanks

#2 Astrojensen

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Posted 05 August 2021 - 02:14 PM

 

the rings seemed to be only appearing in the lower part of the star, was this due to the seeing or maybe astigmatism or smth like that?

This, plus the fact that you saw as much as three or four rings around Polaris strongly suggests that the scope is badly miscollimated and may also have spherical aberration.

 

 

Clear skies!

Thomas, Denmark


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#3 Buqibu

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Posted 05 August 2021 - 02:38 PM

Its a refractor, and views through it are quite sharp, i doubt it would have severe spherical aberration or be misscollimated

#4 Astrojensen

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Posted 05 August 2021 - 02:46 PM

Its a refractor, and views through it are quite sharp, i doubt it would have severe spherical aberration or be misscollimated

And what do you have to compare it with? How do you judge its sharpness?

 

The fact that you saw so many diffraction rings, and only to one side, on a modestly bright star such as Polaris, VERY strongly suggests that something isn't as it should be in that telescope. Views may still appear fairly good, but much in your reports indicates that it could be better. 

 

In a good 4" refractor in excellent seeing, Polaris is a sharp disk, surrounded by a single, faint, unbroken ring. 

 

 

Clear skies!
Thomas, Denmark


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#5 Phil Barker

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Posted 05 August 2021 - 02:47 PM

The collimation will be out if its not even rings in the centre of the field.  If it shows even in one part that should be the centre and the tilt of the lens needs adjusting.   Makes quite a difference and there are collimating tools to sort this as long as you have adjustment screws.

 

The collimating tool is inexpensive and easy to use

 

https://agenaastro.c...refractors.html

 

I own one and have used it on several refractors over the years.  Its easy to use.

 

As some of us are aware with refractors there can be a big difference between quite sharp and a top end apo sharp smile.gif


Edited by Phil Barker, 05 August 2021 - 02:50 PM.


#6 Bonco2

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Posted 05 August 2021 - 03:00 PM

Strongly agree, the telescope needs to be collimated. Does the lens cell have adjusting screws?

Bill




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