Airy Disk interpretation

[Wiu-Wiu]

I have been using the airy disk to check collimation of my scopes, easy peasy: just keep trying to get the dot in the center and all rings concentric.

Recently, someone showed me how to recognise seeing by using a defocused star: you can actually see some kind of "wind" passing through the light. 

 

Now, I upgraded my comacorrector and eyepieces, and got into a whole other league. 

When using extreme magnification, (bright) stars tend to stop being points, but rather become fiery blobs. When trying to reach the best focus, the airy disk transforms in a propellor-like shape, with a bunch of dark lines over it, The best way to describe this is a thumbprint. 

Is this telling me something about my optic system? I have been looking into it, and thought one could interpret these to define the level of the optics, but I can't seem to find anything like it when looking for pictures that show what I saw, or explain me how to interpret the airy disks hidden signs.

 

Can anyone shed any light on this? 

[Peter Drew]

It's quite common to confuse the Airy disc with extrafocal rings. Extrafocal rings can be seen even at low magnification and the expanded star "disc" can be used to examine the collimation and the degree of ambient turbulence as you have found. Observers with suitable experience can also use this procedure to check the optical quality as deviations from perfection show up as anomalies in the rings. The Airy disc itself is seen only under high magnification and excellent seeing conditions, the larger the optics the more difficult it is to see this. More often than not, the disc will be irregular in shape and in some degree of motion. The "propeller" shape can be due to the optics not being cooled to the ambient temperature or even the mirror retaining clips being too tight.  

[Wiu-Wiu]

Thank you for the reply;

I should have specified more details: it’s a 12” dob, and he wasn’t cooled, indeed, as I was just trying to find the right distance for my CC - and do some quick testing. 

The mirror clips don’t touch the mirror, it is resting on 3 see-saws (6 point layup)

so was I looking at something in particular or was this just random noise? 

[westmarch]

Hi,

I found this article helpful. Do any of the illustrations resemble what you are seeing? 

https://popastro.com/documents/PA_jan-mar2009_p12-13_telescopetopics_startesting.pdf

Hth

John

[Wiu-Wiu]

Interesting link, thank you!

I can't find anything quite like it, but if I have to guess based on the pictures it would be some combination of bad seeing and insufficient cooling. Yet the dark lines on it were way more prominent.