Collimating question /Ocal collimation - Forums

Topic starter

Feb 6, 2023

Help need before i go stir crazy , scope is a 200pds ,fitted with a baader steeltrack focuser , now i inputted ocal camera focus code into text file ,for the life of me i cannot align the primary edge and get the donut centred at the same time ,see pictures , ive alinged focuser tube using the off set , aligned secondary etc but for the life of me i can not get the primary aligned , checked with a collimation cap and i can see the clips are equal but donut slightly out , inserted concentre and donut bang on , though outer edge of primary looks slightly out through concentre corresponding to what i see in the Ocal ,so how can i get the primary mirror/clips to look centred and the Donut to centre at the same time or could it be the donut on primary isn’t quite centred ( yet to check and verify)secondary with primary blocked

Primary edge blue circle

=running
Quote

philhilo

Feb 6, 2023

I think you have things out of sequence. 1st make sure the secondary is centered in the tube i.e adjust the spiders as required. Then make sure it under the focus tube i.e in and out along length of OTA, then not twisted i.e presents as a circle, then centered on the primary - however if your primary is wildly out of kilter then it wont look right……I know this as I have a 200PDS and went through all the steps and couldn't get it to look right. Then I looked at the primary mirror cell - I could get 2 fingers between it and the back of the OTA on one side, and none on the other. Instead of it being at approx 90 degrees to the OTA tube it was probably at 80 or less (or 100 depending on how you measured it). I adjusted it until it was roughly 'flat' and then got the secondary centered on the primary and then adjusted the primary - job done.
Hope that helps, alas not used the OCAL collimator, always done it manually ( or avoided it as it is a permanent set up - until I took it to a star party which involved an hours drive down country lanes, bump,bump,bump). I use Astro Baby's guide (doen on a 200PDS) if you despair of the Ocal, but it helps to understand what you are doing.

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Dan Watt

Feb 6, 2023

Every import primary I've seen has the center mark sticker slightly off, definitely check that before you pull all your hair out.

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Matthew Sole

Feb 6, 2023 · Edited: Feb 6, 2023

Hi Dave,

I think it looks like you'll need to go back and make further adjustments to the tilt and rotation of the secondary mirror to better center the primary in the secondary reflection. You'll likely find this to be an iterative process as you rotate the secondary and then need to re-adjust the tilt screws to bring the primary back into center view.
Once you've got the primary bang in the center of the secondary and have adjusted the primary center spot, you might find you need to go back and adjust the secondary again. There can be a bit of back and forward and you've got to be fairly precise with your alignment of the outer circle edges against each mirror.

You might also find that as you adjust and tighten down the secondary screws, this has the effect of rotating the secondary in an undesirable way. Pretty hard to deal with but careful and gradual adjustment and tightening is the way.

Having said all that, I would say you'll not notice any residual collimation issues from what you have achieved so far. It'll likely be fine for your purposes and just leave with some off-centered illumination from the residual rotation error that'll mostly be noticeable in your flats but should calibrate out just fine.

If you get everything in alignment with the OCAL and your primary donut is off, then it is very likely not centered and you can trust the OCAL. The OCAL doesn't need a center donut but if you use a laser or similar to do quick collimation checks, then it can be good to fix up.

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Jacob Heppell

Feb 6, 2023

If the rotational position of the secondary isn't quite straight on with the focuser then the primary will never look centered. It won't make a difference to your ability to collimate and take images but for the sake of achieving an even illumination across your FOV it's best to get the vertical and rotational position of the secondary as accurate as possible. The tool I use for centering secondary mirrors is a concenter eyepiece. The process is somewhat iterative and I'll get the hotspot centered with the concenter, assess how centered the primary is to the focuser (basically the second picture in your post), then if the centering isn't good, I'll make a small rotational adjustment (the hotspot will now be off-center), re-center the hotspot and again assess the primary/focuser centering. Once it looks pretty good with the concenter eyepiece I'll then do a proper collimation with a cheshire and autocollimator. After that, I'll again check centering with the concenter. A few iterations and you'll be set. It sounds like a lot I know but properly centering a secondary is something you only have to do whenever you remove/reinstall the secondary. Otherwise you do it once then forget it.

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Dave B

Topic starter

Feb 7, 2023 · Edited: Feb 7, 2023

Thanks guys I will have another look , collimating isn’t new to me but I like the Ocal because I don’t need to squint and every thing is really clear and you can focus the camera , I was thinking I needed to adjust focuser tilt which looking at my 130pds is showing a similar issue as 200pds , adjusting the stock dual focuser is a fiddly thing so I can’t see skywatcher bothering to check focus tube alignment so I will tinker with that this week and see if I can get that aligned as adjusting does seem to work , on the baader Steeltack can adjust focus tube but I won’t fiddle with it until I can confirm on the 130pds that this is the issue , I will definitely check donut centering .

Dave