Yet another RC collimation thread - Forums

Topic starter

Dec 11, 2021

Hello everyone,
I've recently sold some of my gear and bought a RC 8 from TS Optics, with the CF tube option (rebranded GSO I believe). I've been fiddling with collimation for a while, succeeding sometimes, failing miserably on other times (especially when trying to perfect a good collimation with a star test and end up beyond the point of no return).

Tools I've got: Takahashi collimation telescope, farpoint 2" autocollimator (had it from when I once had a newt).
Focuser: stock focuser is used for collimation, ESATTO 2" for imaging. That's my only solution for now, until I get custom adapters to connect the Tak to the ESATTO.

Yesterday was one of those nights that any attempts to improve collimation went the other way under the stars. So I started from scratch during the day, primary mirror reset done (screws tightened to 0 position), aligned the secondary with it (using the Tak, I don't have a laster) and then loosened 1 turn each screw on the primary to give me some margin for later collimation. I removed the baffle extension.

Note: it is impossible to see anything related to the primary except for shadows with the Tak, unlike all the documents I've seen only. I am not sure why.

So my starting point was this (taken through the Tak):

The black disc labeled #1 and the black circle label #2 move when adjusting the primary, but it is impossible to know if they are concentric, because they don't leave a light circle between them (like all documentations). It's hard to differentiate black on black without a camera setup (which I don't have).

Moving on, I inserted my autocollimator and made the primary concentric, the highlighted very faint circle below moved with the primary, so I used that as a reference to center my primary (taken through the autocollimator):

Then after some iterations back and forth between the autocollimator (for the primary) and the Tak (for the secondary), I ended up with the below taken through the autocollimator (disregard the outer white ring, it's impacted by my hand not being perfectly aligned while holding the phone):

Looks perfect? or should be really close for fine tuning under the stars? But the problem is, I now have to rescrew the baffle extension and I was very careful to avoid touching the secondary. I rechecked with the Tak, and it needed the slightest adjustment. But now with the collimator, things got weird, whatever that thing is in the center is skewed? but Tak confirms the secondary is on point still:

At this point it's impossible for me to check if the primary is still doing fine (it should since I haven't touched it) while the baffle extension is in place.

I appreciate if someone with experience on this would pitch in to help me with few questions:
- is the method I am using during the day a valid one? I never found anyone using the autocollimator for this
- what changed when I inserted the baffle back? is the baffle blocking some of the light or the collimation went bad?
- if you don't agree with the methodology here, what's your experience with daylight tools? What is needed to achieve almost perfect (won't ask for perfect) collimation during daytime?

I am familiar with the DSI method, which I will try to follow tonight if it's clear and to validate what I did above. But I also read somewhere that it isn't applicable to GSO RC's because the primary and the focuser are not separate units

Helpful Engaging

SemiPro

IOTD Staff

Perfect post

Dec 11, 2021 · Edited: Dec 11, 2021

The way I do it is I get it as close as I can during the day and then finish it off at night. Actually after the first time you align it mechanically during that day you probably won't have to do it again unless you really mess it up later!

At night, I adjust the primary mirror to get an even doughnut, and then you adjust the secondary to get the stars in the corners looking all the same. I think that's the basics of the DSI method? If it is, it works. I still use the default focuser.

https://astroimages.weebly.com/collimating-an-at8rc.html
https://www.skydude.me/at8rccoll.html

And here are my results: https://www.astrobin.com/7q69z2/

Not perfect, but acceptable. It could be better with some tweaking though.

