AstroBin is the home of astrophotography - Share full resolution images, preserve acquisition details, discover equipment, discuss techniques, and learn from the community.Discover AstroBin

Possible collimation issues with SCT

Acquisition Deep Sky M51 Messier Objects
12 replies•217 views
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!

Well written Respectful Concise Engaging
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?

Well written Helpful Concise Engaging
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.

Well written Helpful Concise Engaging Supportive
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?

Well written Concise
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?

Well written Respectful
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.

Helpful
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. :)

Well written Helpful Respectful Concise Engaging Supportive
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

Well written Helpful Respectful Engaging
Mihai Barbu avatar
Mihai Barbu avatar

For spot on collimation you need to check your meteoblue for a clear calm good seeing night first.

I used this method for collimating a 9.25” SCT.

  1. There is the defocused star collimation method but I personally don’t use just that. That is only the begining. You defocus a star and eyeball the secondary to be in the center on a defocused star.

  1. Then you focus with a bahtinov mask. You should have 3 spikes and they need to be symmetric. To quantify it there are some bahtinov analyzers in imaging software and I would suggest you get the focus down to when it bounces on one side or another of focus, when you start seeing the seeing. Nina has some of these for free and Sharpcap paid. Both work great.

  2. You then put a tri-bahtinov mask and asses the collimation. You should have 9 spikes and they need to be symmetric. But to know which screw to collimate you need a mask for the tribahtinov mask that is alligned with those screws. You can find these covered Tri-bahtinov masks on Thingieverse. It is a a plastic cap that ellimates the other 2 screws.

  3. You then just use method 2 but instead of using the focuser you use the collimation screw that is free and not masked. You use the bahtinov analyzer because you have a bahtinov form when you use the cover. Repeat for all screws. Then you’re done. Your unmasked tri-bahtinov should show a perfect form and the tribahtinov shows focus and collimation at the same time. And the collimation is done at the focus point with your imaging camera which is great.

Helpful
Steven Zuhoski avatar
Steven Zuhoski avatar

My experience with the Celestron 0.63 reducer almost exactly matches skyzorg’s experience. Here’s some data I measured on my C925 around 5 years ago. I settled on a back focus of 92.5 mm (FL~1395mm) and I have been using it since then with good stars (533 and 294 sensors). I’m still using the same back focus on a new replacement C925 with similar results.

📷 Screenshot 2026-06-01 092929.jpgScreenshot 2026-06-01 092929.jpg

Well written Helpful Respectful Concise Engaging Supportive
Tony Gondola avatar
Tony Gondola avatar

Mihai Barbu · Jun 1, 2026, 08:34 AM

For spot on collimation you need to check your meteoblue for a clear calm good seeing night first.

I used this method for collimating a 9.25” SCT.

  1. There is the defocused star collimation method but I personally don’t use just that. That is only the begining. You defocus a star and eyeball the secondary to be in the center on a defocused star.

  1. Then you focus with a bahtinov mask. You should have 3 spikes and they need to be symmetric. To quantify it there are some bahtinov analyzers in imaging software and I would suggest you get the focus down to when it bounces on one side or another of focus, when you start seeing the seeing. Nina has some of these for free and Sharpcap paid. Both work great.

  2. You then put a tri-bahtinov mask and asses the collimation. You should have 9 spikes and they need to be symmetric. But to know which screw to collimate you need a mask for the tribahtinov mask that is alligned with those screws. You can find these covered Tri-bahtinov masks on Thingieverse. It is a a plastic cap that ellimates the other 2 screws.

  3. You then just use method 2 but instead of using the focuser you use the collimation screw that is free and not masked. You use the bahtinov analyzer because you have a bahtinov form when you use the cover. Repeat for all screws. Then you’re done. Your unmasked tri-bahtinov should show a perfect form and the tribahtinov shows focus and collimation at the same time. And the collimation is done at the focus point with your imaging camera which is great.

All these ultra fine tuning steps are certainly valid but considering the magnitude of the OP’s issues he might not need to guild the lily to quite that degree until he sorts out his larger issues.

Willem Jan Drijfhout avatar
Willem Jan Drijfhout avatar

@Celestron2 Do you have a possibility to rotate any part of the imaging train? If so, you can take two images, with a rotation of e.g. 90° between them. If not, perhaps adding a 0.5/1mm spacer may do the trick.

If some or all of the issues show up in the same stars, but different corner of the frame, it is caused before the point of rotation (e.g. collimation, mirror alignment, etc).

If some or all of the issues show up in different stars, but in the same corners of the frame, the problem is after the point of rotation (back focus, tilt, etc).

And keep in mind the possibility of having multiple issues, that need attention separately.

Well written Helpful Concise Engaging Supportive
Alex Nicholas avatar
Alex Nicholas avatar

Looks like (as many have said) backfocus, but also tilt, as the stars in the top left are ‘sharp’ but show abberations, and the stars in the bottom right look WELL out of focus…

If you’ve got tilt, you don’t have collimation, even if the scope is perfectly collimated, tilt (even if the camera is tilted) means your imaging train is not all properly collimated.

One thing at a time though… Ensure your backfocus is correct as per Celestron’s specs for the .63x reducer (I can’t remember what that is, its been a long time since I owned my C11XLT). Once that is sorted, collimate the scope carefully and re-test. If your results are good - then tilt may not have been a concern, if the results are still showing in focus at one end, out of focus at another, I’d start investigating the imaging train for tilt and try to resolve that.

Well written Helpful Respectful Engaging Supportive