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How many diffraction rings?

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#1 Kon Dealer

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Posted 29 April 2018 - 09:10 AM

When I focus my refractor on a bright star, like Arcturus, I get a nice, sharp Airey disc, surrounded by 3, symmetrical diffraction rings. 

Is this too many?



#2 DLuders

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Posted 29 April 2018 - 09:31 AM

The number of diffraction rings don't seem to be important, compared to the fact that the rings are SYMMETRIC.  There are 6 diffraction rings on the very same star in this video:   https://youtu.be/bZV_g57pto4?t=1m12s



#3 garret

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Posted 29 April 2018 - 09:33 AM

I'm not the professor here but according to Harold Richard Suiter book 'Star Testing' (1994) page 29 the Spherical aberration of your telescope is a bit high, you should see just one...

 

Garrett



#4 DLuders

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Posted 29 April 2018 - 09:37 AM

...Spherical aberration of your telescope is a bit high, you should see just one...

Hmmm, there are good pictures of Spherical Aberration (with differing numbers of diffraction rings) starting 1/2 way down the webpage  http://www.telescope...g_telescope.htm



#5 gfeulner

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Posted 29 April 2018 - 09:48 AM

When I focus my refractor on a bright star, like Arcturus, I get a nice, sharp Airey disc, surrounded by 3, symmetrical diffraction rings. 

Is this too many?

Are you refering to Arcturus in focus or like it shows in the video where Arcturus is out of focus?

 Gerry



#6 JamesMStephens

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Posted 29 April 2018 - 09:50 AM

I think Garrett and DLuders is right, if more energy appears in the rings than the central disk might indicate SA and a more detailed star test would be profitable.

 

This is a tangential question for CN people, I'm curious about something.  What is the faintest star that shows a diffraction ring visually in your scope?  Yes, I know they're always there and in a given scope are identical for every star, but I'm interested in what people see visually.  


Edited by JamesMStephens, 29 April 2018 - 09:51 AM.


#7 Richard Whalen

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Posted 29 April 2018 - 10:00 AM

Under excellent conditions I have seen one faint diffraction ring at high magnification around the E & F stars in the trapizium in my 8" f15.5


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#8 Eddgie

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Posted 29 April 2018 - 01:06 PM

In a perfect unobstructed instrument only a small amount of energy goes into the first diffraction ring, and second and third rings receive almost too little energy to be easily see.

 

Operative word here is "easily."

 

On a very bright star under very dark sky conditions, seeing three rings would probably be possible.   

 

Here is an Aberrator 3 simulation of a perfect 4" telescope under perfect conditions, and I can easily the first two rings in the model, and the third ring is faint, but visible.

 

Rings 2.png

 

Now any small amount of SA would make the outer ring brighter, but to really know if you have SA, you would have to do a star test with a 33% obstruction.  Other than that, there is not really any way to know if you have SA and even if you do, to know how much SA is present.

 

(There are other methods of testing like Double Pass Ronchi - single pass is almost useless - Roddier testing, and interferometer testing.  Star testing though is something that anyone that takes the time to learn to do properly can do with no special equipment).

 

But clearly, under excellent conditions, you should be able to see three rings, thought the outer ring should be difficult.


Edited by Eddgie, 29 April 2018 - 01:13 PM.

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#9 elwaine

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Posted 29 April 2018 - 01:25 PM

Seems to be terminology confusion here.

 

The Airey Disc and it's diffraction rings are seen in a well focused star image when viewed under high magnification. That is what the OP and Eddgie are referring to. DLuders is describing the diffraction rings seen in an out of focus star image viewed under moderate powers of magnification. 


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#10 Eddgie

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Posted 29 April 2018 - 07:45 PM

Yes, the OP was very clear and specific to say that the instrument was in focus.

 

 

The Youtube video of an out of focus star did not appear to me to have anything to do with the OPs question because he said he was in focus and could see the Airy disk and sharp diffraction rings. 

 

If he is in focus and his skies are dark, and the star bright enough, It might be possible to see the third ring, but it would be extremely dim in an excellent scope and perhaps a bit brighter in a scope with some SA or spherochromatism or combination of the two.

 

Had to know anything about whether the OPs scope has a problem or not.   If there is any doubt, further testing may be required to determine if an error exists.

 

If the third ring is not particularly easy to see though, then I would not worry about it. 



#11 Eddgie

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Posted 29 April 2018 - 07:50 PM

Also, it may be necessary to tilt an LCD monitor or adjust brightness and contrast to see the three rings in the simulation shown above.  Oddly I can't see the three rings on my Alienware laptop, but can see them on my cheap Lenovo.


Edited by Eddgie, 29 April 2018 - 07:53 PM.

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#12 Peter Besenbruch

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Posted 30 April 2018 - 01:06 AM

When I focus my refractor on a bright star, like Arcturus, I get a nice, sharp Airey disc, surrounded by 3, symmetrical diffraction rings. 

Is this too many?

On a very bright star like Arcturus three rings is normal when in focus.


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#13 T1R2

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Posted 30 April 2018 - 01:31 AM

Try a star like Deneb or another 2-3rd mag star or even slightly fainter, I'm sure you will see a big difference


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#14 mikeDnight

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Posted 30 April 2018 - 02:20 AM

Diffraction rings go on forever, but in a well figured refractor the first diffraction ring is the brightest and often the only one that stands out. Nearly all the light in a refractor is pumped into the Airy disc, so the diffraction rings become almost invisible, but they are there!