Tom's Pics

Dec 11, 2021

Hi,

I feel your pain. I have a 14" CF GSO RC that I have struggled with for 15 months. I have bought every collimation tool known to mankind and of note, the last one I tried really made a difference. It is a Hotech Advanced CT Laser Collimator. It did take a couple of attempts to get the hang of how to use the tool, and to add and reduce complexity, I had to bring my scope inside the house to collimate it as I couldn't get the proper distance inside by backyard observatory. Here is a link to my website with pics and a write up of what I did. I did speak with the manufacturer who was very helpful in guiding me through the process. The scary part that was not scary at all, was I needed to remove the secondary mirror to collimate the focuser to the tool. I was reluctant to touch "that center screw" that everyone warns about knowing distances are critical in this scope. I was able to remove it from the 4 fins it is suspended from avoiding that screw all together. I will say this at least for my 14" CF GSO, it looks super cool, and optically it might be awesome, but mechanically it sucks. One of my most common mistakes was snugging up the adjustment push/pull screws throws it out of collimation. I could see this with the ACT tool as I kept it on and watched as I tightened everything up. It takes a very soft touch. I wish you luck.

And to add insult to injury, I have always been struggling with getting my flats to correct properly. I stumbled on a YouTube video that identified a design flaw with the GSO RC's, at least the CF ones that the baffle is about 4" too short and needs to be 9mm smaller in aperture! Sigh. I was able to design and 3D print a baffle extension and reducer to spec and amazingly it made a huge difference with the flats and now they correct like they should. I'm not sure it applies to you buthere is the videoin case other RC'ers are interested. I can provide the 3D print file in (.stl) format if anyone has my scope and wants it.

All the Best,
Tom
http://jropics.com

Helpful Engaging Supportive

Brian Boyle

Dec 11, 2021

Hi Michel,

I have a GSO RC8 that collimate using the Tak only. For both primary and secondary. I find it works well and I get good images over a full frame sensor at the native 1600mm focal length, when using the TS 2inch field flattener. Without this, the non-flatness of the RC field more than 7-10mm off-axis can really spook you when you are doing star tests!

However, I find that I have to unscrew the entire primary baffle in order to do the best possible job. This allows you to line on the 3rd reflection of the spider vanes with the second reflection of the vanes, as per the instruction sheet for the Tak that came with it.

Now getting that baffle off is really challenging for someone with big hands or forearms. I am a tall person (2m) with large hands and forearms, so I don't find this easy at all. Squeezing past the 2ary to grip the 1ary baffle at the bottom in order to unscrew it is my least favourite part of the collimation.

And I can't do it without touching the 2ary support. However, I have found that even although I do press on the 2ary support, it doesn't change the collimation once I put the baffle back in.

I have also tried using a Hotech laser with the concentric circle screen, but I find that did not give good results i.e. when I collimated "perfectly" on the bench, the star test showing the collimation was way out. [I spent a frustrating week on this]

In contrast I found the, once I removed the entire primary baffle, the collimation with the Tak was relatively easy and robust. Personally, I would be a little careful of collimating different mirrors with different collimators. Its rare to find different types of collimators giving exactly the same results, and you might find yourself in an iteraive loop that never converges. Particularly with an RC.

Hope this helps.

Brian

Helpful Engaging Supportive

Michel Makhlouta

Topic starter

Dec 11, 2021

Thank you each and everyone on of you for all of the replies, I appreciate that a lot!

What I've done during the day ended up with good results, not perfect, but I am reluctant to fiddle with it. Sometimes we just need a win in this hobby, I will let this be mine over the RC for now. Here are the results, which are much better than before (native FL, 294MM bin 2x2, Lum filter, unity gain):

The way I do it is I get it as close as I can during the day and then finish it off at night. Actually after the first time you align it mechanically during that day you probably won't have to do it again unless you really mess it up later!

At night, I adjust the primary mirror to get an even doughnut, and then you adjust the secondary to get the stars in the corners looking all the same. I think that's the basics of the DSI method? If it is, it works. I still use the default focuser.

https://astroimages.weebly.com/collimating-an-at8rc.html
https://www.skydude.me/at8rccoll.html

And here are my results: https://www.astrobin.com/7q69z2/

Not perfect, but acceptable. It could be better with some tweaking though.

I think your results are excellent! It seems you nailed it! What you described is the DSI method, that's correct. Thanks for sharing the other two links, it seems that's the way to do this. I somehow manage to make things worse at night.

