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RC collimation with no laser or stars

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#251 Ancient Photons

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Posted 04 March 2023 - 10:43 AM

Don't worry about the focal length - it's a guideline, not exact, and the impact on the image is minimal compared to collimation and tilt. Get the focuser squared, then adjust collimation. If you're still worried about mirror spacing, you'ld then need to put the sensor at exactly the specified backfocus (which should be written on the back of the primary mirror) and adjust the mirror spacing until you are in focus. The central bolt is not the mechanism to do that. Instead, the secondary is mounted in a ring that can be adjusted for mirror spacing. 

100% agree. Based on extensive reading, the effect is minimal compared to tilt/collimation.



#252 astrobananeck

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Posted 05 March 2023 - 05:15 PM

Thanks Rasfahan and Chad, this thing about not touching the focal length seems to be a large consensus in the community and yes I think you're right when you say I should worry about tilt and collimation first. I agree seeing makes an enormous difference, both in guiding (with my ZWO AM5 mount I use 1s guiding exposure times) and in the subs.

 

I am still waiting for the REEGO (cheshire-like) that I ordered so I'll report back when I have received it and tested it. I really like your method, Chad, so I'm going to try it even though I can't easily remove the baffle.

 

I guess the blog description will also be clearer once I see the actual pattern in the cheshire (in particular the difference between the focuser/primary tilt and the primary/secondary decollimation; I still struggle to understand how to separate them visually since they seem to have the same effect).

 

Thanks again for your wise words.


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#253 Ancient Photons

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Posted 06 March 2023 - 08:59 PM

Thanks Rasfahan and Chad, this thing about not touching the focal length seems to be a large consensus in the community and yes I think you're right when you say I should worry about tilt and collimation first. I agree seeing makes an enormous difference, both in guiding (with my ZWO AM5 mount I use 1s guiding exposure times) and in the subs.

 

I am still waiting for the REEGO (cheshire-like) that I ordered so I'll report back when I have received it and tested it. I really like your method, Chad, so I'm going to try it even though I can't easily remove the baffle.

 

I guess the blog description will also be clearer once I see the actual pattern in the cheshire (in particular the difference between the focuser/primary tilt and the primary/secondary decollimation; I still struggle to understand how to separate them visually since they seem to have the same effect).

 

Thanks again for your wise words.

I also use an AM5, and boy - the 1-second exposures really provide a good indicator of seeing conditions. The good news is that even when my guiding gets above 1" on nights of bad seeing, the stars generally stay round. Usually, though, I'm getting between 0.5 - 0.8".

 

I hope you're able to get a good view in the Cheshire to try out the method. I'd definitely prioritize aligning the primary and secondary mirrors if you don't have a reasonable view of the outer ring of light to align the focuser/tilt. If that's the case, maybe remove the focuser collimation/tilt ring altogether, then align the primary and secondary and see what you get. Without that collimation ring on, and the focuser attached directly to the tube, you should be able to at least see the outer ring to assess the degree of focuser tilt/misalignment.

 

Keep us posted!



#254 astrobananeck

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Posted 06 April 2023 - 07:02 PM

So just an update on my long collimation story: I got the REEGO (cheshire-like) and indeed it was immediately apparent that the secondary mirror was off - since the sky was cloudy for a few weeks in a row I started with the indoor collimation and first fixed the secondary alignment by turning slightly the secondary screws. Then, since I couldn't see the white outline in the cheshire, I removed the focuser and replaced the original, shorter, crayford; with that, I brought the cheshire closer to the primary and could see the outline on the edge of the circle. The primary was slightly off so I recentered it by turning the primary screws. I messed up a little because I didn't quite understand how those collimation screws work - in fact one is a push and the other is a pull screw so you have to turn the first one slightly CW and the other CCW by the same angle (but for some reason I thought one was just blocking the other screw from turning).

