Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Collimation question................

  • Please log in to reply
33 replies to this topic

#1 astrogeek64

astrogeek64

    Vostok 1

  • -----
  • topic starter
  • Posts: 171
  • Joined: 30 Apr 2017
  • Loc: Chesapeake, Virginia

Posted 16 May 2018 - 12:51 PM

If you are having trouble getting a sharp focus, even thought a star test produces perfectly concentric rings, could it be that in making multiple adjustments over time you have moved the aggregate distance between primary and secondary mirrors? Thus throwing the system out of focus? I'm thinking such a movement, even of it's just a MM or so could cause focusing issues.

 

Am I wrong?

 

If not, does anyone have any ideas on a quick diagnosis to ensure this is the issue, before I spend an evening moving the primary in and out, evenly on all three points until it clears up?

 

Dave



#2 xiando

xiando

    Fly Me to the Moon

  • *****
  • Posts: 5985
  • Joined: 27 May 2015
  • Loc: Cloudy NEOhio

Posted 16 May 2018 - 12:58 PM

I don't *think so, as the focuser should provide leeway to accommodate such shifts,  but still, after many collimations, it may be wise to "reset" the primary and start fresh anyay (as well as the secondary) . Over the course of many field collimations, I walked my primary to the point where the screws were almost falling off a couple of times. I *do have ample focuser adjustment though, so maybe it can happen on a scope with a limited range of focus



#3 havasman

havasman

    Cosmos

  • ****-
  • Posts: 7799
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 16 May 2018 - 01:14 PM

If that is the case then you should be having trouble coming to focus because you are running out of travel on your focuser. If you still have travel left when you are unable to focus then there's likely another cause.

Are you getting good cooling/temp equilibration?

If you find that relocating the mirror solves your problem then you can avoid repeating the failure by only using the same two adjustment knobs each time you collimate.


  • AhBok likes this

#4 macdonjh

macdonjh

    Gemini

  • *****
  • Posts: 3021
  • Joined: 06 Mar 2006

Posted 16 May 2018 - 01:27 PM

If you are having trouble getting a sharp focus, even thought a star test produces perfectly concentric rings, could it be that in making multiple adjustments over time you have moved the aggregate distance between primary and secondary mirrors? Thus throwing the system out of focus? I'm thinking such a movement, even of it's just a MM or so could cause focusing issues.

 

Am I wrong?

 

If not, does anyone have any ideas on a quick diagnosis to ensure this is the issue, before I spend an evening moving the primary in and out, evenly on all three points until it clears up?

 

Dave

If you have a Newtonian, I doubt it, for the reasons xiando and havasman state.  You said you have a good star test, were you able to get sharply focused images during the same night you star tested?  Were your fuzzy images on a different night?  I ask because my guess is your problem is related to seeing, or as havasman suggested temperature imbalance.  If you're getting a fuzzy image, make sure you're not trying to observe an object above a neighbor's roof, try not to set up on concrete or other paving, make sure your scope is at thermal equilibrium, all that stuff.


  • Jon Isaacs likes this

#5 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6096
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 16 May 2018 - 01:46 PM

If you are having trouble getting a sharp focus, even thought a star test produces perfectly concentric rings...

I don't understand--to star test your scope you would need to be able to achieve a "sharp" focus.

 

Is this issue happening with your 8-inch f/4 Newtonian?

What tools are you using to collimate your scope?

Is the focus issue visual or imaging related?


  • astrogeek64 likes this

#6 havasman

havasman

    Cosmos

  • ****-
  • Posts: 7799
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 16 May 2018 - 01:54 PM

I don't understand--to star test your scope you would need to be able to achieve a "sharp" focus.

waytogo.gif

 

i think it is a very small subset of the people who say they star test their Newts that actually understand how to do or interpret the test.

I never claim to have star tested my scopes though I have done some version of that test several times and have used it to verify and/or tweak collimation (per info on Loptics.com) because I'm just not firmly confident in my execution or reading of the test.



#7 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • Posts: 7287
  • Joined: 21 Oct 2006
  • Loc: California

Posted 16 May 2018 - 06:45 PM

 even thought a star test produces perfectly concentric rings, 

 

Are you sure you are performing the star test correctly?

