help! I have no idea what this set of images means

[MarkInSpace (Mark)]

I have a new (used) RC-8 CF. The out of focus star seems good after collimation (see pic).
But...
look at the ugly asymmetrical flat and what is causing these airy disk like effects on the big stars in the m20 image?
Is the scope out of collimation? If so, why can I see fine details like the M20 jet? What else could it be?

[bojan]

Dirt on sensor.

[MarkInSpace (Mark)]

I don’t mean the donuts on the flat I mean the fact the flat is asymmetrical , but the out of focus Star looks good, but in the image you can see airy disks on the stars.

[bojan]

It could be internal reflections... What kind of source did you use to obtain it?
I have un-even flats when using my Canon 60D (BTW, I am using clear sky before sunset as a source) - mirror is a bit in the light path, causing some vignetting.

[MarkInSpace (Mark)]

I use sunset sky as my flat, with a tight white piece of material on top.
I can see the asymmetric vignette in my images as well.

[JA]

Hi Mark,

The uneven illumination shown in your flat image could be due to some misalignment of an internal light baffle in the telescope coupled with an image circle that is close to the margins of the sensor.

Best
JA

[MarkInSpace (Mark)]

JA - that's good thinking! The RC has a large central baffle - maybe it isnt seated correctly. I'll check...

[The_bluester (Paul)]

I would not panic too much. Flats tend to be extremely heavily stretched. If you reduce the stretch level you may find that you won't see that effect very strongly at all.

This is the same flat from me 10"F4 Newt (A luminance) with no stretch and with the default stretch in Astro Pixel Processor.

[Shiraz (Ray)]

I'm with JA on the baffle misalignment

fwiw (since I don't have an RC8), some opinions re the stars;

The star test pattern looks nicely symmetrical, but you may need to take it closer to focus to see if there are any misalignment or figure errors. In case you haven't already used it, this is a good resource for faults and star testing. https://www.telescope-optics.net/dif...berrations.htm

Apparently, collimating these things is an art. If you haven't already seen it, https://www.iceinspace.com.au/forum/...d.php?t=120591 edit. Just read your earlier thread on collimation - pity the secondary dot is missing - I would be putting one on the mechanical centre of the secondary and using it with your TS device to collimate the scope as per the video..

The dim stars are tiny and the nebula is really well resolved - you had good seeing and the scope is functioning well enough to get most of the energy where it is needed.

The intermediate stars have a slightly trefoil type halo, which is a bit asymmetric . suggest that you look at the mirror mounting systems - trefoil generally means that something is distorting one of the mirrors in 3 places. maybe the adjusting screws are so tight that they are distorting the secondary plate a little? I don't know how the primary adjusters work, but could they be so tight that they are distorting the mirror?

The bright star has a distinct diffraction pattern - the pattern looks large and may be zonal or spherical aberration. I guess this might result from misalignment not properly correcting SA? If you can't find any pinching/distortion anywhere on either mirror and the scope is properly collimated, then the error is in the optical figure of an element.

The spyder diffraction pattern shows what is probably strong secondary diffraction from the end mounts of the vanes. You will have to live with this unless you can modify the mountings to get the big end brackets out of the light path (assuming that is the issue)

Despite all of this, you are getting really nice detail on the nebula and the scope is obviously working pretty well. The stars maybe look a little unusual and definitely could do with a tidy up, but the nebula is the thing that matters - even if you have to accept the current level of performance, you still have a useful imager. Professional telescopes can have stars that are a bit untidy - and it doesn't seem to matter too much.

https://esahubble.org/images/heic2018b/.

https://esahubble.org/images/heic1903a/

Cheers Ray