Are these halos caused by good seeing and diffraction patterns?

Yep that looks like an airy disk. Have the same thing on my SCT as you can see on the bright star on the bottom right. A good way to confirm is to see if they rainbow out with different color filters. If you see a rainbow its diffraction, if you don’t its reflection.

📷 Airydisk.png

As per usual advice, keep the frames with round sharp stars and toss the rest. They really shouldn’t change much assuming guiding is good and the collimation isn’t changing.

I would stack up the frames you want to keep and then see what it looks like. My final image has the star I showed before as the orange one at the bottom. With normal star processing you aren’t stretching them that far so the rings will likely only be subtle. Not a big enough deal in my opinion to remove them or do any special type of processing to deal with it.

https://app.astrobin.com/u/Young_Astronomer?i=hij89n#gallery

John Hayes · Sep 29, 2025, 03:28 AM

Anthony,

It is entirely possible to see the Airy patten with a 4” inch refractor under good seeing. You definitely need stable conditions to image it using a long exposure. It looks like you got it…so good stuff!

The patten that Naveen imaged with his 8” SCT is due to diffraction but it’s unlikely that his image is showing the separation between adjacent diffraction rings. The PSF of an obscured aperture is given by the difference between two Bessel function that have slightly different spatial frequencies in the rings. That creates a beat frequency between the two patterns that has a larger spacing between the rings than what you would get from an unobscured aperture of the same diameter. That makes it much easier to see the rings. In fact, the ring pattern that Naveen showed probably has very nearly the same spacing as for your 4” scope. So, the seeing still had to be pretty good to see it but it’s an apples to oranges comparison. They are both relatively hard to see unless the seeing is good but the image from the 8” probably isn’t really showing the fully resolved Airy pattern.

John

Thanks for the info john I guess I should have mentioned this was with my cdk14 , and like I said I can clearly see a difference in the halo extent when switching filters so it all seems to line up.

This is an OIII sub from a recent project I just completed and I wasn’t sure if this was a mix of the airy pattern and some diffraction micro lensing effects because this is the center of OU4 and the star is rather brightOIII halosmall.png

Anthony Grillo · Sep 29, 2025, 03:34 AM

I should have mentioned this was with my cdk14 , and like I said I can clearly see a difference in the halo extent when switching filters so it all seems to line up.

This is an OIII sub from a recent project I just completed and I wasn’t sure if this was a mix of the airy pattern and some diffraction micro lensing effects because this is the center of OU4 and the star is rather brightOIII halosmall.png

Ah…that’s very different. In that case, it is extremely unlikely that you are seeing the fully resolved Air pattern. You are seeing the aliased pattern, which is still caused by diffraction but that’s not the same as resolving the full pattern. What you imaged does not appear to be due to mirco-lensing, which produces a more rectangular pattern instead of a radially symmetric pattern. The brightness of the star is certainly making the pattern brighter and easier to image.

BTW, if this pattern is objectionable, it should be possible to mostly eliminate it by apodizeing the central obscuration. The hard part is that you’d have to add a gradient filter going from T=1 to T=0 over maybe 1/2” around the secondary mirror to eliminate the sharp shadow around the obscuration. This could be done with a thin film or maybe with a specially made optical pellicle. It would be interesting to try to compute this maybe using a Gaussian or triangular apodizing filter. It’s probably not worth much. It’s just an interesting way to maybe get rid of these sort of fringes.

John

John Hayes · Sep 29, 2025, 03:28 AM

The PSF of an obscured aperture is given by the difference between two Bessel function that have slightly different spatial frequencies in the rings. That creates a beat frequency between the two patterns that has a larger spacing between the rings than what you would get from an unobscured aperture of the same diameter.

That’s a beautifully intuitive explanation! I’m familiar with the equation for the obscured Airy pattern but I had never thought of it in terms of the beat frequency.