RC collimation

These versions of the rc have the focuser attached to the back plate without a tilt adjustment that’s separate from the primary. I can get a tilt plate but I’d like to get things closer before I worry about that.

What do you think about this approach:

https://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf

Matthew Proulx · Oct 4, 2025, 08:31 PM

Tony Gondola · Oct 4, 2025, 08:24 PM

What do you think about this approach:

https://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf

This is the approach one should use once a decent collimation is achieved with tools. Tools only get you so far. This method is the icing on the cake. Once you learn this you will understand which mirror causes what kind of aberration and it will be easier to solve and collimate in the future should you have to.

In short : Main mirror to adjust coma of star (you want full illumination of the out of focus star)

Secondary to balance the corners of the image.

Then check main again, repeat if needed.

That’s pretty much the plan for tonight. Moonlight will make it easier to find my Allen wrenches when I drop them :)

Tobiasz · Oct 4, 2025, 09:32 PM

For the daylight collimation I use the same tools as you with the help of my eyes and some distance to the back of the scope. What I mean is described within the collimation guide of Tommy Nawratil's blogpost about the GSO RC. (LINK to the image, Source: Blogpost)

You have to make sure your laser is collimated as well. So when you align your laser against the secondary your laser dots should be spot on but you should also see the center mark reflection on the crosshair plate of your laser. Only then you can expect your focuser plane and your secondary to be in line mechanically. Everything else means tilt in your focuser and/or your laser.

After that I align the primary mirror like it is described in the image of Tommy above. Repeat as often as needed until your eyes cannot make out any errors anymore.

This way you should achieve a 90-95% collimation (IF your focuser sits tight and is stable enough) and only very minimal changes should be required under the stars.

Concentrate on an on-axis star first. The poisson spot should be in the center already and only the secondary shadow of the defocused star should show slight displacement. Fix it with the primary collimation screws and then check the “balance” of your abberations in your field. The bigger the sensor the better you can see differences in your field (I use a Canon EOS 6D for that).

All your astigmatic stars should point straight to the center like in the guide from DSI. You may see it better with defocused stars.

Repeat until you're happy.

Good luck!

That’s a good article, I’ve never seen the stand back and look approach to adjusting the secondary before but I love it’s simplicity. I’ll give that look early tonight and see how far off I really am. I’m a little confused when he talks about aligning the spider vanes though. What exactly is he doing there?

Tobiasz · Oct 4, 2025, 09:53 PM

When you're standing back you will see the spider vanes in the reflection of the secondary mirror, too. If your primary is aligned well, they should form a perfect X.

ahhh, ok. Not offset from each other.

Tony Gondola · Oct 4, 2025, 04:19 PM

I am going through the process of collimating a GSO/Skywatcher style RC. When I first got it, it was slightly out so I decided to line everything up on the bench using a Cheshire and laser. At that point everything looked perfect through the Cheshire. First look at the stars and I had a field full of comets, my bench alignment made things far worse then it was to begin with. After a LOT of undirected tweaking of both the primary and secondary I was able to get to this:

📷 star test 3.jpgThere’s still some on-axis coma with the P dot not centered. The the off axis astigmatism is also unbalanced. I think my next move is as follows:

1. With a star centered, adjust the primary to remove the remaining on axis coma.

2. Balance the off axis astig. with adjustments of the secondary.

3. Repeat if needed.

   Am I on the right track here?

Hi Tony

I’ll follow your thread because I’m going through a similar experience.

Earlier this month, I tried the Cheshire + laser + DSI method and was able to reach this level: 📷 NGC281-300x300-session_1_session_2_session_3_session_4_session_5.jpg
(This image is a stack of 300 images)

I’m not sure whether the issue is with collimation or backspacing. The stars in the corners seem to be radiating inward, yet according to the DSI method, the image appears balanced—or at least that’s what I think. I should mention that I have installed a tilt plate, though I haven’t made any adjustments to it yet.

