Star Collimation of RC

[JC17 (John)]

Hi everyone
I have a 10" RC that is also fitted with a Moonlite focuser (with the collimation adaptor plate). I have tried collimating the optics many times utilising a Takahashi collimating scope and also the Howie Glatter laser. Those steps appear to be reasonably successful however when I re install the OTA onto my mount and connect my cameras, I see that the defocused stars donuts are slightly brighter on one side. This messes up SGP autofocusing routine that picks the bright sides as in-focus stars amongst a lot of out of focus stars.
My question is when trying to fine tune the collimation using a star - do I adjust the primary mirror, the secondary mirror or the focuser collimating ring?
Any tips welcome as I have now lost all remaining hair!

[glend (Glen)]

I hope you have never touched the RC primary, as once changed, even slightly, there is a world of pain to get if back. There are many threads on various forums (most on Cloudy Nights) dealing with RC collimation problems. Most RC collimation issues can be traced straight back to the owner, who messed it up in the first place. Research well, before you touch anything. And never buy a used RC, chances are many are sold because of collimation destruction.

PS, if this is a GSO RC, they are prone to focuser misalignment (even with a Moonlight), and the collimation ring can only fix it in one position ( as you rack the focuser in and out, you can watch your laser dot migrate back and forth across the field).

[Stonius (Markus)]

Originally Posted by glend

I hope you have never touched the RC primary, as once changed, even slightly, there is a world of pain to get if back.

The thing is, there's no guarantee that the primary hasn't shifted in shipping, or that the factory set it correctly in the first place. If you're going to have an RC, (or any equipment really) it's best to fiddle and get to know what everything does so you can get yourself out of trouble. But I do think there is a lot of fear and disinformation about how difficult RC's actually are.



Simply put, if you mess with the primary spacing the result will be a ten-fold shift in focus and (IIRC) worsening field curvature. No biggie. You'd have to adjust it quite a *lot to make it completely unable to achieve focus (like cranking the primary by a centimeter or so, which is a lot).


Assuming you haven't done this, all you have to do is solve some plates to get a sense of your real focal length and the direction it needs to be adjusted in to get the spacing back within the spec of your mirror. If you have CCD inspector, so much the better, but it isn't necessary.



I like the simplicity of the DSI method for evaluating issues in collimation. Essentially you adjust the primary (yes, the primary!) until there is no on-axis coma. Any residual coma in the edge of field should be adjusted out via the secondary.


Since your bright edges are effecting the *entire field, I would say the primary is precisely what requires adjusting (with apologies to Glen).

Originally Posted by glend

And never buy a used RC, chances are many are sold because of collimation destruction.

I don't really think that's very fair. A lot of people rely on collimation methods that rely on physical aspects of the scope being aligned with the optical axis, which is not necessarily the case (baffle tubes, or centre-dots, for example). That's why I avoid the Tak collimation scope. And as mentioned before, if your problem affects the entire field and you're too scared to touch the primary, the problem will never go away no matter how much you wrestle with the secondary or focuser plate.



If you have the holographic attachment for the Howie Glatter, a good sanity check is to cut out some translucent paper or film (I use a white translucent plastic film they use for diffusing lights) and place it over the end of the OTA. It will need to be a doughnut shape to accommodate the secondary screws. The circles from the holographic paper will project onto the paper allowing a quick alignment check. For me, this works better than projecting on a wall because a) there's no guarantee the wall is orthogonal and b) there are no walls in the bush. :-)

Originally Posted by glend

PS, if this is a GSO RC, they are prone to focuser misalignment (even with a Moonlight), and the collimation ring can only fix it in one position ( as you rack the focuser in and out, you can watch your laser dot migrate back and forth across the field).

But wouldn't that kind of banana-ing imply that the Moonlight focuser was not tracking straight? They do have adjustments for that sort of thing, though I've never had to mess with mine. I can't for the life of me think of a situation that would cause that to happen in the scope without something flexing or moving that shouldn't, like a focuser plate that is not locked down?


