ritchey chretien collimation

[Joe Acosta]

Hi all, 

So a friend of mine loaned me an astro tech 6 inch ritchey chretien telescope.  He said it was collimated.  I end up with lots of donuts though.  I use the same camera on other scopes and do not end up with these donuts.  I do use a field flattener with it. 

Does anyone know how to collimate ritchey chretien and have the tools to do it?

thanks
Joe

[Brad Templeton]

I have the same scope, but don't use it as I just have other instruments that will do better for me.  I have bob's knobs on it, that does help with the collimation, but it's more complex than a newt.     Though in theory if you get it right an R-C is a good astrograph.       If anybody wants to make a reasonable offer on a barely used one, let me know.   I also have the flattener.

[Francesco Meschia]

Donuts are perfectly normal for any obstructed instrument. The question is whether the donut is “centered”.

Francesco

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[Joe Acosta]

Main donut is justoff center. Like just below center of object and then all over are other donuts. 

Flats don't fix this either. 

I've since motorized my dobsonian, and don't see same issues, so might start switching imaging to the dobsonian. 

J

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[4b...@templetons.com]

You get a donut only when out of focus.  Each point of light is a donut.  Can be handy collimating at night.  In day you use other methods.  To observe or shoot, you focus and donuts turn to points

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[Joe Acosta]

Hmm, I use bahtinov mask to focus and focus looks good.  I have noticed a certain tilt in the whole backend focuser today.

Wondering if that tilting is causing the issue and how to fix it 

[Francesco Meschia]

Tilt would change the size of the donuts across the field.

Why don’t you share a sample frame?

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[Aris Pope]

Are you at the correct back focus with the extension tubes? 

Aris

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[Joe Acosta]

Yes,. Only way to get focus. 

Joe

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[Aris Pope]

And at your best focus you're still getting donuts? How many extension tubes are you using? I know it comes with three but I think you only need two for imaging.

If the back focus is correct then you should not be getting donuts. I would double check your backspacing requirement for the corrector and make sure that you are at the correct distance. 

If the back focus is correct maybe the next thing is focuser tilt adjustment. I do not have an RC anymore but when I had one I was researching quite a bit. From my understanding the focuser  tilt adjustment is the third adjustment that you need. 

GSO Focuser Collimation Ring / Tilt Adapter - For 6" and 8" RC Telescopes with M90x1 Focuser Thread https://share.google/2zHypw15xJ4vvdkZ6

On Sun, Sep 7, 2025, 1:42 PM Joe Acosta <furryj...@gmail.com> wrote:

Yes,. Only way to get focus. 

Joe

On Sun, Sep 7, 2025, 12:48 AM Aris Pope <arispo...@gmail.com> wrote:

Are you at the correct back focus with the extension tubes? 

Aris

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Aris

[furryjoebear]

hopefully this image goes across the wire :)

I opened one of my fits images and then auto stretched in siril and youi can see donut near m57 and then other donuts all over the place

Joe

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[Daniel Vancura]

Hey, is this image calibrated with flat frames yet? Looks to me like some dust/smudge that should be corrected with matching flats. Had something similar once also where dew started building on my scope and didn’t match the flats.

[Aris Pope]

That definitely looks like a flats issue. 

Aris

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[Francesco Meschia]

I agree with Aris, these “donuts" are shadows of dust specks– and from the size I’d say they are on the camera optical window. Judging from the vignetting, I think that flats were not used – and therefore there’s nothing that can correct dust motes.

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[Joe Acosta]

This is a light frame. So no flats used. 

I'd think if this was from the camera I'd see this when I use the camera with my dobsonian or my refractor. Only see it on the rc6.

I tried lights with stacking and the lights didn't do anything with this. 

Joe 

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[Aris Pope]

Flats will correct  dust motes throughout the optical train so if it is not the camera sensor then it could be a secondary or primary mirror.

Aris

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[Francesco Meschia]

Dust tends to move, flats are normally considered not to be good anymore whenever the camera is removed from the OTA

“I tried lights with stacking and the lights didn’t do anything” makes no sense – did you mean you tried flats?

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[Brad Templeton]

It's the camera.  I've had these.  Flat frames will fix this 

There is a calculator online that with tell you. Based on the size of the donut, just how has far the dust is above your sensor.  So that and you can know where it is, like on your filter. Or the glass over your sensor etc.

[Mark Westling]

Hey Joe, I asked ChatGPT about your image and got this (see https://chatgpt.com/share/68bf40aa-db50-8007-8f34-6658aabc84d4):

My query: Diagnose this image taken with a AT6RC telescope. Comment on star shape and any "donuts"

ChatGPT said:

I’ve taken a close look at your image from the AT6RC, and here’s a diagnostic summary:

1. Star Shape

• The stars across the field look mostly round, which suggests your collimation and guiding are fairly good.

• There is some elongation visible in a few stars, but it is mild. This could be due to:

  • Slight polar misalignment

  • Guiding corrections not fully tuned

  • Wind or vibration during exposure

• Importantly, the elongation is not strongly directional across the frame, so it’s not a severe tracking issue.

2. Donuts / Shadow Artifacts

• I can clearly see a dark “donut” ring above the Ring Nebula (M57) in the frame.

• This is a classic dust mote / dust shadow from debris on either your camera sensor window or (more likely) the optical train (filters, reducer, or field flattener).

• The shape is well-defined, meaning it’s fairly close to the sensor.

• There also appear to be fainter ring-like shadows elsewhere, which reinforces this diagnosis.

3. What This Means

• These dust donuts are normal in raw astrophotos but should be calibrated out with flat frames. Flats map these illumination differences and allow them to be subtracted during preprocessing.

• If you aren’t already taking flats, this is a good reminder—they’ll also fix uneven field illumination and vignetting.

4. Next Steps

• Collimation: Your AT6RC collimation looks pretty good since stars are symmetric, but you could fine-tune with a defocused star test if you want perfection.

• Dust cleanup: Either physically clean the sensor window / filters or rely on flats (the safer route—avoid overcleaning optics).

• Guiding: If you’re chasing perfectly sharp stars, review guiding logs (PhD2 RMS stats) and polar alignment.

✅ Overall, this is a solid capture of M57 with your AT6RC—round stars, well-defined nebula, and only minor calibration issues.

Do you want me to walk you through a flat-frame setup with your AT6RC + ASI294MC Pro so you can easily remove those donuts in future stacks?

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[Joe Acosta]

So same camera, different scope. Much shorter subs, but no signs of donuts.

Joe

[Brad Templeton]

The doughnuts are out of focus images of the telescope including the obstruction of the secondary mirror.     In a refactor, they will just be round darkened disks.   The spikes suggest a reflector though.

[Aris Pope]

Did you try the RC without the corrector?

Aris

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[Joe Acosta]

I tried it with and without the flattener and same results.  I tried with and without flats, same thing. Unless my flats were bad, but they did fine with my refractors.

J

[Francesco Meschia]

On Sep 28, 2025, at 19:30, Joe Acosta <furryj...@gmail.com> wrote:

Unless my flats were bad, but they did fine with my refractors.

Meaning that you used the same flats for the refractor and for the Ritchey-Chretien?

[Joe Acosta]

No I take flats at the end of the imaging session. Bias and darks can be reused, Flats can't 

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