Can you elaborate? Approximately how many rings did you see? 3? 4? Or too many to count?

 

Refer to the attachment:

First image is for a star that is defocused too much. Rings appear concentric.

Second image is for the exact same setup but with a slightly defocused star (same star).. Now the rings are incomplete.

Third image is the the exact same setup but with a cheshire tool inserted to show how grossly the scope is miscollimated.

 

The point: Many will defocus a star too much and get the impression they have a well-collimated scope whenin fact their scopes are grossly miscollimated.

 

star_collimation.jpg

 

Jason


  • Jon Isaacs, Asbytec, havasman and 4 others like this

#8 astrogeek64

astrogeek64

    Vostok 1

  • -----
  • topic starter
  • Posts: 171
  • Joined: 30 Apr 2017
  • Loc: Chesapeake, Virginia

Posted 17 May 2018 - 12:21 PM

I don't understand--to star test your scope you would need to be able to achieve a "sharp" focus.

 

Is this issue happening with your 8-inch f/4 Newtonian?

What tools are you using to collimate your scope?

Is the focus issue visual or imaging related?

I referring to focus post star test. The scope in question is an F/9.75 - 200mm Maksutov Cassegrain. (Vixen VMC200L)

 

I have an Astrosystems laser, a 2" Farpoint Cheshire........ But, AFAIK, those are of little to no use with this type pf scope.



#9 astrogeek64

astrogeek64

    Vostok 1

  • -----
  • topic starter
  • Posts: 171
  • Joined: 30 Apr 2017
  • Loc: Chesapeake, Virginia

Posted 17 May 2018 - 12:25 PM

Are you sure you are performing the star test correctly?

Can you elaborate? Approximately how many rings did you see? 3? 4? Or too many to count?

 

Refer to the attachment:

First image is for a star that is defocused too much. Rings appear concentric.

Second image is for the exact same setup but with a slightly defocused star (same star).. Now the rings are incomplete.

Third image is the the exact same setup but with a cheshire tool inserted to show how grossly the scope is miscollimated.

 

The point: Many will defocus a star too much and get the impression they have a well-collimated scope whenin fact their scopes are grossly miscollimated.

 

 

 

Jason

 

Thanks. I go with the slightly defocused. 2-4 rings max. The next time I do the testing, I'll take some shots. That might help,

 

Also, I've just realized I may be in the wrong forum.......... LOL! Sorry about that.



#10 Jacob.Redshift

Jacob.Redshift

    Sputnik

  • -----
  • Posts: 31
  • Joined: 02 Mar 2018

Posted 17 May 2018 - 07:04 PM

On a related collimation issue... I have a question: If collimation appears lined up well when looking through the focuser (I have yet to star test because of weather), but putting in a laser collimator I get a de-centered laser beam (by about 1.5 inches), can I assume that my focuser tube is not quite aligned?

 

When I give the focuser tube a little pressure from one side (from the ground side, pushing skyward when the scope is aimed at the horizon), it pushes the laser more toward the center of the primary-- AND when pushing like this the beam on the secondary gets pushed more toward the center of the secondary. It's definitely not hitting the center of the secondary by 3/8th of and inch. And it's not off center on the optical axis toward the mirror or away from the mirror, but is off center toward skyward when the scope is horizontal. Before I start shimming it, I considered the laser may itself be off (I have yet to use it on another telescope, as it is second-hand and came with the dob I'm discussing here. I don't have another one). I even rotated the laser inside the focuser to see if it spiraled around, but it seems pretty dead on and not floating or moving or arcing anywhere no matter what orientation it is sitting within the focuser.

 

Here's my action list, play by play.

 

1. With my eye dead center of the focuser, I have aligned the secondary mirror so I see the primary perfectly centered. Like I said, no cheshire, but I've got a decent eye for that sort of thing.

2. Then, I aligned the primary so it is aimed squarely at the secondary. I got the central mark to appear dead center on the secondary.

Everything appears nicely placed concentrically set out from there-- BUT I throw in the laser collimator, and it's 1.5 inches off from center.