Sorry to disagree with you Glen, but I guess a range of opinion is what forums are all about. :-)



Cheers, Markus

[multiweb (Marc)]

Originally Posted by JC17

Hi everyone
I have a 10" RC that is also fitted with a Moonlite focuser (with the collimation adaptor plate). I have tried collimating the optics many times utilising a Takahashi collimating scope and also the Howie Glatter laser. Those steps appear to be reasonably successful however when I re install the OTA onto my mount and connect my cameras, I see that the defocused stars donuts are slightly brighter on one side. This messes up SGP autofocusing routine that picks the bright sides as in-focus stars amongst a lot of out of focus stars.
My question is when trying to fine tune the collimation using a star - do I adjust the primary mirror, the secondary mirror or the focuser collimating ring?
Any tips welcome as I have now lost all remaining hair!

Are you saying the concentric rings have different brightness in and out of focus or the actual defocused star has a ring that's brighter on one side and is not symetrical?

Also take off the focuser tilt adapter for now. So just thread the focuser to the back as is.

[Sayfog (Alistair)]

Follow the deep sky instrument method of collimation after getting it close with the tools, they're a nice help but ultimately nothing compares to a star test. (except for I guess some fancy optical test setup, but that's not really practical for most of us)

[Paul Haese]

Star testing an RC and doing the required final adjustment will involve moving the primary. If all your preliminary work is done properly then it should be an ever so slight star test adjustment.

Just to be clear you need to first align the focuser with the secondary mark (use a laser for this and use the focuser adjustment screws or a focuser collimation ring), then the secondary to the primary and then finally align the primary to the secondary (all done with a Takahashi Collimation scope).

I would recommend doing the adjustments when the scope is on the mount. Any knocking or bumping must be assumed to affect collimation and hence even a new scope will require collimation. Be also careful to just lightly tighten screws. If you apply too much pressure, you will cause pinching of the optics. I also therefore don't agree that you should never touch the primary. The reason the screws are not glued tight is because it is expected that you will need to adjust them to obtain perfect collimation.

It can be a little tricky to get collimation right but the results can be fantastic when you do get it right. If you can still get it, there is a video from Ken Crawford which shows how to collimate an RC with a Tak scope.

Hope that helps.

[JC17 (John)]

Originally Posted by multiweb

Are you saying the concentric rings have different brightness in and out of focus or the actual defocused star has a ring that's brighter on one side and is not symetrical?

Also take off the focuser tilt adapter for now. So just thread the focuser to the back as is.

Hi Marc
Yes the donuts are slightly brighter on one side. In focus stars look ok but out of focus stars are brighter on one side. SGP picks up the bright spots as stars soo it fails the auto focus most times.
Thanks for the tip - I’ll see if I can remove the focusing ring - I did try to set it back to zero adjustment - ie set all screws back to zero then out one turn in an attempt to get a starting point.
Having three points of adjustment increases the complexity,

[JC17 (John)]

Originally Posted by Paul Haese

Star testing an RC and doing the required final adjustment will involve moving the primary. If all your preliminary work is done properly then it should be an ever so slight star test adjustment.

Just to be clear you need to first align the focuser with the secondary mark (use a laser for this and use the focuser adjustment screws or a focuser collimation ring), then the secondary to the primary and then finally align the primary to the secondary (all done with a Takahashi Collimation scope).

I would recommend doing the adjustments when the scope is on the mount. Any knocking or bumping must be assumed to affect collimation and hence even a new scope will require collimation. Be also careful to just lightly tighten screws. If you apply too much pressure, you will cause pinching of the optics. I also therefore don't agree that you should never touch the primary. The reason the screws are not glued tight is because it is expected that you will need to adjust them to obtain perfect collimation.

It can be a little tricky to get collimation right but the results can be fantastic when you do get it right. If you can still get it, there is a video from Ken Crawford which shows how to collimate an RC with a Tak scope.

Hope that helps.

Hi Paul
Thanks for the response. I have tried many procedures, the Takahashi procedure has been successful in the past but recently I noticed guide stars had tails. I checked out and followed YouTube procedures (Stephen Kirk and Allen Mitchell) and digested the OPT and DSI procedures too.
I’m thinking that I may need to persevere with the DSI procedure for the final stage because I am reasonably confident that the laser collimating (using the Howie Glatter) is pretty close.
I was just unsure what part of the train needed the adjustment.
The other concern is the laser collimating is completed indoors on my desk, then I re mount my OTA onto my mount and then reattach my cameras so that is likely to change things a little.
Cheers
John

[JC17 (John)]

Originally Posted by Sayfog

Follow the deep sky instrument method of collimation after getting it close with the tools, they're a nice help but ultimately nothing compares to a star test. (except for I guess some fancy optical test setup, but that's not really practical for most of us)

