Jump to content


Photo

Collimation Eyepiece Test-double discs? Help

  • Please log in to reply
11 replies to this topic

#1 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 06 August 2014 - 05:38 PM

Hi,

I just received a 70mm X 500mm  rokinon achromat ota that I was hoping would be a grab and go or super finder.   Its quite inexpensive.  I decided to put my collimation chesire EP in it to see how it looked.  I racked out the focuser (plastic) and I see 2 airy discs, side by side, but each have very good concentric rings!   The 2 airy discs are touching / overlapping each other just a small amount.  I checked the chesire on my 102mm achro and it looks just fine.

So how can this be?   What can I do?  I loosened the objective's retaining ring, shook it a bit and retightened it to just touching.  No change.  It has 3 spacers which look OK.   

I loosened the 3 focuser bolts and moved them around and it did change the discs closeness to each other, but I really had to put a lot of pressure on the focuser to the side to do this.

Any ideas?

Thanks

Bob



#2 choran

choran

    Vanguard

  • *****
  • Posts: 2360
  • Joined: 28 Dec 2012

Posted 07 August 2014 - 08:32 AM

If the discs are overlapping, you are very very close and any improvement might not be detectable visually.  Another thought:  Have you tried the collimating eyepiece in your diagonal as well as straight through without using the diagonal?  It may be closer when you use the diagonal.  If not, some diagonals allow a bit of adjustment internally.  *I am assuming your scope does not have collimation screws in the objective cell.)  Sagging of the draw tube can also cause the discs not to lay perfectly upon one another, so perhaps some adjustment of the draw tube tension might help.  If the views are acceptable, don't worry too much about it.

Some people try to adjust the focuser housing placement by filing the screw holes a bit larger to allow a bit of play.  I would be a little wary of that, and realize that close enough might very well be fine. Another thing to try (though you have done something very similar) is to point the scope toward the zenith, then loosen the lens retaining ring, and gently tap on the outside of the cell for a minute or two, sort of drumming on the tube with your fingers.  Then, with the scope still in that position, gently tighten the ring until it just touches.  If the lenses have become tilted slightly in the cell, this might get them settled down to the square position where they belong.


Edited by choran, 07 August 2014 - 08:53 AM.

  • spongebob@55 likes this

#3 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 07 August 2014 - 09:18 AM

If the discs are overlapping, you are very very close and any improvement might not be detectable visually.  Another thought:  Have you tried the collimating eyepiece in your diagonal as well as straight through without using the diagonal?  It may be closer when you use the diagonal.  If not, some diagonals allow a bit of adjustment internally.  *I am assuming your scope does not have collimation screws in the objective cell.)  Sagging of the draw tube can also cause the discs not to lay perfectly upon one another, so perhaps some adjustment of the draw tube tension might help.  If the views are acceptable, don't worry too much about it.

Some people try to adjust the focuser housing placement by filing the screw holes a bit larger to allow a bit of play.  I would be a little wary of that, and realize that close enough might very well be fine. Another thing to try (though you have done something very similar) is to point the scope toward the zenith, then loosen the lens retaining ring, and gently tap on the outside of the cell for a minute or two, sort of drumming on the tube with your fingers.  Then, with the scope still in that position, gently tighten the ring until it just touches.  If the lenses have become tilted slightly in the cell, this might get them settled down to the square position where they belong.

Ok, great ideas.  Let me try some of those.   I was doing it w/o the diagonal and have a couple with which to try that with.  I've done the filing once with another ST80 and wasn't too happy doing that.   I just wish that these (all) scopes would be better so we didn't have to futz around with them new, but I know that that's unrealistic.   Thanks for your answer!

Bob



#4 choran

choran

    Vanguard

  • *****
  • Posts: 2360
  • Joined: 28 Dec 2012

Posted 07 August 2014 - 03:57 PM

It's f7, so a bit forgiving of collimation, though not to extent that an f15 would be, for example.  Some of the guys get very good at fine tuning, and take the doublet apart and rotate the lenses against one another until they get the best view possible.  Some also experiment with different spacing and so forth.  It is possible that if one spacer is too thick that it can throw things off as well.  Bottom line, overlapping circles is pretty darned good.  It's also possible that the Chesire can be off a bit, either due to its own minor error, or due to a registration error in the tube.  Have you tried rotating the cheshire and seeing if the two circles move?  If they do, it may just be the cheshire that is giving a tiny bit of a false reading.  In any event, you are very close, and even if you don't get it picture perfect, it's good to go, I'll bet.



#5 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 18 August 2014 - 03:57 PM

Well it seems like if I move the focuser to one side, it will get both discs to combine with each other.  Of course the space between the end of the tube and the 'shoulder' of the plastic focuser goes from nil to about 2mm.  It now star tests ok, but I get bending of the outer rings at all three spacers.  The Chesire checks out well on my 102mm refractor so I know its not the problem. Time to blacken the edges of the objective and see if there's any issues with the spacers.   Everything else is OK.   I'll have to try testing it again.  Its a nice small scope and would love for it to work up to its potential.

Thanks for your ideas.

Bob



#6 choran

choran

    Vanguard

  • *****
  • Posts: 2360
  • Joined: 28 Dec 2012

Posted 19 August 2014 - 08:56 PM

Have you tested with a diagonal in place?



