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Collimation and Airy Disk

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#1 garyp1936

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Posted 07 November 2010 - 10:27 PM

I admit I am confused about the Airy Disk. I have read several articles about collimating an SCT--some of them show the airy disk as a central WHITE disk (David Knisely, Thierry Legault) while others (Uncle Rod's Zen Collimation, Steve Walters) show the disk as BLACK. My 8SE at 300-400X, both inside and outside of focus shows a DARK central disk surrounded by alternating light and dark rings. The disk appears centralized and the rings form concentric circles. Is the collimation on the mark? Thierry Legault say the rings should be of "decreasing brightness", but he show the disk as white. So where lies the truth--white disk or black disk???

#2 MrJones

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Posted 07 November 2010 - 10:54 PM

The Thierry Legault page at http://www.astrosurf...ult/collim.html explains it pretty well for SCTs.

First set of figures are defocused low magnification where the "hole" in the middle is the secondary.

The second figures are high magnification defocused, except for the middle ones which are in focus.

The third set of figures are in focus Airy patterns where Fig. A is what you want to be seeing.

The Airy disc is the center spot of Fig. A, the Airy pattern is the diffraction rings around it.

There are better people here than me to tell you the rest but I've been playing with higher magnification and just slight defocussing as some people here suggest for fine collimation and it seems to work well.

#3 Jim-M

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Posted 07 November 2010 - 11:11 PM

I'll try, as I am still new at this and was confused myself at the start.
The airy disk is the central bright spot at perfect focus. In near perfect seeing, and high magnification (300-400x with the 8SE) you may see the faint light concentric circles around the airy disk that are the diffraction rings.

The black disk seen inside or outside of focus, is an artifact (the shadow) of the secondary I believe. Uncle Rod describes this as the "donut hole". It is not an airy disk, but is very useful for rough and medium collimation. The final collimation steps are done with the airy disk at focus, if the seeing is excellent. See Choosing and Using a New CAT, pages 226,227. I don't get many days with the seeing good enough to really see the airy disk and diffraction pattern well enough for this final collimation step. Hopefully you will have better nights.

#4 Eddgie

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Posted 08 November 2010 - 09:16 AM

Jim has it right.

The Airy Disk is only visible when the telescope is in or very near perfect focus.

Due to the wave nature of light, the circlar opening at the front of the tube formed by the ring that holds in the front corrector is defined as the "Aperture".

Since the ring is round, as light passes though the opening, some will be very slightly diffracted.

The result is that the telescope is not able to focus the light from a star to a perfect point.

Instead, it form the ring into a tiny spot which is very bright at the center, and fades around the edges.

IT finally fades to the point were there is no light, then again due to the wave nature of light, the intensity starts to brighten and form a ring around this central spot. We call this the first diffraction ring, and as the name implies, there are other rings beyond this, but the are usually to dim to be easily seen.

As you move the focuser, the shadow of the secondary mirror will start to emerge. This shadow is visible as a dark circle that grows larger and larger as you defocus. This pattern is called the Fresnel pattern.

If you look clostly at this time, there will be a tiny little point at the center called the Poisson point. You can also collimate using the Poisson point with only one bright ring around it.

This is actually more accurate than using a "Doughnot." The Dougnut should only be used for rough collimation.

If seeing isn't good enough to see an in-focus image, after doing a rough collimation, turn the focuser so that the pattern collapses until there is a VERY bright ring and maybe a very faint ring, with the Poisson point visible as a bright little spot in the center.

Collimate again so the the Poisson point is as perfeectly centered in the bright ring as possible. This will usually get your collimation much closer than using the doughnut method.

So the in focus image of a star is normally a very small disk with a faint ring around it called the Airy pattern, and the docused disk with several rings showing is called the Fresnel pattern.

The Centering the shadow in the Fresnel pattern is sufficient for rough collimation, but doint the final collimation in focus is the most accurate.

If the seeing is not good enough to see the Airy pattern while in focus, defocus a TINY amount until yo see a bright ring and the Poisson point, and tweak until the Poisson point is as perfectly centered in the bright ring as possible.. This will get you pretty darn close.

#5 garyp1936

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Posted 08 November 2010 - 11:34 AM

Thanks for the replies. I was mistakenly looking for the airy disk when the scope was OUT of focus. :foreheadslap: Now I know. Thanks again.

#6 gnowellsct

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Posted 09 November 2010 - 12:27 AM

The bigger the aperture the less likely you are to see the Airy. On the C14 group with 800+ members, only one person claimed to have seen an Airy and then only once. On the Bigdob Yahoo group I asked the same question. There, no one had seen an Airy disk (in their larger scopes: I can't *avoid* seeing it in my refractors). If you want to see an Airy look through a four or five inch refractor.

regards
Greg N

#7 Eddgie

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Posted 09 November 2010 - 09:24 AM

I see Airy pattern frequenly in my C14 during the summer, and the rest of the year, if you can stay up quite late so that the atmosphere settles and the optics cool, it is not unusual to see it.

It is rarely steady though. Almost always in motion.. The first ring forms and breaks, or there will be a shimmering where the ring seems to pulsate like a wave rolling in front of it.

On my C14, the ring is quite even and very faint in comparision to many of the SCTS (and even some refractors) I have owned. The start test for my C14 indicates a really high level of spherical abberation correction, so the only contributor to the first' rings brigtness is the central obstruction, which is also a bit smaller as a percentage of aperture than any other commercial SCT in production.

It is amazing to view with the telelscope when the conditions are like this.. I have split doubles that wouldn't even resolve in my 6" APO. I mean CLOOOOOOOOSE doubles.

So, easy to see Airy pattern when conditions are decent, but rare to ever see one where the first ring is completly steady. Have seen a steady first ring, but it is very rare. Seeing a disturbed first ring though is pretty common for me. Scope needs to be cooled though, and atmosphere has to be fairly steady.

And when it all works, the scope gives the "Refractor like" view that refractor people talk about all the time. The big aperture, seeing, and cooling are the main reasons that the SCTs are often denounced as being good telescopes. Sometimes it is the optical quality, but most SCTs are at least decent. The softness of stars that people report has NOTHING to do with optical quality. Even a BAD telescope will show a clear Airy pattern when the other conditions are right.

#8 Don Trinko

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Posted 09 November 2010 - 09:30 AM

Thanks for asking this question. I have always had a problem seeing the "airy disk". From the above information I was looking for something that was not their under my focus conditions. Thanks; Don T.

#9 deSitter

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Posted 09 November 2010 - 10:06 AM

A scope with astigmatism or a really uneven (not rotationally symmetrical) figure cannot by definition form a symmetrical Airy pattern - other than that one technicality, what Eddgie said. A scope with terrible SA will show a fantastic system of rings that merge into each other on passing best focus like a kaleidoscope.

-drl






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