Hi Alistair
Thanks for your advice - that is pretty much what I am thinking too, I was concerned about what to adjust and being aware that playing with too many things will not help!
Cheers
John

[JC17 (John)]

Thanks for the advice everyone.
I admit that I was quite concerned about messing with the primary mirror so spent weeks reading and watching YouTube videos before starting the journey.
I first tried the Takahashi scope that I had successfully used previously but my guide stars in particular (OAG) had tails.
So then I had several goes using the Howie Glatter laser and to be honest the desktop results looked good. However once I put my OTA back onto the mount and connected my cameras the images confirmed that more tweaking is required.
I am also thinking that I should be following the final stages of the DSI method - I avoided that because it looked quite complex. I may be able to utilise an artificial star (initially at least) to make it easier.
Cheers
John

[multiweb (Marc)]

Originally Posted by JC17

Hi Marc
Yes the donuts are slightly brighter on one side. In focus stars look ok but out of focus stars are brighter on one side. SGP picks up the bright spots as stars soo it fails the auto focus most times.
Thanks for the tip - I’ll see if I can remove the focusing ring - I did try to set it back to zero adjustment - ie set all screws back to zero then out one turn in an attempt to get a starting point.
Having three points of adjustment increases the complexity,

Do you have a webcam or planetary camera? Could you post a few pics of the airy pattern at focus, then IN and OUT of focus?

[JC17 (John)]

Hi Marc
I’ll try and get some images when I get a chance over the next few nights. Not looking too hopeful though😩

[JC17 (John)]

Hi Marc (and anyone else!)
I will upload a few images to show what I am seeing. Clearly the star donuts are brighter to the bottom left - both inside and outside of focus. The secondary shadow is also offset to the bottom left. So now to work out what that really means and how to fix it so any tips would be welcome.

[Paul Haese]

Hi John, that is definitely collimation error. So you'll need to collimate on the mount and be more particular. All the rings must be concentric.

[multiweb (Marc)]

Originally Posted by JC17

Hi Marc (and anyone else!)
I will upload a few images to show what I am seeing. Clearly the star donuts are brighter to the bottom left - both inside and outside of focus. The secondary shadow is also offset to the bottom left. So now to work out what that really means and how to fix it so any tips would be welcome.

Oh well easy. What Paul said.

[xthestreams (Paul)]

Originally Posted by JC17

Hi Marc (and anyone else!)
I will upload a few images to show what I am seeing. Clearly the star donuts are brighter to the bottom left - both inside and outside of focus. The secondary shadow is also offset to the bottom left. So now to work out what that really means and how to fix it so any tips would be welcome.

Yup clearly out of collimation - the DSI method is your friend (in fact I've come to the conclusion that once you've roughly centred the axis and spot, throw away the Tak and go DSI - it's surprisingly quick once you've got the hang of it).

As a few have mentioned, if yours in the older solid tube model with the primary attached to the focuser, then you might have challenges keeping it in collimation - if you send a photo of the back plate through, we can probably figure that out.

The GREAT thing about these scopes is that once you've mastered collimation a). pretty much everything else is easier after this and b) as Paul mentioned, you will get some LOVELY images!

Once you've reached final collimation you will want to check the spacing - preferably with a Ronchi, otherwise plate solve to determine your actual focal length and compare that to the stated specs (2000mm from memory) which can then be adjusted either via the knurled ring on the secondary or careful adjustment of the primary (moving all three screws in/out the EXACT same number of turns) or getting really courageous and adjusting the central screw on the secondary (VERY MUCH NOT RECOMMENDED UNLESS YOU ARE BRAVE/FOOLISH).

Chris Woodhouse book has an excellent section dedicated to the fine art of RC collimation, I would consider it mandatory reading. (the whole book in fact)

[JC17 (John)]

Thanks Paul & Marc
Yep I figured that the collimating is just not quite there.
I’ll persevere with the DSI method when the clouds nick off!
I have the carbon fibre tube model with the Moonlite focuser tilt plate so have three things to move which increases complexity.
My focal length from plate solving is 2030 mm.
It’s a journey and like most of this hobby its about working out how to do something then doing it a few times and try not to forget what and how you did it!

[Stonius (Markus)]

"its about working out how to do something then doing it a few times and try not to forget what and how you did it!"


I can very much relate to this.


Markus