#7 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 20 August 2014 - 09:43 AM

Well, I took off the focuser and put a 90* angle on it and the tube is not square.   I've had to extend the holes a bit to get the concentric circles to joint up.   Problem is, that it doesn't seem to hold.  Just now I looked at it with the cheshire and its out again.  The same if I put on the diagonal.   I do this with the focuser racked out all the way.   Then when I rack it in, it gets worse.  Ug.  I know this is an inexpensive scope, but still, I should be able to get it to some kind of decent collimation.   I was out the other night and looked at Saturn and just before it focused, it was sort of comet with tail looking.   I didn't look at any stars unfortunately.   Oh well, back to the drawing board.  :mad:



#8 choran

choran

    Vanguard

  • *****
  • Posts: 2360
  • Joined: 28 Dec 2012

Posted 20 August 2014 - 02:03 PM

Well, I took off the focuser and put a 90* angle on it and the tube is not square.   I've had to extend the holes a bit to get the concentric circles to joint up.   Problem is, that it doesn't seem to hold.  Just now I looked at it with the cheshire and its out again.  The same if I put on the diagonal.   I do this with the focuser racked out all the way.   Then when I rack it in, it gets worse.  Ug.  I know this is an inexpensive scope, but still, I should be able to get it to some kind of decent collimation.   I was out the other night and looked at Saturn and just before it focused, it was sort of comet with tail looking.   I didn't look at any stars unfortunately.   Oh well, back to the drawing board.  :mad:

Sad truth is sometimes there is just so much movement and misalignment along the whole trail, you just have to settle for the best you can get.  You can go crazy shimming this and that, and filing holes, etc. The diagonal itself can be miscollimated.  There can be slop in the focuser and/or draw tube.  There can be registration errors, i.e., eyepieces can be tilted, the diagonal can be tilted in its holder.  As you point out the tube might not be square at one or BOTH ends.  My advice is don't obsess over it too much.  I would probably try to get it as close as possible with only your cheshire in the draw tube, with no accessories or adapters beyond what you need to get the cheshire in play.  You might be able to artificially "square" the tube if it's not off by much by using small strips of masking tape on the "low" spots, and then just fan the focuser assembly as tight as you can against your artificially squared surface, and tighten the screws holding it to the OTA the best you can. 



#9 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 20 August 2014 - 07:26 PM

 

Well, I took off the focuser and put a 90* angle on it and the tube is not square.   I've had to extend the holes a bit to get the concentric circles to joint up.   Problem is, that it doesn't seem to hold.  Just now I looked at it with the cheshire and its out again.  The same if I put on the diagonal.   I do this with the focuser racked out all the way.   Then when I rack it in, it gets worse.  Ug.  I know this is an inexpensive scope, but still, I should be able to get it to some kind of decent collimation.   I was out the other night and looked at Saturn and just before it focused, it was sort of comet with tail looking.   I didn't look at any stars unfortunately.   Oh well, back to the drawing board.  :mad:

Sad truth is sometimes there is just so much movement and misalignment along the whole trail, you just have to settle for the best you can get.  You can go crazy shimming this and that, and filing holes, etc. The diagonal itself can be miscollimated.  There can be slop in the focuser and/or draw tube.  There can be registration errors, i.e., eyepieces can be tilted, the diagonal can be tilted in its holder.  As you point out the tube might not be square at one or BOTH ends.  My advice is don't obsess over it too much.  I would probably try to get it as close as possible with only your cheshire in the draw tube, with no accessories or adapters beyond what you need to get the cheshire in play.  You might be able to artificially "square" the tube if it's not off by much by using small strips of masking tape on the "low" spots, and then just fan the focuser assembly as tight as you can against your artificially squared surface, and tighten the screws holding it to the OTA the best you can. 

 

 

Yeah, I'm starting to feel like 'oh well', you tried.   But anyway.....I moved the focuser again to the best concentric circles I could get and then tightened the screws.  In order to hold those positions, I put some Elmers in the gap between the end of the aluminum tube and the focuser housing.  If it holds, it should be the best it'll get......if it holds.

Thanks

Bob



#10 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 8286
  • Joined: 22 Feb 2010
  • Loc: You can't see me...

Posted 20 August 2014 - 09:08 PM

I only use the sight tube to adj. the objective. I shoot a laser, in the focuser,  thru the objective with a paper in the front marked with a center spot to align the focuser.



#11 choran

choran

    Vanguard

  • *****
  • Posts: 2360
  • Joined: 28 Dec 2012

Posted 21 August 2014 - 04:25 PM

Spongebob, here's good article on refractor collimation that might be of interest.

http://philjay2000.t.../adventures.pdf



#12 spongebob@55

spongebob@55

    Viking 1

  • *****
  • Posts: 961
  • Joined: 26 Dec 2011
  • Loc: Bergen Co. New Jersey

Posted 08 September 2014 - 08:01 PM

Well I've moved the focuser back and forth a bit, elongated 2 screw holes and have spot glued the 'gap' between the tube and the shoulder of the focuser, so it won't move back.   I got good concentric rings.........so......

Last night I decided to take it out and do a star test and have some fun with it.   I got a nice star test, with only the outer most ring being slightly deformed at the 3 places where the spacers are between the 2 objective glass.

Even though the 97% full moon was out, along with 25% clouds, I had a blast with this little 70mm.   Double cluster, M31, M35, M57, Mizar, Albireo, and the Moon of course.  Just lovely. Amazing how much fun you can have with a small aperture refractor.

Thanks for all your help and comments.

Bob








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics