'Donut' diffraction ring

[oxygene (Michael)]

Hello!

And welcome to my first post! I've been reading threads on this forum for a while but it was only once I hit a snag that I thought I would join in.... how selfish of me!

Anyhow....

I've hit an issue that I've scoured the net for and have not found any solution. I have an image (attached) that shows multiple donuts along the Dec axis (also the RA axis but not as pronounced. The image is overexposed but it is also apparent when I median 50 or so well-exposed images together as well. (Easier to see in the overexposed though)

I am thinking it is an issue somewhere in the optics... either reflections off the side of the optical tube or reflections off the filter. It looks somewhat like a reflection of the lens and secondary mirror but I cannot be sure, and nobody on the net seems to have had this issue!

Has anybody here seen this before or could hazard a guess at what it is?

I have a 12" Meade LX200 Classic with an ST7 with custom scientific RGBC filters.

Thanks in advance and hello everybody!

Michael

[Wavytone]

My first guess is you have a refracting element in the path which has at least one or both sides that are not AR coated, and is not perfectly perpendicular to the optical axis - what you are seeing is multiple reflections off that. The stray reflections are also slightly out of focus meaning there must be a path length difference involved with respect to the primary image.

To my eye the reflections suggest that if there is a clear glass element somewhere in the path and the surfaces are not exactly optically flat and plane-parallel - a few fringes of wedge will produce what you're seeing if the surfaces are not anti-reflection coated.

You mention a filter, let me guess - your OTA sealed with a clear glass UV filter ?

[Don Pensack]

I vote for the eyepiece if this was shot through an eyepiece. If not, and you do have a 1A "Skylight" filter on the back of the scope--REMOVE IT. It isn't necessary for any nighttime use.
Custom Scientific filters are usually Anti-reflection coated, but that doesn't mean the filter slide isn't tilted relative to the optical path.
You can check out some of the sources for the reflections by focusing on the sun's reflection on a ball bearing during the day (it should be a hundred meters or so from the scope).
Also, check the collimation of the scope. Slight miscollimations can cause reflective problems.
Then it's a matter of tracking down the reflections. Due to the small size of the reflections, start close to the focal plane. Check the blackness of the inside of the camera adapter, the inside of the primary mirror's baffle tube, the outside of the primary mirror's baffle tube, the inside of the secondary baffle tube.
If none of those help, it is probably in the camera itself. I've seen several problems like this when there was dirt on the chip--especially dirt particles near a bright star's position (like in your photo).
I had an issue with a reflection in an SCT that led me to flock the inside of the tube, the inside of the secondary baffle, the outside and inside of the primary baffle, and the star diagonal, only to discover the issue was a shiny bevel on the edge of the primary mirror. When blackened with paint it went away.
But that was a ring 1/2 the diameter of the field of view. In your case the highest likelihood is a reflective surface somewhere in the chain.
Just be systematic and track them down one at a time until you determine the cause.
And post here when you find it.

[Terry B]

I get the same thing on very bright objects like planets. Mine is caused by blooming in the KAF0401E CCD. A line of little images will run the full length of the frame if you over expose enough.