Possible collimation issues with SCT

Acquisition Deep Sky M51 Messier Objects
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Celestron2 avatar
Celestron2 avatar

I’ve recently decided to give my Celestron Nexstar (Evolution) 6 OTA another go after a while sat on the shelf since I switched to a smaller refractor.

From a few test shots I managed to get a few nights ago, there seem to be a few issues that need looking into. Besides the obvious coma and vignetting which I can do very little about, there is a focus issue across the field with the stars on the right appearing significantly more distorted than on the left. Is this just collimation or is tilt potentially also an issue here?

I’ve attached a 300s test shot of M51 and a version cropped in on the centre of the image to hopefully clearly show the issues with the stars.

M51 300s test cropped.jpg

M51 300s test.jpg

If needed the equipment/software I’m using with it is:

Canon EOS 600D (APS-C)

Celestron AVX Mount

Celestron f6.3 Reducer

T-adapter for SCT

Altair 250mm Guidescope + Altair GPCAM130

QHY Polemaster

NINA/PHD2

Any help would be greatly appreciated!

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Tom Boyd avatar
Tom Boyd avatar

To first order, looks to me like you have back focus issues. Your camera appears to be too far away from our reducer. What is the recommended back focus of the Celestron reducer you are using?

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Tony Gondola avatar
Tony Gondola avatar

Those Celestron reducers can be pretty rough but that’s another story. For your issue I would remove the reducer for now and make sure your basic collimation is spot on. Then I would double check that you have the proper back focus distance between the reducer and the image plane of the camera. Then and only then would I start looking at tilt. You can very easily start chasing your tail of you don’t break these problems down, first things first.

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Celestron2 avatar
Celestron2 avatar

Tom Boyd · May 31, 2026 at 02:14 PM

To first order, looks to me like you have back focus issues. Your camera appears to be too far away from our reducer. What is the recommended back focus of the Celestron reducer you are using?

It should be 105mm from the face of the reducer as far as I can tell. The combination of DSLR (44mm) SCT T-adapter (50mm) and T-Ring (10mm) should be enough?

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Celestron2 avatar
Celestron2 avatar

Tony Gondola · May 31, 2026 at 03:33 PM

Those Celestron reducers can be pretty rough but that’s another story. For your issue I would remove the reducer for now and make sure your basic collimation is spot on. Then I would double check that you have the proper back focus distance between the reducer and the image plane of the camera. Then and only then would I start looking at tilt. You can very easily start chasing your tail of you don’t break these problems down, first things first.

I’ve been considering removing the reducer and trying it without for a bit. I removed it to check the collimation which is better than it was but probably still needs some work. Would a laser collimator be any help with this?

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Tony Gondola avatar
Tony Gondola avatar

Celestron2 · May 31, 2026, 03:57 PM

Tony Gondola · May 31, 2026 at 03:33 PM

Those Celestron reducers can be pretty rough but that’s another story. For your issue I would remove the reducer for now and make sure your basic collimation is spot on. Then I would double check that you have the proper back focus distance between the reducer and the image plane of the camera. Then and only then would I start looking at tilt. You can very easily start chasing your tail of you don’t break these problems down, first things first.

I’ve been considering removing the reducer and trying it without for a bit. I removed it to check the collimation which is better than it was but probably still needs some work. Would a laser collimator be any help with this?

No, do it by observing an out of focus star at the center of the field. The shadow of the secondary should be centered in circular pattern. Make an adjustment, recenter focus in a bit tighter and repeat until you are close to in focus.

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Dave Stirling avatar
Dave Stirling avatar

If you decide to collimate using the method Tony describes, one thing you could look at before you bust out your screwdriver is to ensure the secondary mirror is properly set. I believe the Nexstar has a similar retaining ring for the secondary as other Celestron SCTs. If it’s been sitting for a while, might be worth loosening the ring and wiggling the mirror assembly ever so slightly to see if it clicks in more firmly. Ask me how I know that fiddling with the screws before you check the mirror is set is a fruitless exercise. :)

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skyzorg avatar
skyzorg avatar

Looks like part of your issue is back focus. I had the exact same setup as yours, using the 9.25 SCT XLT with a Canon 600D and the Celestron x0.63 reducer, and found that the 105mm back focus recommendation was way off. With 105mm I was getting a plate-solved focal length of 1321mm — no where near the nominal 1480mm it should be, and the stars were streaked/rotated in the corners. After a lot of experimentation I settled on 80mm back focus, which yielded way better shaped stars in the corners, and a focal length of 1463mm. It seemed like anywhere between 75 and 90mm back focus produced OK results.

I got the inspiration to start experimenting from this James Lamb video:

https://www.youtube.com/watch?v=bxDLEwXXudk

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