 

The first time I did the above procedure, I backed off from my eye-balling confidence and did the procedure only using the laser-- from aligning the secondary so it centered the laser dot on the primary, and then aligning the primary to center the reflected laser dot back into the collimating eyepiece target. BUT, when I pulled out the laser collimator after using it to center everything, looking down the focuser tube clearly shows misalignment of both the secondary and primary. Pretty heavy misalignment. 

 

I'm thinking the only thing left is the angle of the focuser tube pushing the laser beam off kilter, whereas my eye isn't affected by the slight angle of the focuser tube.

 

Another clue-- the laser aims and strikes off-center of the secondary, actually in the skyward side

 

Anyone experience this before? 

 

Thanks for sticking with me on that run-down!



#11 jtsenghas

jtsenghas

    Gemini

  • *****
  • Posts: 3326
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio --Bloomdale

Posted 17 May 2018 - 08:49 PM

I don't *think so, as the focuser should provide leeway to accommodate such shifts,  but still, after many collimations, it may be wise to "reset" the primary and start fresh anyay (as well as the secondary) . Over the course of many field collimations, I walked my primary to the point where the screws were almost falling off a couple of times. I *do have ample focuser adjustment though, so maybe it can happen on a scope with a limited range of focus

It appears to me that the original poster is getting the appropriate advice on collimation from the experts, so I won't comment on those matters further.

 

As far as avoiding running the adjustment of the primary to either extreme over time with collimation maintenance as described by Xiando, I have a simple bit of advice:  ADJUST ONLY TWO OF THE THREE COLLIMATION SCREWS FOR SUBSEQUENT TWEAKS AFTER AN ORIGINAL COLLIMATION THAT PUTS THE FOCUSER HEIGHT IN YOUR DESIRED RANGE.

 

If you have any eyepieces that require most of your adjustment either in or out, you might be able to accommodate them by advancing or retracting all three primary mirror collimation screws the same number of turns before fine tuning them. If you want to use a Paracorr, you will need a bit more in focus than your eyepieces alone would need.  That's just one example for which you may need to shift your overall focuser range. 

 

Once you have everything fine-tuned, however, the general practice of of leaving one collimation screw alone (in my case the one furthest from my focuser), will help you to avoid gradually walking your mirror forward or back.


Edited by jtsenghas, 17 May 2018 - 08:58 PM.


#12 macdonjh

macdonjh

    Gemini

  • *****
  • Posts: 3021
  • Joined: 06 Mar 2006

Posted 18 May 2018 - 07:22 AM

On a related collimation issue... I have a question: If collimation appears lined up well when looking through the focuser (I have yet to star test because of weather), but putting in a laser collimator I get a de-centered laser beam (by about 1.5 inches), can I assume that my focuser tube is not quite aligned?

 

It is entirely possible.  One way to check that is to remove the spider and make a mark on the inside of your OTA opposite the focuser.  The easiest way to do that is tape a piece of white paper to the inside of your scope.  Then take a strip of paper that wraps all the way around your scope.  Mark where the paper strip overlaps itself, which will mark the "beginning" and "end" of the paper and give you an accurate measurement of the circumference of your OTA.  Then, holding your two marks together, fold the paper strip in half to give yourself an accurate measurement of half the circumference of your OTA.  Then, starting at the center of your focuser, hold/ tape that strip to the inside of your OTA.  Then you'll have a mark opposite your focuser.  Finally, measure from the end of your OTA, parallel with the length of your OTA, to the center of your focuser and transfer that mark to the opposite side as marked by that strip of paper.  Now you have a mark, or target, 180o opposite your focuser.  You can now use your laser to help you adjust your focuser until the laser hits that target.  Reinstall the spider and recollimate your scope as normal.