Last week, I travelled with the scope to a Bortle 2 site (a rough, hilly drive with the scope kept on the back seat—EAF and camera still attached, resting between two pillows 😄). Things seem to have worsened since then—the corners have degraded noticeably. 📷 2025-10-01_03-02-42__-9.80_180.00s_0012.jpg
(single frame). I did not try to tweak collimation because of the bad weather.

I have not followed the DSI method to the hilt. I removed the On-Axis Coma by adjusting the primary mirror following the DSI method. But after that, I simply adjusted the secondary mirror and centred the dot. This was more because I did not want to waste a night, and the subs looked more or less “balanced” to me.

About the tilt plate: I have not made any adjustment yet to the tilt plate itself. However, my tilt improved quite a bit after installing the plate. The aberration inspector in NINA showed an improvement in the tilt measurement.

One resource to share: https://lambermont.dyndns.org/astro/rc/
It collates all the articles on RC collimation on the internet. Ignore if you have already seen this.

Do share if you manage to fix the collimation. I will get on to it when the clouds move away.

Clear skies!

Tobiasz · Oct 5, 2025, 08:01 AM

A good collimation does not translate into a perfect flat field. The mirror design will still suffer from field curvature and astigmatism. If I look at your profile, you’re using a 6 inch RC with an APS-C camera and without a flattener right? Then it is expected to see pointy stars off-axis with such a big sensor.

You have to crop in or use an additional flattener.

That’s what I even thought that I may need an additional flattener to correct the corners. However, the source of confusion was also the NINA Aberration Inspector (Hocus Focus), as it suggested that my backspacing might be off by 170+ microns. Do you think that assessment is correct?

Tobiasz · Oct 5, 2025, 08:39 AM

Rajat Kumar · Oct 5, 2025, 08:23 AM

Tobiasz · Oct 5, 2025, 08:01 AM

A good collimation does not translate into a perfect flat field. The mirror design will still suffer from field curvature and astigmatism. If I look at your profile, you’re using a 6 inch RC with an APS-C camera and without a flattener right? Then it is expected to see pointy stars off-axis with such a big sensor.

You have to crop in or use an additional flattener.

That’s what I even thought that I may need an additional flattener to correct the corners. However, the source of confusion was also the NINA Aberration Inspector (Hocus Focus), as it suggested that my backspacing might be off by 170+ microns. Do you think that assessment is correct?

Without a flattener you don’t have to keep track of correct “backspacing” as there are no corrective optics (like lenses) in the light path. This would only apply if you use a flattener (109mm for the 2 inch, 89,5mm backfocus for the 3 inch version).

Another topic would be mirror spacing, but you will need a Ronchi eyepiece for that and not NINA.

Noted. Even I thought the same. Thanks!

Resetting the collimation bolts can work to get something like an “inital” position of everything.

How tight are your bolts screwed in? Once I managed to tighten the secondary screws so much that I even had astigmatic stars on axis.

Tobiasz · Oct 5, 2025 at 08:39 AM

Without a flattener you don’t have to keep track of correct “backspacing” as there are no corrective optics (like lenses) in the light path. This would only apply if you use a flattener (109mm for the 2 inch, 89,5mm backfocus for the 3 inch version).

Another topic would be mirror spacing, but you will need a Ronchi eyepiece for that and not NINA.

That’s mostly right; however, you still want the BWD to be roughly correct to avoid introducing SA, although SA varies VERY slowly with spacing changes. As for the field curvature, you can introduce a small focus bias to deal with the field curvature. I can’t recall the best zone but if you focus around the 50% zone, it will introduce an insignificant amount of defocus at the center but it will reduce the field astigmatism by a noticeable amount. Then, BXT acts like a “digital” flattener to remove the residuals. I’m planning to remove the flattener in my ASA600 RC to eliminate halos and this is how I’ll operate going forward.

- John