 

Fast forward to a few days later when the stars were visible again: my first picture was disappointing with all stars displaying strong coma; since I had a few weeks to study collimation theoretically, I knew this was a sign of decollimated primary so I then used a defocused star to try and recenter it as much as possible; after about an hour using trial and error I managed to get relatively round and even stars. One last step was just retouching the secondary alignment a bit - since I had moved the primary quite a bit, the initial indoor secondary collimation was no longer good.

 

Last step, I took a few test shots (after leaving the telescope out for 2 hours to cool down) and used CCD Inspector to measure the results. Collimation error is now down to 1.7" in CCD Inspector while the original value was over 10". Also the tilt is now down to a value I no longer need to correct with a tilt corrector. Overall it's been a rough experience but I managed to make it work, or so it seems. Now shooting M51 despite the full moon, just to see if my resolution has improved, too.

 

Thanks for your support during my learning process!


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#255 astrobananeck

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Posted 06 April 2023 - 07:09 PM

I also use an AM5, and boy - the 1-second exposures really provide a good indicator of seeing conditions. The good news is that even when my guiding gets above 1" on nights of bad seeing, the stars generally stay round. Usually, though, I'm getting between 0.5 - 0.8".

Right now (with full moon shining on the gear) I'm at roughly 0.40-0.50" RMS, even though sometimes there's a quick jump up to 0.7"-ish but that's really because the sky is flooded with light and my guidescope has a hard time attaching to a star. I love the AM5 except for one thing: when I do polar alignment and release the altitude knob, my telescope tends to slide down under its own weight so I have to hold it, adjust altitude ever so slightly, re-tighten the knob, measure, rince & repeat... this makes my polar alignment a bit longer than with my previous mount. However this is relatively minor and the ease of use, resistance to wind and guiding stability make up for this small inconvenience.


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#256 Ancient Photons

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Posted 06 April 2023 - 07:46 PM

Fast forward to a few days later when the stars were visible again: my first picture was disappointing with all stars displaying strong coma; since I had a few weeks to study collimation theoretically, I knew this was a sign of decollimated primary so I then used a defocused star to try and recenter it as much as possible; after about an hour using trial and error I managed to get relatively round and even stars. One last step was just retouching the secondary alignment a bit - since I had moved the primary quite a bit, the initial indoor secondary collimation was no longer good.

 

Glad you're getting it tuned in! I highly recommend you check the primary alignment using only a Cheshire and a bright flashlight. Just remove the baffle extension and shine the flashlight into the Cheshire. You should see something like this:

 

3oqxIgYl.jpg

 

The black ring the red arrow is pointing to should be concentric - this indicates a well-aligned primary. When you used the REEGO to align the primary, you aligned it to the far outer ring - but if there's any tilt in your focuser, this is a terrible reference.

 

Anyway, sounds like you're getting it worked out, so if it ain't broke....

 

And yes, the AM5 has so many positives that the minor negatives don't even bug me. The portability of it really saves my back - I can carry it outside fully loaded, which means I'll be way more motivated to image when the skies are clear. My EQ6R Pro was HEAVY.


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#257 astrobananeck

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Posted 06 April 2023 - 08:07 PM

One issue I have with the RC6 is that the internal baffle (the one over the hole inside the primary) is impossible to remove without disassembling the telescope because there is not enough space in the secondary spider to push an (adult) hand through and into the tube. So I sadly cannot use this simple method (I believe on an RC8 it's easier to reach through the spider and grab the baffle to remove it)...

 

I'll post a raw light sub - right now I think I'm pretty close to the stage you call "if it ain't broke..." haha!


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#258 Ancient Photons

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Posted 06 April 2023 - 08:14 PM

One issue I have with the RC6 is that the internal baffle (the one over the hole inside the primary) is impossible to remove without disassembling the telescope because there is not enough space in the secondary spider to push an (adult) hand through and into the tube. So I sadly cannot use this simple method (I believe on an RC8 it's easier to reach through the spider and grab the baffle to remove it)...

I'll post a raw light sub - right now I think I'm pretty close to the stage you call "if it ain't broke..." haha!