 

When I give the focuser tube a little pressure from one side (from the ground side, pushing skyward when the scope is aimed at the horizon), it pushes the laser more toward the center of the primary-- AND when pushing like this the beam on the secondary gets pushed more toward the center of the secondary. It's definitely not hitting the center of the secondary by 3/8th of and inch. And it's not off center on the optical axis toward the mirror or away from the mirror, but is off center toward skyward when the scope is horizontal. Before I start shimming it, I considered the laser may itself be off (I have yet to use it on another telescope, as it is second-hand and came with the dob I'm discussing here. I don't have another one). I even rotated the laser inside the focuser to see if it spiraled around, but it seems pretty dead on and not floating or moving or arcing anywhere no matter what orientation it is sitting within the focuser.

 

Sounds like your focuser has a sloppy fit between the body and the draw tube.  I had the same problem with the plastic focuser that came with my Orion XT6i.  Just like you're contemplating, I shimmed it with plastic sheet from a local hobby store and solved the problem.

 

Here's my action list, play by play.

 

1. With my eye dead center of the focuser, I have aligned the secondary mirror so I see the primary perfectly centered. Like I said, no cheshire, but I've got a decent eye for that sort of thing.

2. Then, I aligned the primary so it is aimed squarely at the secondary. I got the central mark to appear dead center on the secondary.

Everything appears nicely placed concentrically set out from there-- BUT I throw in the laser collimator, and it's 1.5 inches off from center.

 

I think your theory below is correct: you're sighting down a "crooked" focuser.  Besides, even with a "good eye for that sort of thing", a laser shines a straighter line than you can hold with your eye.

 

The first time I did the above procedure, I backed off from my eye-balling confidence and did the procedure only using the laser-- from aligning the secondary so it centered the laser dot on the primary, and then aligning the primary to center the reflected laser dot back into the collimating eyepiece target. BUT, when I pulled out the laser collimator after using it to center everything, looking down the focuser tube clearly shows misalignment of both the secondary and primary. Pretty heavy misalignment. 

 

That's the correct order for collimating a Newtonian.  If you could post pictures of what you describe as "pretty heavy misalignment" it would help.

 

I'm thinking the only thing left is the angle of the focuser tube pushing the laser beam off kilter, whereas my eye isn't affected by the slight angle of the focuser tube.

 

Another clue-- the laser aims and strikes off-center of the secondary, actually in the skyward side

 

Anyone experience this before? 

 

Thanks for sticking with me on that run-down!

Just to add a bit of personal experience to what's in red above: when I took delivery of my latest scope a couple of years ago I got a good lesson in collimation from a friend of mine.  I didn't want to, but he forced me to check the focuser first.  I couldn't believe that an adjustment like that wouldn't have been done by the maker of the scope, or that the fabrication wouldn't have been good enough that the focuser wouldn't be square.  It wasn't.  Turns out I had to spend way more time squaring the focuser (which I'd never done before, I hate push-pull screws) than I did collimating the mirrors.  Once I'd done things in the proper order, though (square the focuser, center the mirrors, collimate the secondary, collimate the primary) everything went smoothly without wasted effort and my scope gives wonderful images.


Edited by macdonjh, 18 May 2018 - 07:26 AM.

  • Jacob.Redshift likes this

#13 Jacob.Redshift

Jacob.Redshift

    Sputnik

  • -----
  • Posts: 31
  • Joined: 02 Mar 2018

Posted 18 May 2018 - 08:50 AM

Macdonjh,

 

Thanks for those fantastic details, suggestions, and information on your experience!

I am going to try one more procedure for confirming the rotation of my secondary isn't contributing to the problem, as someone elsewhere suggested might be the case, and if I can get that part of the procedure real "true" so-to-speak, then I may have fixed it, or at least given myself the best alignment for the correct amount of shimming I may have to do if, when I do your measurements, I confirm a misalignment of the focuser tube.

 

I will get this "squared" away! hah. 

 

I'll keep you updated.

Thanks, again!



#14 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • Posts: 7287
  • Joined: 21 Oct 2006
  • Loc: California

Posted 18 May 2018 - 10:36 AM

1. With my eye dead center of the focuser, I have aligned the secondary mirror so I see the primary perfectly centered. Like I said, no cheshire, but I've got a decent eye for that sort of thing.

Not the most accurate method but it will get you close enough

 

2. Then, I aligned the primary so it is aimed squarely at the secondary. I got the central mark to appear dead center on the secondary.

