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Viewing through the Collimation cap

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#26 Vic Menard

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Posted 12 October 2018 - 07:13 PM

...That was great advice, Vic. Those steps seem to have helped me make some progress. How does it look now? 

A little better. You actually applied a bit too much correction--but we may be able to use it.

 

If you think it through, the secondary mirror tilt adjustment screw (green circle) moved the primary mirror center spot toward the primary mirror (green arrow). If you find the secondary mirror tilt adjustment screw between the two spider vanes at 12 and 3 o'clock (red circle) and turn it the same direction, it should move the center spot closer to the center (red arrow). You should also see the spider vane end attachment at 12 o'clock disappear. You'll know you're very close when the spider vanes all look the same and the bright primary mirror reflection appears "centered" in the secondary mirror. If you rack the focuser out a bit, it should make the bright primary mirror reflection look larger in the secondary mirror, which will make it easier for you to see the alignment improvement.

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#27 Vic Menard

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Posted 12 October 2018 - 07:19 PM

And to clarify--the secondary mirror tilt adjustment screws I'm talking about are located behind the secondary mirror (not the primary mirror like the circles I provided might seem to indicate). The circles show where the screws are located relative to the front aperture of the OTA.


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#28 Vic Menard

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Posted 12 October 2018 - 09:15 PM

...You need to tilt the far end of the secondary mirror (the end closest to the primary mirror) away from the focuser side of the OTA. This will bring the primary mirror center spot into the center of the field of view...

I got a PM from Jason who caught my mistake--to bring the center spot into the center of the field of view you need to tilt the far end of the secondary mirror toward the focuser side of the OTA--push the cross hairs, not the center spot...

 

My apologies, although the OP seems to have found the right direction on his own! That's what I get for being in a hurry. Fridays!


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#29 SteveG

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Posted 14 October 2018 - 01:33 PM

That's the last thing I want. Thanks for the heads up.

 

 

I can't seem to make it any better than the following picture (I can't spot much offset from my original position tbh). The spider vane end attachment has been stubborn. Another thing I noticed is that whatever adjustments I make, I'm unable to get the last spider vane attachment into the view (the one opposite to the stubborn one).

 

attachicon.gif 2.jpg

Just so you're clear, collimation is not done by adjusting spider vanes. The vanes should be very tight, and perfectly straight across, with the center exactly in the center of your optical tube.



#30 druhela

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Posted 14 October 2018 - 02:06 PM

Thanks guys for all the help! I'm quite near perfect collimation, if I'm not wrong. But I'm running with what I have for now. Star tested on Capella, and it doesn't seem all that bad. I've got the whole process in theory. Have also come to the conclusion that it's really hard to make the adjustments of the secondary accurately without using a pointing tool. So I'm going to acquire a cheshire+sight tube combo tool soon. Once I do, will attempt another collimation and post pictures on this thread for you guys to see how that looks.

Edited by druhela, 14 October 2018 - 02:07 PM.

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#31 Asbytec

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Posted 31 December 2018 - 04:59 AM

Hi,

 

I am in desperate need for help to understand why I can not see all six primary clips. I can only see the top 3 and no matter what, I can not center the view to see the clips. Why is this happening?

 

Secondary adjustment doesn't work, I have rotated that thing almost a complete 360.

 

<snip>

I ran across this thread and read your post. I had the same problem. Did you manage to fix it? Vic mentioned the solution, I believe, when he talked about the "three circles" in post #4 and others have touched on the solution, too. 

 

This is not a "no matter what" condition, you can and will see the three clips and, more importantly, the entire primary. If you can imagine hand holding a small mirror directly under the focuser, you can rotate and tilt it so all three clips are visible - meaning the entire primary mirror is visible, too. This is where your diagonal needs to be and you get there by tilt and rotation of the secondary (and maybe some longitudinal movement, too, if necessary). However, to me, it's not so much about the clips, they are just along for the ride. It's about seeing the third, innermost circle - the reflection of the primary's edge. If you tilt and rotate the hand held mirror until the entire mirror is visible, you will see all three, four, or six clips and importantly the entire primary mirror. This is where the diagonal needs to be to start, so we need to put it there.  

