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Faint rings around bright stars

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9 replies to this topic

#1 SupernovaDust

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Posted 06 August 2020 - 04:27 PM

Hello,

 

On my Celestron Evolution 6 I see faint rings close around bright stars as if the star would be out of focus, like the rings we use for collimation. When I do the donut test, meaning out- and infocusing to collimate, everything looks fine, dark dot in the center and rings are not skewed. But when I try to focus on the star I cannot get rid of a few rings very close to the star, no matter how carefully I try to focus. 

 

Is this normal or does anyone have an idea what might be causing this? I don't see such rings on my 12" dobson but other than that I didn't notice anything to be off, planets and moons are sharp and less bright stars are pin point in my SCT.


Edited by SupernovaDust, 06 August 2020 - 04:28 PM.


#2 Migwan

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Posted 06 August 2020 - 05:23 PM

How high of power were you using?  Could be airy disc, which for me tend to be more visible on smaller scopes, but at higher powers. 

 

jd


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#3 macdonjh

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Posted 06 August 2020 - 06:19 PM

Do you see a wide, faint, blurred ring, or a thin, sharp, well-defined ring?  If the latter, that's probably the first diffraction ring, caused by the secondary mirror.  If the former, it could be fog on your corrector or eye piece.

 

My guess is you don't see it your Dobsonian because you notice the diffraction spikes from the spider.


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#4 jgraham

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Posted 06 August 2020 - 07:48 PM

What you are seeing is the first and brightest diffraction ring. There are more, but they get faint fast. The central obstruction in your Cat is probably larger than your Newtonian (relative to the size of the primary) and that pushes more energy into the diffraction rings. No big deal, just physics. :)

#5 Nippon

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Posted 06 August 2020 - 07:58 PM

If it's a dot with a complete ring around it congratulations you have a well corrected and collimated scope:)


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#6 SupernovaDust

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Posted 07 August 2020 - 01:05 AM

Okay thanks for your replies, after you guys mentioned it I googled diffraction ring and that's exactly what I see in higher magnifications. I was worried because I feared I damaged something when I exchanged the collimation screws for thumb screws. I learned something new, topic can be closed :D


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#7 Eddgie

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Posted 08 August 2020 - 04:47 PM

Okay thanks for your replies, after you guys mentioned it I googled diffraction ring and that's exactly what I see in higher magnifications. I was worried because I feared I damaged something when I exchanged the collimation screws for thumb screws. I learned something new, topic can be closed laugh.gif

Physics and the wave nature of light, all presented to you at once.  The diameter of the ring is a function of the aperture of the telescope.  The larger the aperture, the smaller the diameter of that ring (at a given power).  This is why with all other things being equal, a bigger scope will have better contrast transfer than a smaller scope. 

 

While the diameter of the maximum brightness of the ring will not change with an obstruction or with spherical aberration,  the brightness of the ring will be affected by these characteristics, with each of these causing the ring to be brighter, and that means the contrast transfer of the scope will be lowered. 

 

You can tell a great deal about a telescope from the appearance of the rings. 


Edited by Eddgie, 08 August 2020 - 04:48 PM.

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#8 gnowellsct

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Posted 08 August 2020 - 11:13 PM

How high of power were you using?  Could be airy disc, which for me tend to be more visible on smaller scopes, but at higher powers. 

 

jd

No, not for you.  For everyone.  

 

The Airy disk is one of the things that I had really struck me about viewing through refractors.  I see the Airy all the time in my 80 and 90 mm scopes.  Much less often in my 130 mm, but it's still there to be seen.

 

I don't have a 6 inch so I can't comment.  

 

But I'm pretty sure that once you get to 8 inches and higher, you will see an Airy disk rarely if at all.  

 

Greg N


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#9 Supernova74

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Posted 11 August 2020 - 01:27 PM

Well the first route of call is double check the collomation of your celestron Evo 6.it also sounds familer to the anomaly’s which show up in my Meade 12”ACF an halo type pattern around very bright stars this could be a number of things the worse case scenario would be a falty correcter plate which is highly unlikely.if it’s the halo pattern anomaly your seeing on very bright objects ie Vega,Arcturus this is caused by stray light hitting the baffle inside the scope and for now this does not bother me as it only happens on a handful of objects.im not necessarily saying it is this as you have to troubleshoot all causes.is it defraction on stars your seeing is the scope achieving focus ok.if possible try to post a few pictures I think it would be a lot easier.it could even be normal in what you are seeing.



#10 Bean614

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Posted 11 August 2020 - 06:37 PM

Well the first route of call is double check the collomation of your celestron Evo 6.it also sounds familer to the anomaly’s which show up in my Meade 12”ACF an halo type pattern around very bright stars this could be a number of things the worse case scenario would be a falty correcter plate which is highly unlikely.if it’s the halo pattern anomaly your seeing on very bright objects ie Vega,Arcturus this is caused by stray light hitting the baffle inside the scope and for now this does not bother me as it only happens on a handful of objects.im not necessarily saying it is this as you have to troubleshoot all causes.is it defraction on stars your seeing is the scope achieving focus ok.if possible try to post a few pictures I think it would be a lot easier.it could even be normal in what you are seeing.

??????....!

In case you weren't following,  it has been clearly determined that what the OP was seeing was not only perfectly normal for his scope (perfect diffraction rings) but an indication that his optics are excellent!

   So, there are NO anomalies,  NO faulty corrector, NO collimation issues, NO stray light issues, etc., etc.


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