 

Unclear what you meant by the above. How do you know that primary is aimed squarely at the secondary mirror? Getting the central mask to appear dead center on the secondary does not mean the primary optical axis is pointing at the focuser center. Besides, if you align the secondary and primary mirror reflection -- as you stated in step#1 -- then it is implied the central mask  will appear dead centered in the secondary mirror. 

 

Everything appears nicely placed concentrically set out from there-- BUT I throw in the laser collimator, and it's 1.5 inches off from center.

 

I am not surprised. Eyeballing can't beat laser accuracy. You did state that your laser collimator is decently axially aligned but has little play in the focuser, but that can't justify the 1.5" error. I would attribute most of that error to the inaccuracy of your eyeballing. You will subconsciously displace your eye to align the primary mirror reflection with the secondary mirror. See the illustration below:

 

5053368-cheshire_laser_mismatch.JPG

 

QUOTE:The first time I did the above procedure, I backed off from my eye-balling confidence and did the procedure only using the laser-- from aligning the secondary so it centered the laser dot on the primary, and then aligning the primary to center the reflected laser dot back into the collimating eyepiece target. BUT, when I pulled out the laser collimator after using it to center everything, looking down the focuser tube clearly shows misalignment of both the secondary and primary. Pretty heavy misalignment. (Hmmm, looks like CN are now limiting the number of formatted QUOTE blocks to three)

 

 

That is actually a common observation that is reported in this forum periodically. What is not well-understood by many is that laser collimators do not center/round the secondary mirror under the focuser (unless you use a holographic attachment). The reason is simple: The laser beam does not interact with the secondary edge; therefore, it can't provide feedback on how centered/rounded the secondary mirror is.

 

May I suggest following the procedure outlined in the follow post:

 

https://www.cloudyni...ment/?p=5260727

 

Jason


  • airbleeder and Jacob.Redshift like this

#15 Jacob.Redshift

Jacob.Redshift

    Sputnik

  • -----
  • Posts: 31
  • Joined: 02 Mar 2018

Posted 30 May 2018 - 10:35 AM

Thank you for that detailed response! I realize I wasn't clear as you commented in your "quote 2", but I followed your advice, and a few others here, and I've definitely improved/corrected what I'm doing with my step-by-step processes. I got a Cheshire to Collimate with now, and see the improvement. I think my secondary was not rotated correctly, as I adjusted that as well as its tilt to finally get my nice concentric alignment as viewed through the Cheshire, and then the primary adjustments were done after that. Using the Cheshire to check and redo and check as needed really seemed to work better now.

 

I was was able to get out for a star test, and saw the following. 1. I'm centered nicely now ( thank you, everyone, for your help in getting me there). I am really appreciative of the time folks take to help out another observer. 2. I have a dirty mirror. 3. I can detect the oval shape of astigmatism when focusing in and out past focus.

 

So, being excited that I corrected my collimation processes, I decided I would clean the mirror. Also, thinking that the retaining clips my be the culprit for the astigmatism, I figured it was a good time to get in there and release them and clean around them nicely, and reinstall them properly.

 

Happy with that work, I hoped the retaining clips were causing the issue with the astigmatism, and I went back to Collimate again. Went faster this time, but I'm still having a **** time with the primary adjustments with the locking screws tending to foul up whatever adjustments I'm making with the adjustment screws. It's a lightbridge 12" telescope. I finally released all of the locking screws until there was a gap between them and the mirror holder, then adjusted the adjustment screws. That helped, and I got things collimated again. I retightened the locking screws very gently, checked I was still collimated, and things looked good.

 

I did another star test, and Idefinitely reduced the astigmatism, though I still see it a tiny bit. Also, I can now see a tiny amount of what looks like mirror pinch. It's ever so slight, but I think it's there. Is this from not enough tightening on the locking screws? I really did just bring them to touch the holder with a gentle amount of pressure. Does a 12" lightbridge mirror need the locking screws and the adjustment screws to spread out the force holding the mirror? Should the locking screws be tightened as stiffly as the adjustment screws feel? I had to pack it in and stop fiddling due to time constraints, but the more I thought about it, the more I recall that one of the adjustment screws was stiffer than the other two. Perhaps that's causing the mirror pinch?  Unless anyone has some different thoughts, I think my next trial will be to leave the locking screws and loosen the tension on the adjustment screws, hopefully keeping the mirror positioned the same, but relieving any tension. Thanks for your continued insights, Cloudynights community!