 

I think the trick is to start small and work up. What I mean is, draw the focuser inward until you can see the entire primary mirror from the peep hole in the collimation cap or site tube. Center the primary reflection as best you can using tilt and rotation. Then back off a little so the image of the primary expands a little, then tweak it again until it's better centered within the secondary edge. Then pull the focuser further out until the primary reflection is very nearly the same size as the secondary and tilt and rotate it again, as needed. At this point, and assuming the secondary is concentric with the bottom of draw or site tube (the first two circles), then the third circle - the primary mirror reflection - will be concentric with them. All three circles will be concentric and you will see the mirror clips (really, you see the clips because the primary reflection is well centered in the secondary). 

 

At this point, if you have a site tube or combo tool, tilt the secondary until the cross hairs hit the (well placed) primary center mark. You might induce some rotation error, just fix it. (A milk jug washer comes in handy for tiny rotation error). Repeat the tilt until the cross hairs are on the center mark with the mirror reflection centered in the diagonal. It should be fine through the full range of focuser motion. If you centered the primary mirror in the diagonal at the maximum focuser outward motion, then it should be very well centered as the focuser is moved inward. The clips will be easily seen all around and you know you have centered the field illumination (where you can see the entire primary mirror.) And with the cross hairs over the center mark, you have good focuser axis alignment.

 

You're almost done. :)

 

Disclaimer: I am new at this, too, about a month into my week long exercise with a combo Cheshire/site tube and some star test confirmation. But, getting results by watching, reading, thinking through, and performing. :) 


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#32 Icaro

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Posted 31 December 2018 - 08:21 AM

Decided to take the plunge and possibly screw up a perfectly collimated scope, in order to learn the process. And it seems I’ve done an spectacular job of screwing things up because last night all I saw when pointing at the sky was blackness in my EP.

So looks like I’m going to be spending lots of time today with this thread nearby as a guide, to see if I can bring light once more into my scope.

This is not as simple to learn as some seem to think,, for a newbie anyway. It’s down right frustrating as a matter of fact. I can only hope that once I get the hang of it,, it’s no big deal.

#33 Asbytec

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Posted 31 December 2018 - 11:29 AM

I think it's a huge help to understand what's going on. Basically you center the diagonal under the focuser for field illumination.

Second, you tilt the secondary to point the focuser axis to the center of the primary. The site tube peep hole, cross hairs, and primary center mark define that axis. So does a laser.

Doing these two things at the same time can be tricky. That's the hard part.

Lastly, bring the primary axis to bear at the center of the eyepeice defined by the Cheshire ring or collimation cap peep hole. That's it. All else is detail in how to do it.

It does take some practice and trial and error, but once it comes it does become easy.

Edited by Asbytec, 31 December 2018 - 11:37 AM.


#34 Sky_LO

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Posted 03 January 2019 - 09:18 PM

Decided to take the plunge and possibly screw up a perfectly collimated scope, in order to learn the process. And it seems I’ve done an spectacular job of screwing things up because last night all I saw when pointing at the sky was blackness in my EP.

So looks like I’m going to be spending lots of time today with this thread nearby as a guide, to see if I can bring light once more into my scope.

This is not as simple to learn as some seem to think,, for a newbie anyway. It’s down right frustrating as a matter of fact. I can only hope that once I get the hang of it,, it’s no big deal.

I have a nextstar I bought used.   Got a laser collimator and the red dot was coming right out of the end of the tube and did not even hit the secondary mirror because the primary was so far off.    Bottom line is after adjustments, I "saw blank sky" as well.   But I knew my collimation was fairly good.   It turned out that a significant collimation adjustment required me to readjust the finder.  You may be ok on the collimation but need to re-align the finder.  This is what I experienced and hope it helps.    

-Lauren




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