Ah, that's right, you have an RC6. Bummer you can't remove the baffle extension! Looking forward to seeing your sub, and hopefully once you get everything aligned it will hold for a long time.

#259 astrobananeck

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Posted 07 April 2023 - 05:21 AM

OK so it's not perfect but still much better than before. Visually the stars are reasonably round (when pixel peeping I can still see one brighter edge in each star, meaning there is residual coma maybe - but it could also have other causes).
 
CCD Inspector, run over 11 uncalibrated subs (taken with ASI ZWO 533MC) and then 20 uncalibrated subs (with ASI ZWO 1600MM) to average out any local glitches, indicates collimation at 1.8" with the 533 and 0.0" with the 1600. Below a certain value, according to the manual, the results given by CCD Inspector can become a bit random so here I'm not giving credit to a defined value, instead let's say it's <2" which is good enough for me. The field curvature could be just a little bit more centered on the 533 so I was going to tweak the secondary alignment just a tiny little bit with the REEGO but then with the 1600 set it looks centered (the absolute value is higher which is expected on a larger sensor) so again, I'm probably in the "good enough" zone here.
 
2023 04 07 121550
2023 04 07 115929
2023 04 07 122842

 


Edited by astrobananeck, 07 April 2023 - 05:37 AM.


#260 DevonRob

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Posted 24 May 2023 - 03:23 AM

New to this thread and new RC8 owner. Actually, I'm not too unhappy with my early bench collimation steps so far - definitely a learning process, but one I seem to be getting better at. Plan to try the Hall of Mirrors idea next too. One thing I do struggle with a bit is the final star test - although a couple of posts in this thread have made me realised I'm probably not picking a bright enough star, defocusing enough, and exposing long enough. One Q - does NINA have a tool which overlays circles and a centre cross like shown in earlier post for ease of use when out imaging:

 

https://www.cloudyni...n-star-example/

 



#261 DevonRob

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Posted 24 May 2023 - 03:31 AM

Thanks Gerrit! That's a good question - the decoupler is collimate-able separately from the primary mirror cell. It has much larger push/pull screws attached to some aluminum that has been securely bolted to the backplate. The guy who machined it cut out "jelly bean" slots so that you can still access the primary collimation bolts through the decoupler. If you haven't checked out nateman_doo's thread on it, here it is. He has lots of excellent pictures of the design:

 

https://www.cloudyni...mation-adapter/

 

It's the best solution I've seen to fully decouple the focuser - way better than the collimation ring GSO sells. So, essentially once I received the mirror cell with decoupler, I reassembled the telescope, aligned my mirrors using the Cheshire, aligned the focuser (decoupler) using the Cheshire, then checked the mirrors again (which needed a very slight adjustment).

 

Here's what mine looks like - I put black foam tape around it to mitigate potential light leaks:

V8OrMyL.jpg

 

Love this adaptation - sadly I'm not nearly mechanically minded enough to craft my own. Pity no one has made a kit form for DIY muppets like me.


Edited by DevonRob, 24 May 2023 - 03:32 AM.


#262 lambermo

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Posted 24 May 2023 - 04:02 AM

Love this adaptation - sadly I'm not nearly mechanically minded enough to craft my own. Pity no one has made a kit form for DIY muppets like me.

nateman_doo can make one for you, he has helped several ppl already in that thread https://www.cloudyni...mation-adapter/


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#263 DevonRob

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Posted 24 May 2023 - 04:25 AM

nateman_doo can make one for you, he has helped several ppl already in that thread https://www.cloudyni...mation-adapter/

I may just contact him!



#264 DevonRob

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Posted 24 May 2023 - 05:14 AM

One Q - does NINA have a tool which overlays circles and a centre cross like shown in earlier post for ease of use when out imaging:

 

https://www.cloudyni...n-star-example/

Answering my own Q here - but this looks to be just the job:

 

https://www.cloudyni...pe-collimation/




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