#16 Mike G.

Mike G.

    Apollo

  • *****
  • Posts: 1194
  • Joined: 17 Jun 2013
  • Loc: Oberlin, Ohio

Posted 30 May 2018 - 02:06 PM

get some stiffer springs for the primary mirror and put the locking screws in your desk drawer. neither my 12" or 8" have the locking screws installed.  I collimate before each use (usually a very minor adjustment) and then check at zenith  and at horizon.  the mirror doesn't shift as long as I remember to put the shims back in the mirror cell after cleaning...


  • SteveG likes this

#17 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 38009
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 01 June 2018 - 04:04 PM

Second that advice: there are aftermarket springs from many sources (e.g. Farpoint, Bob's Knobs, et.al.)

Replace the springs with stiff ones that hold collimation and eliminate the locking screws.



#18 vhinze

vhinze

    Sputnik

  • -----
  • Posts: 27
  • Joined: 10 May 2009

Posted 02 June 2018 - 10:09 PM

I have been getting the hang of collimating my new Obsession 20". I am fortunate that it came with the primary and secondary center marked. Being a truss tube 'scope, I can drop the shroud so I can see the surface of the secondary. This makes it simple to affirm that the collimating laser is striking the secondary dead center and, if not, move the secondary (via its threaded stalk) towards or away from the primary or rotate the assembly until the secondary center is being struck by the laser. Verify the centering of the secondary visually with the cheshire eyepiece or a collimating cap, ignoring the reflected image of the primary at this point

.

Step one (assuming the focuser is square with the UTA): center the secondary under the focuser, as just discussed.

 

Step two: use the secondary tilt screws to aim the laser at the primary center mark.

 

Step three: use the primary screws to return the laser to the center hole of the laser.

 

Step four: star test for concentricity.

 

If in step three, the laser return is out the front of the UTA, use something like a sheet of butcher paper across the muzzle of the UTA  to let you guide it onto the secondary. Once there, you'll see the laser spot on the side of the UTA, hopefully near the focuser tube. Steer it into the tube and onto the laser target screen. All this is best done with a quality collimating tool like the Glatter. Cheaper tools like the Orion Lasermate may exhibit consideable runout when rotated in the focuser, limiting the accuracy you may hope to achieve. I saw fit to go with the TuBlug, presently being built by Starlight Instruments..

 

This seems to work for me. Everybody understands the need  for a marked primary. Seldom is it mentioned that a marked secondary can eliminate much angst. Not too hard to do, just carefully demount the secondary flat and trace its outline. Find the x-y centerline, mark it, cut a tiny hole in this paper template and use it to transfer the mark to the secondary. This procedure ignores secondary offset, but most "experts" seem to agree that secondaries are generally sized generously enough that this can be done with negligible impact on field illumination. In practice, following the above three step procedure, I can run my focuser in and out and see the entire secondary (and primary) in the focuser tube.


Edited by vhinze, 02 June 2018 - 10:55 PM.


#19 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 38009
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 02 June 2018 - 11:38 PM

I have been getting the hang of collimating my new Obsession 20". I am fortunate that it came with the primary and secondary center marked. Being a truss tube 'scope, I can drop the shroud so I can see the surface of the secondary. This makes it simple to affirm that the collimating laser is striking the secondary dead center and, if not, move the secondary (via its threaded stalk) towards or away from the primary or rotate the assembly until the secondary center is being struck by the laser. Verify the centering of the secondary visually with the cheshire eyepiece or a collimating cap, ignoring the reflected image of the primary at this point

.

Step one (assuming the focuser is square with the UTA): center the secondary under the focuser, as just discussed.

 

Step two: use the secondary tilt screws to aim the laser at the primary center mark.

 

Step three: use the primary screws to return the laser to the center hole of the laser.

 

Step four: star test for concentricity.

