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I don't see diffraction rings ( "Airy discs" ) at all

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#1 Buqibu

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Posted 27 July 2021 - 05:57 AM

Hello, I've read that stars through a telescope show up as small discs with diffraction rings around them. I have a 4 inch achromatic refractor and have never been able to see this. Could it be the optics are sub-par, or has the seeing never been good enough? The second seems unlikely, as I've tried it on multiple occasions, through high magnifications, but always just saw a deformed smudge, not a defined disc. Thanks for any advice!
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#2 Supernova74

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Posted 27 July 2021 - 06:09 AM

Hi and welcome.

firstly what scope have you got?

well even tho refractors have relatively low cool down times to reach the thermal equilibrium should really only take 30

minites tops.from your discription it sounds like atmospheric dispersion your noticing in your 4” refractor.so I would make sure the scope is away from any heated sources ie a house or building (especially roof tops) as heat rises and is a possibility it’s effecting the image.i would also advice you don,t attempt to view a star test low in the meridian as this would give you the worse possible results.if you plan to do a star test I would personally view a star overhead in the zenith as thats where the atmosphere is most stable.and finally you will not achieve text book airy disks as if you was viewing in a APO telescope,compared to a achromatic telescope.


Edited by Supernova74, 27 July 2021 - 06:12 AM.

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#3 Buqibu

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Posted 27 July 2021 - 06:23 AM

That could be a reason, I have only observed in a balcony over roofs and houses. Will try some day in a better environment
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#4 Migwan

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Posted 27 July 2021 - 06:37 AM

Also, it takes a magnification around 40 to 50 times your aperture, to make them more apparent.   As for how seeing effects the diffraction rings and the airy disk, have a look at THIS.


Edited by Migwan, 27 July 2021 - 06:39 AM.

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#5 TOMDEY

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Posted 27 July 2021 - 07:37 AM

Migwan's link there is very good. You will have a better chance of seeing it if you use a green filter and go to high magnification on a bright star. 200x with your 4-inch telescope should do it. The Airy Pattern (J1(x)/x)2 will be quite small and probably shimmering a lot... even under very good seeing conditions. For bigger telescopes it is angularly smaller, extremely bright... and rare.

 

50-60mm scope >>> pattern is common under good conditions --- core is about 3.6 arc-sec in diameter

4-inch scope >>> pattern is common under very good conditions --- core is about 2.0 arc-sec in diameter

6-inch scope >>> pattern is not uncommon under great conditions --- core is about 1.4 arc-sec in diameter

12-inch scope >>> pattern is fleeting, even under great conditions --- core is about 0.7 arc-sec in diameter

 

If you use a white star unfiltered, you will see pretty coloration in the pattern, even with an entirely apochromatic reflecting telescope. This is the "natural" coloration of the Airy Pattern. With my trusty old 12.5-inch Cave Astrola under great conditions, the miniscule Airy Disc persisted nicely and reliably resolved a half-arc-sec on those good nights.

 

Note to tire kickers:  I used the approximations that λ = 0.5 μm (greenish), and core diam is as typically perceived photopic, larger than photometric Full Width Half Max (FWHM)... perceptions may vary.

    Tom


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#6 spereira

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Posted 27 July 2021 - 07:42 AM

Moving to Refractors.

 

smp



#7 Alan French

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Posted 27 July 2021 - 07:44 AM

In addition to a green filter, adding an aperture stop, perhaps 1-inch in diameter, will help negate the effects of seeing, and make the Airy disk larger and easier to see, requiring less magnification to make it visible. A good preview of what you should see with full aperture and a higher power when the seeing cooperates.

 

Make sure the hole in the aperture stop is nice and round and has smooth edges. 

 

Clear skies, Alan


Edited by Alan French, 27 July 2021 - 08:27 AM.

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#8 Asbytec

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Posted 27 July 2021 - 08:28 AM

That could be a reason, I have only observed in a balcony over roofs and houses. Will try some day in a better environment


Balconies and roofs /can/ cause a lot of thermal activity and distortion or poor local atmospheric seeing conditions that adversely affect your scope's images. I strongly suspect this is why you see a smudge rather than a disc. As Tom says above and as you alluded to high magnification, you should be at about 200x or more. The disc is pretty small. A better environment will certainly help, and your scope should be thermally stable.

Edited by Asbytec, 27 July 2021 - 08:32 AM.

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#9 TOMDEY

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Posted 27 July 2021 - 08:38 AM

In addition to a green filter, adding an aperture stop, perhaps 1-inch in diameter, will help negate the effects of seeing, and make the Airy disk larger and easier to see, and requiring less magnification to make it visible. A good preview of what you should see with full aperture and a higher power when the seeing cooperates.