Tom's Pics:
Hi,

I feel your pain. I have a 14" CF GSO RC that I have struggled with for 15 months. I have bought every collimation tool known to mankind and of note, the last one I tried really made a difference. It is a Hotech Advanced CT Laser Collimator. It did take a couple of attempts to get the hang of how to use the tool, and to add and reduce complexity, I had to bring my scope inside the house to collimate it as I couldn't get the proper distance inside by backyard observatory. Here is a link to my website with pics and a write up of what I did. I did speak with the manufacturer who was very helpful in guiding me through the process. The scary part that was not scary at all, was I needed to remove the secondary mirror to collimate the focuser to the tool. I was reluctant to touch "that center screw" that everyone warns about knowing distances are critical in this scope. I was able to remove it from the 4 fins it is suspended from avoiding that screw all together. I will say this at least for my 14" CF GSO, it looks super cool, and optically it might be awesome, but mechanically it sucks. One of my most common mistakes was snugging up the adjustment push/pull screws throws it out of collimation. I could see this with the ACT tool as I kept it on and watched as I tightened everything up. It takes a very soft touch. I wish you luck.

And to add insult to injury, I have always been struggling with getting my flats to correct properly. I stumbled on a YouTube video that identified a design flaw with the GSO RC's, at least the CF ones that the baffle is about 4" too short and needs to be 9mm smaller in aperture! Sigh. I was able to design and 3D print a baffle extension and reducer to spec and amazingly it made a huge difference with the flats and now they correct like they should. I'm not sure it applies to you buthere is the videoin case other RC'ers are interested. I can provide the 3D print file in (.stl) format if anyone has my scope and wants it.

All the Best,
Tom
http://jropics.com

That's one expensive collimator, and it seems to be well recommended ever for hyperstar and sct setups. Such an investment makes sense if it lives up to its reputation and can handle multiple OTAs. I will look into it, but this is more of a long term solution. I will keep in mind that trick for removing the secondary, that's much better than having one more problem about mirror spacing to worry about. As for the baffle part, I came across that video and I am not sure if TS Optics or GSO corrected that, but there is an extension installed on the baffle, it's about 10-15 cm? I am not sure if that was of the original design or added later to correct this particular problem. I am afraid I didn't get to the flats part yet for this scope.

Brian Boyle:
Hi Michel,

I have a GSO RC8 that collimate using the Tak only. For both primary and secondary. I find it works well and I get good images over a full frame sensor at the native 1600mm focal length, when using the TS 2inch field flattener. Without this, the non-flatness of the RC field more than 7-10mm off-axis can really spook you when you are doing star tests!

However, I find that I have to unscrew the entire primary baffle in order to do the best possible job. This allows you to line on the 3rd reflection of the spider vanes with the second reflection of the vanes, as per the instruction sheet for the Tak that came with it.

Now getting that baffle off is really challenging for someone with big hands or forearms. I am a tall person (2m) with large hands and forearms, so I don't find this easy at all. Squeezing past the 2ary to grip the 1ary baffle at the bottom in order to unscrew it is my least favourite part of the collimation.

And I can't do it without touching the 2ary support. However, I have found that even although I do press on the 2ary support, it doesn't change the collimation once I put the baffle back in.

I have also tried using a Hotech laser with the concentric circle screen, but I find that did not give good results i.e. when I collimated "perfectly" on the bench, the star test showing the collimation was way out. [I spent a frustrating week on this]

In contrast I found the, once I removed the entire primary baffle, the collimation with the Tak was relatively easy and robust. Personally, I would be a little careful of collimating different mirrors with different collimators. Its rare to find different types of collimators giving exactly the same results, and you might find yourself in an iteraive loop that never converges. Particularly with an RC.

Hope this helps.

Brian

I think that's what I was missing the whole time... The whole baffle comes out? I am only unscrewing hat looks like an extension, only about 10cm long. Only after that I can see the primary in the autocollimator, but never with the Tak. Now it makes sense. If you don't mind to double confirm that the whole thing unscrews. Also, how far is the tak from M90 thread at the back of the scope? does it have to be very close or near focus point or it doesn't matter once the baffle is removed?

