Airy Disc

[Stub Mandrel]

The Wikipedia article on Airy Disc popped into my mind last night after I focussed on Altair then removed my bahtinov mask, as I could see a flickering ring around teh central part.

I took some video, then stacked 50% for about 2500 frames.

Then I hit it with registax wavelets. Looks like my collimation is a tad off, but not too bad?

 

[Ben the Ignorant]

53 minutes ago, Stub Mandrel said:

Looks like my collimation is a tad off, but not too bad?

Sorry but it looks rather bad. If you can, try again with a defocus on both sides, defocus till you see only two or three rings around the central dot. More defocus is too forgiving and harder to read; two rings might make the figure too small and too much blurred by turbulence, but the three rings pattern is usually large enough to be readable, and steady enough.

[Stub Mandrel]

22 minutes ago, Ben the Ignorant said:

Sorry but it looks rather bad. If you can, try again with a defocus on both sides, defocus till you see only two or three rings around the central dot. More defocus is too forgiving and harder to read; two rings might make the figure too small and too much blurred by turbulence, but the three rings pattern is usually large enough to be readable, and steady enough.

Problem is that I could only make out one ring without the stacking and processing because of the seeing.

[Ben the Ignorant]

Nope, I'm talking about the rings you see when you defocus. Try it, you will learn something new. The defocus rings are always easier to see than the rings around the focused star because they are much larger. Try different amounts of defocus and you will see. A large defocus makes them disappear save for the external ring, they look like a continuous disk. The smaller the defocus the more collimation errors are plain to see, until the pattern becomes so small (very near focus and at focus) it's hard to read.

Learning to interpret defocus patterns is essential for a telescopist.

[vlaiv]

I'm not sure how much is it down to collimation.

Image is a bit too stretched to show properly. Can you do another one, but this time try not to "burn out" first ring?

Besides central airy disk position, good indicator of collimation is intensity of first ring - if it is uniform then collimation is close to perfect. On the other hand if it is broken in one place or missing a half then collimation is quite off.

These images will be a good guide:

Lower part of first ring is "missing" (very low brightness, and it appears as broken, other rings too).

Here are some real images (not simulated) of airy disk that show different aberrations (most due to seeing):

So while above image may seem like a bit poor collimation, it is not necessarily so, bad seeing / stacking artifacts and even tube currents can distort central airy disk and break rings in certain way.

[Stub Mandrel]

Here you go, one untouched stack, one stretched a bit:

Are these hinting at a pinched mirror?

 

[vlaiv]

10 minutes ago, Stub Mandrel said:

Here you go, one untouched stack, one stretched a bit:

Are these hinting at a pinched mirror?

 

It kind of looks like pinched optics, but rather slightly. It can also come from tube currents. Maybe even mirror clips can produce similar effect.

Just ran a sim to see if mirror clips can produce something similar - but no, they are too small to have such effect on airy disk.

So I would say two possible causes - either it is indeed pinched optics, or scope was not really properly cooled and some tube currents were still present.

Given that you were just focusing the scope - that would indicate beginning of the session, so some residual heat might still be there.

To be absolutely sure - do the same thing next time (before focusing) - but defocus to show couple of rings and record it.

You can stack those as well and examine result, or you can observe live video to see if there is any "shimmering" of image to indicate tube currents (it looks a bit like smoke rising on one edge of defocused image).

There is a great video to show this effect (although defocus is much bigger here):

Alternatively if you are worried about state of your optics, why don't you do Roddier test yourself?

You have all the gear that you need, and WinRoddier 3.0 is free and works well - just take your ASI120 and shoot defocused star (in and out focus), stack (there are couple of options that you need to tweak in stacking but there is excellent guide how to do it) and import in program - it will give you airy disk as well as all the parameters of optical system (wavefront diagram, etc ...).

[Stub Mandrel]

I have no view on how good my optics are. I know when out of focus I get concentric circles. But I think I need to skip the x3 barlow for tests, and choose steadier seeing. These are the 100 best frames (Wow just discovered I can upload an MP4 in an Avi container 143kB, rather less than  the >half gig original)

[vlaiv]

1 hour ago, Stub Mandrel said:

I have no view on how good my optics are. I know when out of focus I get concentric circles. But I think I need to skip the x3 barlow for tests, and choose steadier seeing. These are the 100 best frames (Wow just discovered I can upload an MP4 in an Avi container 143kB, rather less than  the >half gig original)

 

2018-06-29-2248_2 Altair_pipp_pipp.avi

This movie puts things into perspective

Most of distortion is down to seeing and the way stacking works, it probably does not reflect true airy disk of optic faithfully. But it does show that stacking + wavelets works wonders to recover most of the detail

As for optics, like I've said - Roddier test is really interesting to do - not complicated at all, almost like a short planetary session (x2 short movie of around couple of hundred of frames each, a bit of stacking and after running a program with produced stacks). If you are curious and want to give it a go, just go for it, there are really nice docs (pdf) to be found online (yahoo roddier group is best source for both software and documentation). If you wish, I'll be more than willing to provide pointers and pass on my experience with this technique to help you get started.

[Stub Mandrel]

Thanks, I'll capture some test footage next time I'm focusing up.

[NickK]

You could use that stacked raw PSF and deblur images affected with it... 

[Stub Mandrel]

3 hours ago, NickK said:

You could use that stacked raw PSF and deblur images affected with it... 

I did wonder about that, of course it's hugely over-exposed compared to the planetary images. This is a processed image, same night, same setup and same unbinned/undrizzled pixel scale:

[NickK]

Oversaturation does mean you loose measured information, just loading the unstretched PSF into PI & the image, channel splitting, running seperate RGB deconv, then recombining (loosing some detail in png) but looking carefully I think it could work but I think trying to source from PNG has already killed it: