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Kaptain Klevtsov

Diffraction rings - what do they tell you?

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I was having a little play last night with the Tal looking down the little whatchamacallits that came with it. I aimed it at Arcturus and messed about with the focus to check the collimation. As I understand it, the out of focus image should form a series of rings, and if these are concentric then all is well with collimation. They were perfectly symmetrical, but there were some narrow rings and a wide ring. Can I interpret these rings as meaning something, or do they just happen? I've Googled about a bit and, apart from learning how to debunk ghostly orb images, I've not managed to learn a lot. Any tips, links or opinions?

I'll grab a photo of the rings if there is anything significant to be deduced from the rings once it stops raining.

Kaptain Klevtsov

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IIRC the thin rings were the optics and the thicker one was caused by the central obstruction. You're right, they should be concentric on both sides of focus when the scope is collimated and the mirrors are correctly figured.

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The diffraction rings in a centrally obstructed scope show a number of things. The central dark circle is clearly the secondary mirror. The concentric rings are, in effect, interference patterns with the incoming wavefront and the edges of the aperture. As such, they become thinner as they approach the innermost ring on a more or less logarithmic scale.

All sorts of information about the quality of your optics can be gleaned from careful study of these rings as they appear on both sides of focus. I highly recommend a book called, "Star Testing Astronomical Telescopes" by someone named Suter (?) Richard? Bruce? Something, can't recall. It's quite dry at times, but really shows you how to star test your scope and troubleshoot if you have spherical aberation or just tube currents.

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Kaptain,

This is my first post on this site and I,m sorry to say that it has to be a negative one. I,ve been waiting for Sutor's book "Star testing etc" since March, I wanted to take it on holiday at Easter! I,ve ordered it from Earth and Sky in Norfolk but so far they have been unable to obtain a copy from the States. Not to cast aspersions on a man of your rank but be warned that this is supposed to be quite a difficult book. Maybe it's as well that I didn't get it on time, it could have spoiled the holiday. As far as I know Earth and Sky will be your only hope.

Regards Dave Cooke

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Thanks Dave for the info. I've got loads of books that I don't understand already so if another turns up I'll have a huge pile. :)

I remembered today the Geoptik artificial star review in Practical Astronomer had loads of out of focus images, I'll have to read all of that and see if anything means anything to me.

Kaptain klevtsov

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AM has explained it very well KK. Light from a star, comes from a distance so far off, it can be considered a point source. However perfect the optical system, the star cannot be focused to a single point. Instead, the light forms a small disc. The waves of light about .4 to .65 microns in length, are wriggling about in all directions laterally. The stone in the pond analagy. The disc is surrounded by dark rings which are caused by waves cancelling each other out, the crest of one, falling on the hollow of another. Between the dark rings, are the bright ones, where the waves reinforce one another. crest upon crest. The bright spot of light and it's attendant rings, is called the diffraction pattern, or disc, and is formed by each tiny area of the mirror reflecting rays back to the same spot. The more perfect the mirrors surface, the smaller and brighter the disc will be, and the better the image formed. Of course you don't need me to tell you I am talking about the Airy Disc. named after the man who calculated it's size George Airy in 1834

More Info if you need it.

Ron. :)

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If you have more Ron, I'd be interested. I've looked at the article in Practical Astronomer and some of the images are a bit similar to what I think I saw as I was only looking to see if the rings were concentric. As I recall, there were a series of rings of varying widths (or thicknesses?) depending on how far in or out I moved the focuser. I'm just curious what it all might mean a the article refers to undercorrection and other stuff I know nothing about.

Kaptain Klevtsov

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KK, the interference patterns you see in the unfocused image are not really telling you anything to enable you to quantify your optics if that is what you are seeking to do. That information lies in the focused image, and the surrounding 16% or so of the light that is not in the Airy disc.

The human eye is not sensitive enough, although there is something about using a bright star with faint companion which will allow some conclusion to be drawn, without resorting to lab tests of the optics.

You mentioned earlier about a collection of books which you have, and not got around to reading, or something like that.

Astroman made reference to a book also, which would illuminate the subject for you. (No Pun Intended Here).

If you are keen to follow up on this, and rather than get into a long drawn out thread on this subject, I have a book by Allan Mackintosh.

Advanced Telecope Making Techniques, which I am sure you will appreciate. You are welcome to borrow it for as long as you need it.

Just let me know by PM, and I will post it off to you.

Ron.

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That's most kind of you Ron, and I will indeed PM you my contact details shortly.

What caught my eye specifically today when reading the Practical Astronomer article is the bit where it states that things such as astigmatism and surface roughnes can be assessed by checking the out of focus images. Not that I could do anything about such errors, but it would be interesting to compare the different 'scopes that I have.

Kaptain Klevtsov

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That's most kind of you Ron, and I will indeed PM you my contact details shortly.

What caught my eye specifically today when reading the Practical Astronomer article is the bit where it states that things such as astigmatism and surface roughnes can be assessed by checking the out of focus images. Not that I could do anything about such errors, but it would be interesting to compare the different 'scopes that I have.

Kaptain Klevtsov

PM sent KK. :)

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I know this isn't a public library but could I be next on the list Ron? :)

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KK,

The patterns you see in the PA article sound very much like the ones you'd see in Sutor's book. Barkis is correct also in that only lab tests such as Focault tests can determine exact quality. The out of focus images can point to problems, but not measure them. Some OOF images may look like several different problems, but they can help sometimes.

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I know this isn't a public library but could I be next on the list Ron? :)

Sure thing Gaz. Just make arrangements with KK, and have him post it to your home address when he has finished with it. Don't pressure him though, he hasn't even got it yet :)

:o

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Don't pressure him though, he hasn't even got it yet :)

:o

I was going to call round at the weekend...you think thats a bad idea? :)

KK, just let me know when you're done, no rush!! :D

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