 

Make sure the hole in the aperture stop is nice and round and has smooth edges. 

 

Clear skies, Alan

Yes! I did that experiment under ideal conditions in the lab using a giant perfect collimator as the source and telescope with a round unobstructed iris as the viewer. The Airy Disc was textbook perfect and I could watch it expand and contract, dim and brighten, as I adjusted the iris. Also gives great appreciation to the quartic intensity relationship. Twice the aperture gives sixteen times the core radiance. I designed and built the parfocal illumination sources for the Dahl Collimator there. That gave me a good excuse for admiring the Airy Patterns. My 36-inch telescope gives an Airy Core over a thousand times more luminous than a six-inch scope... a bright star fleetingly looks like a miniscule dancing carbon arc... dazzling too bright to look at.    Tom

Attached Thumbnails

  • 48 80 Tiny Airy Disc 150.jpg
  • 47 80 Lab Dahl Collimator annotated Tom's Parfocal Sources.jpg

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#10 Migwan

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Posted 27 July 2021 - 10:44 AM

One other thing you might try if you don't have an aperture mask, is using a moderately bright star.   Using Pherked or a similar mag 3 star when up high, I can see the diffraction pattern at 133x in my 5" quite well.  That's only 27x per inch of aperture.     


Edited by Migwan, 27 July 2021 - 10:45 AM.

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#11 Asbytec

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Posted 27 July 2021 - 11:04 AM

I can see the diffraction pattern at 133x in my 5" quite well. That's only 27x per inch of aperture.


Yes, I believe with average acuity we can see the Airy disc fairly certain at near 1mm exit pupil or around 133x in a 5" aperture or 102x in a 4" aperture. I can just barely make it out at a little lower magnification, but being small it helps to recognize what we're looking for. At 0.5mm exit pupil or 50x per inch it's much easier without question. But, you're right, about 25x per inch is enough for most folks depending on our acuity. I normally observe double stars at 50x per inch or higher, so the small disc is very apparent. And of course, the better end of the seeing scale really helps.

Edited by Asbytec, 27 July 2021 - 11:09 AM.

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#12 Buqibu

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Posted 27 July 2021 - 12:28 PM

Thanks for all the advice!

#13 Redbetter

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Posted 27 July 2021 - 12:31 PM

Hello, I've read that stars through a telescope show up as small discs with diffraction rings around them. I have a 4 inch achromatic refractor and have never been able to see this. Could it be the optics are sub-par, or has the seeing never been good enough? The second seems unlikely, as I've tried it on multiple occasions, through high magnifications, but always just saw a deformed smudge, not a defined disc. Thanks for any advice!

Too vague of a description for people to do more than guess.  What particular scope (e.g. focal length and ratio) and what eyepieces have been used with it?  

 

"Deformed smudge" is the one descriptor above that catches my attention, since it implies that the shape at whatever power you are using is not round/uniform.  The specific type of deformation could suggest the type of problem and why the airy disk might be unseen.  Objective alignment collimation (comatic effects), decenter, wedge, pinch, astigmatism, spherical aberration and other things can be evaluated based on the shapes seen when going back and forth through focus at sufficiently high power.  

 

I have seen far more severe problems with a plethora of optical defects in achros than I have with Newtonians, SCT's and Maks combined.  I have a substantial collection of junk achro objectives now.  The lateral chromatic distortions that result from some of the defects in achros add another dimension.


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#14 Migwan

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Posted 27 July 2021 - 05:16 PM

Yes, I believe with average acuity we can see the Airy disc fairly certain at near 1mm exit pupil or around 133x in a 5" aperture or 102x in a 4" aperture. I can just barely make it out at a little lower magnification, but being small it helps to recognize what we're looking for. At 0.5mm exit pupil or 50x per inch it's much easier without question. But, you're right, about 25x per inch is enough for most folks depending on our acuity. I normally observe double stars at 50x per inch or higher, so the small disc is very apparent. And of course, the better end of the seeing scale really helps.

Yep, but I was trying to address the smudge as seen in a sloppy achro, such as my ST120.   If I look at a mag 1 star with it at 133x, it's just a smudge.    Also, I realized I didn't know how much magnification the OP could reach.      
 


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#15 Redbetter

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Posted 27 July 2021 - 11:38 PM

Yep, but I was trying to address the smudge as seen in a sloppy achro, such as my ST120.   If I look at a mag 1 star with it at 133x, it's just a smudge.    Also, I realized I didn't know how much magnification the OP could reach.      
 

Yes, thanks for doing that.  An ST120 should have copious levels of color blur that will impact the airy pattern.  That effect is one of the reasons I asked about focal length and ratio.  With a very fast achro (for the aperture) the level of CA is such that there is considerable blurring of the disk and first diffraction ring.  That is why green filters are useful for star testing when looking for spherical aberration and the like. 