Obviously the brighter the star the brighter the diffraction rings. They should become less obvious as you move out from the Airy disc but if there is a sudden brightening of a ring away from the disc, you have zonal problems. If your diffraction rings are virtually identical either side of focus, then I wouldn't worry about seeing more than the first diffraction ring on a star as bright as Arcturus. 



#15 Asbytec

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Posted 30 April 2018 - 02:52 AM

We have to remember, though the amount of light in the rings is set by diffraction and aberration, the brightness of the first ring is really a function of our visual response. It will appear brighter than it really is at say 7% of the total energy and 84% in the central Airy disc.

Seeing faint rings is also highly dependent on seeing. Its possible someone will note only one ring from Arcturus and never see the second or possibly a third ring disturbed by the slightest seeing.

In my undercorrected and obstucted scope, I see up to five rings on Arcturus when seeing is very calm. Only the first has a significant visual brightness, appearing as bright as the central disc.

There are two moderately bright, very thin rings just outside the first. But often they are just tiny disturbed arcs in all but the absolute best seeing. Outside those two are two more faint rings, the fifth and outermost being the most elusive in seeing.

So, the score may be unobstructed and nearly perfect 3, undercorrected and obstructed 5. Less in average seeing conditions.

#16 daquad

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Posted 30 April 2018 - 11:37 AM

It might be instructive to address the OP's question based on the theoretical intensity of the Airy diffraction pattern.

 

The intensities of the first three rings and the resulting magnitude loss with respect to (wrto) the central maximum are as follows:

 

Data from Jenkins and White, Fundamentals of Optics, McGraw-Hill, 1957.

 

1st ring:   0.0175 wrto central maximum.  Magnitude loss wrto central maximum: 4.4

 

2nd ring:  0.00416,  Magnitude loss: 6.0

 

3rd ring:   0.0016,  Magnitude loss: 7.0

 

For the magnitude loss I used   2.5log(intensity ratio).

 

The magnitude of Arcturus is 0.2.  

 

These values would seem to indicate that for Arcturus the third ring should be easily visible.  

 

For a 6" refractor, the limiting magnitude in good seeing and 6.0 mag skies is around 13.0.  So the diffraction rings of a 6th magnitude star should be barely visible in a 6" refractor in 6.0 magnitude skies in good seeing.  

 

An 8" refractor under the same conditions should go about 0.5 magnitude deeper than a 6", so about 13.5.  The E and F stars in the Trapezium are ~11.0 magnitude, so the first ring  ( about magnitude 15.4) would appear to be not visible in an 8" refractor under 6th magnitude skies in good seeing.

 

However,  it is well known that the theoretical limiting magnitude of a telescope can often be exceeded by a full magnitude or more with good seeing, and high enough power, which darkens the sky background.  So Richard Whalen's observation with his 8" refractor, is certainly one that can be reasonably expected, especially since he used the caveat, "in excellent seeing."

 

Dom Q.


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#17 Kon Dealer

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Posted 02 May 2018 - 02:10 AM

Thanks for all these helpful responses.

I’ll try a fainter star and see what it looks like.

However I think I have a touch of SA. 



#18 mikeDnight

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Posted 02 May 2018 - 02:54 AM

Thanks for all these helpful responses.

I’ll try a fainter star and see what it looks like.

However I think I have a touch of SA. 

After reading about your concerns, and the responses you've had, I took a careful look at Antares the other night through my Tak FC100DC. The seeing was a little wobbly but at X200 there were several very fine diffraction rings visible, each one fainter than the one preceding it. 

It's perfectly normal to see more than the first diffraction ring on brighter stars, so I wouldn't lose sleep over your scopes optical quality. Also, restricting the colour spread in ED or apochromatic optics can give the illusion of SA when there is none. 


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#19 Allan Wade

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Posted 02 May 2018 - 09:05 AM

Seems to be terminology confusion here.

 

The Airey Disc and it's diffraction rings are seen in a well focused star image when viewed under high magnification. That is what the OP and Eddgie are referring to. DLuders is describing the diffraction rings seen in an out of focus star image viewed under moderate powers of magnification. 

That's mostly correct, but the rings visible in an out of focus star image are called Fresnel Rings.


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#20 elwaine

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Posted 02 May 2018 - 09:47 AM

That's mostly correct, but the rings visible in an out of focus star image are called Fresnel Rings.

Thanks for the correction, Allan. The physics of optics is way above my understanding and I am certain that your knowledge runs rings around my own. wink.gif  Yet, even though diffraction can be subdivided into Fresnel diffraction and Fraunhofer diffraction, the rings seen with in-focus and out-focus are commonly referred to simply as a diffraction pattern. Here is just one of dozens of articles referring to those rings as a diffraction pattern (without specifying which type they are.) I'm just happy I can differentiate between the patterns I see with a well focused star at high mag. vs. the in-focus and out-focus rings I see at moderate mag. Beyond that, I'm lost in space.

 

Cheers,



#21 Allan Wade

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Posted 02 May 2018 - 11:35 AM

Very good Larry, we are probably both correct enough to call it splitting hairs.




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