I find that the secondary collimation changes a bit after rescrewing the baffle (impossible to avoid pressing against it gently) but that's easily recorrected with the Tak, since having the baffle doesn't impact secondary.

You're the first one on the web that mentions that the whole baffle comes out. You would think they would have mentioned that in a manual or a guide somewhere...

SemiPro

IOTD Staff

Perfect post

Dec 12, 2021

Well looks like you have it pretty good! Now it just looks like a matter of playing with the backfocus giving all the corners look more or less the same.

Brian Boyle

Dec 12, 2021

Hi Michel,

I agree with SemiPro - what you now have looks good.

I can confirm that the whole thing unscrews you need to grip in a least half way down. I doesn't, of course, come out of the tube (it won't get past the secondary). But just just it inside the tube, lying flat. It does hide part of the reflections, but it is still easy to see what's going on.

I have my Tak mounted on the end of the focuser using the 2inch adapter that came with it. No need to uncouple the focuser from the M90 thread.

It's all quite easy. One you know of course. It took me a couple of weeks to work all this out.

CS Brian

Bradley Watson

Dec 12, 2021

Thanks for posting this problem @Michel Makhlouta and the responses from all, I have followed this with keen interest. I also struggled with my RC, collimating with a Cheshire with the appearance that my RC was collimated. Take the scope out and well, not what I expected. I now understand properly the mechanics behind the scope and the assumptions I had made. This has been most helpful. Looking forward to getting mine up and running, the darkness has been lifted!

Supportive

Michel Makhlouta

Topic starter

Dec 13, 2021

Thank you all again, here are some results from the final stack that night. Same setup, Chroma Ha 3nm filter, 600s, gain 200, less than 2 hours in total:

Full resolution: https://www.astrobin.com/k4bw0g/

Well looks like you have it pretty good! Now it just looks like a matter of playing with the backfocus giving all the corners look more or less the same.

Yep I am happy with the current results, especially that it didn't take any fiddling after my daytime routing. I will stick to it for now and hope for the best each night, until I am confident my nighttime changes don't make things worse. There isn't a backfocus issue, I am not using a flattener or reducer. I think it's the secondary that still needs some work.

Brian Boyle:
Hi Michel,

I agree with SemiPro - what you now have looks good.

I can confirm that the whole thing unscrews you need to grip in a least half way down. I doesn't, of course, come out of the tube (it won't get past the secondary). But just just it inside the tube, lying flat. It does hide part of the reflections, but it is still easy to see what's going on.

I have my Tak mounted on the end of the focuser using the 2inch adapter that came with it. No need to uncouple the focuser from the M90 thread.

It's all quite easy. One you know of course. It took me a couple of weeks to work all this out.

CS Brian

I really appreciate your feedback. I've been trying to wrap my head around the primary collimation with the Tak and I finally have an answer. I have the scope collimation where I want it now. But the next time I am going for it, I am definitely taking the whole thing out, not only the extension. And with removing only the extension, it seems that the autocollimator did a good job, despite not being designed for this type of scopes. I will order from preciseparts an addapter for the Tak to connect to the zwo oag. Easiest connection I can think of to keep most of my imaging train in place.

Bradley Watson:
Thanks for posting this problem @Michel Makhlouta and the responses from all, I have followed this with keen interest. I also struggled with my RC, collimating with a Cheshire with the appearance that my RC was collimated. Take the scope out and well, not what I expected. I now understand properly the mechanics behind the scope and the assumptions I had made. This has been most helpful. Looking forward to getting mine up and running, the darkness has been lifted!

I am glad to hear that Bradley, and I am looking forward to hear some good news from your end. I hope you get it right very soon and share some results!

SemiPro

IOTD Staff

Perfect post

Dec 13, 2021

That's a good looking Soul Nebula! Glad you got it sorted.

Well Written Respectful

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