 

Of course the level of blur depends on the ratio of the 4" being discussed.  In terms of CA, a 4" f/5.9 or f/6.5 will have about the same level of blur to the airy disk as an 80 f/5--although it will be more apparent in the larger aperture for a given star.  They should have noticeably less color blur than an ST120.  A 4" f/9.8 will have considerably less blur than the other 4" achros I listed.  


Edited by Redbetter, 28 July 2021 - 01:04 AM.

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#16 BruceNewEngland

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Posted 29 July 2021 - 07:06 AM

Here’s another thread on this topic.

https://www.cloudyni...4782-airy-disk/

 

You may want to try viewing stars that are close to being overhead the next time, to minimize effects from the atmosphere and hot roofs of nearby buildings.

 

Try an eyepiece that gives over 200 times magnification, even 300x (magnification is the focal length of the scope divided by the focal length of the eyepiece). If your scopes focal length is around 700mm, then try an eyepiece around 3mm, or use a Barlow if you don’t have an eyepiece that short.

 

Also, to get a sense of what to look for, try defocusing the image slightly. Try a little out of focus on each side of perfect. As you move away from perfect, the rings will grow in size (they may be more obvious on one side). If seeing conditions are poor, you might still not see them, try another night, and check the astronomy forecast for seeing in your area so you’ll know when seeing should be ok.

 

https://www.cleardarksky.com/csk/

 

https://in-the-sky.org/weather.php

 

Bruce.



#17 Buqibu

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Posted 29 July 2021 - 10:33 AM

Thank you! But 300x times on a 4 inch achro sounds very optimistic. Wouldn't the blurriness alone eleminate my chances?
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#18 Astrojensen

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Posted 29 July 2021 - 02:03 PM

Thank you! But 300x times on a 4 inch achro sounds very optimistic. Wouldn't the blurriness alone eleminate my chances?

That depends on what kind of 4" achro we're talking about... 

 

 

Clear skies!

Thomas, Denmark


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#19 Buqibu

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Posted 29 July 2021 - 02:29 PM

AstroMaster 102 it is

#20 Redbetter

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Posted 29 July 2021 - 05:25 PM

That depends on what kind of 4" achro we're talking about... 

 

No, it really doesn't.  Diffraction rings should be clear and obvious by 200x with any decent 4".  The OP is not even seeing the diffraction pattern at this point and describing it as a "deformed smudge" so going to 300x would just be blowing up a mess.  

 

Beyond about 200x with a 4" one is only enlarging the diffraction pattern for deeper inspection, achro or apo.  Useful for very tight double stars, but certainly not a starting point for this discussion.

 

Anyway, the OP now says it as an Astromaster 102.  It looks like that comes with a 10mm eyepiece.  That is a 1.6 mm exit pupil, so unlikely a novice will see the airy pattern in a fast achro.  A 2x Barlow should begin to reveal the diffraction pattern.  Unfortunately, it looks like the scope has the basic RACI diagonal, so that will complicate star testing.  This might be the source of the deformed smudge seen at 66x, but we are only getting snippets of information at a time.


Edited by Redbetter, 29 July 2021 - 09:24 PM.

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#21 Voyager 3

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Posted 30 July 2021 - 12:15 AM

If you don't have higher power eyepieces , try pointing the telescope at the brightest stars . Vega will be a good one now . 



#22 Astrojensen

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Posted 30 July 2021 - 01:32 AM

No, it really doesn't.  Diffraction rings should be clear and obvious by 200x with any decent 4".

  

What I meant was that there's a huge difference between something like a 4" f/15 Unitron/Clark/Zeiss/etc. or a 4" f/6 Chinasomething. A 4" f/6 is NOT, by its very nature, going to be able to give as clear an image as a 4" f/15.

 

 

Clear skies!

Thomas, Denmark



#23 Buqibu

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Posted 30 July 2021 - 08:44 AM

I have a barlow and a Celestron Zoom. I've gone up to 165x, still just a smudge. Could be the seeing, could be the scope. Anyway I appriciate the help. I've also tried without the diagonal, same story.

#24 BruceNewEngland

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Posted 01 August 2021 - 09:07 PM

 It’d be easier if you could get above 200x, but in the meantime try looking at the image of the star while adjusting it out of focus on each side of perfect. You should see the rings, then adjust back to perfect - watching the rings shrink. Now that you know what to look for, can you still see them? If not, try higher power, viewing stars nearly overhead, and when seeing is forecast to be reasonably good.



#25 Buqibu

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Posted 02 August 2021 - 04:27 AM

Yeah, I can see the rings when out of focus


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