 

If in step three, the laser return is out the front of the UTA, use something like a sheet of butcher paper across the muzzle of the UTA  to let you guide it onto the secondary. Once there, you'll see the laser spot on the side of the UTA, hopefully near the focuser tube. Steer it into the tube and onto the laser target screen. All this is best done with a quality collimating tool like the Glatter. Cheaper tools like the Orion Lasermate may exhibit consideable runout when rotated in the focuser, limiting the accuracy you may hope to achieve. I saw fit to go with the TuBlug, presently being built by Starlight Instruments..

 

This seems to work for me. Everybody understands the need  for a marked primary. Seldom is it mentioned that a marked secondary can eliminate much angst. Not too hard to do, just carefully demount the secondary flat and trace its outline. Find the x-y centerline, mark it, cut a tiny hole in this paper template and use it to transfer the mark to the secondary. This procedure ignores secondary offset, but most "experts" seem to agree that secondaries are generally sized generously enough that this can be done with negligible impact on field illumination. In practice, following the above three step procedure, I can run my focuser in and out and see the entire secondary (and primary) in the focuser tube.

One small problem.  In a collimated scope, the laser does NOT strike the center of the secondary.  It strikes a point on the long axis that is offset toward the upper end of the secondary.

Maybe by only a few millimeters, but it will not strike the center of the mirror even if the mirror is centered in the tube as long as it is centered under the focuser.

Now if you spot the secondary mirror with the correct offset, you could use the small mark on the secondary to adjust things, but I'm betting you're using a center dot on the secondary.

And that results in miscollimation of the scope because offset is necessary to collimate the scope properly.

Your step three and four could be combined into one step if you use the Barlowed Laser Protocol, which is more accurate than using the return beam of the laser to collimate the primary:

http://www.micosmos....lowed_Laser.pdf

or

http://www.obsession...ation/index.php

 

Why is offset necessary?  Because it is the way the optical axis is correctly pointed at the center of the field at the same time the focuser axis points exactly at the center of the primary.

The process of collimation will automatically build in offset.  If your laser hits the center of the primary and reflects to its source, it will hit the offset point on the secondary, not its center if the secondary is centered under the focuser.

The secondary will be positioned higher in the tube than being centered under the focuser if a center dot on the secondary can be used for both secondary and primary collimation.



#20 vhinze

vhinze

    Sputnik

  • -----
  • Posts: 27
  • Joined: 10 May 2009

Posted 03 June 2018 - 12:15 AM

OK, no problem.

 

I am using the secondary mark put there by Obsession. Presumably, offset was considered when it was done, looks like it's about 1/4 inch up from center on the 3.1 inch secondary, or about 4.5- 5 mm.  Maybe that's why all is well here and you'd lose your bet.

 

I understand the purpose of offset, to ensure uniform illumination.  It's discussed briefly in the last paragraph I wrote. I remark that "experts" suggest that it's not an absolute necessity because secondaries are seldom sized as small as possible. That's what I 've read, I didn't originate that opinion and certainly don't care to defend it to the death.

 

I do insist that it is a great idea to center mark the secondary and your outlined procedure is the textbook perfect way to do it, I acknowledge that. As far as an actual centered dot resulting in a miscollimated scope, I see no profit in arguing the point. I do agree that a centered mark with no offset may result in part of the field being underilluminated if the secondary was sized to minimize the central obstruction. This is where your procedure removes all doubt as I understand it. However, collimation does not "automatically build in" offset. It has to be calculated, then applied and there is a formula for it.

 

As mentioned, I am using a TuBlug. That is a barlowed laser. I did it with and without the TuBlug barlow. It is phenomenally accurate, no doubt. After using the pinpoint Glatter laser to define the focuser axis, align it with the secondary spot then tilt the secondary axis at the primary, I inserted the TuBlug. The TuBlug (invented by Howie Glatter) is a barlow with a tiny lens element not much larger than the laser beam. Kinda smacks of magic how the shadow of the primary do-nut pops up on the TuBlug target (has to do with the collimated laser light, as we know).

Good of you to furnish the link to the Obsession site. Having ponied up for an Obsession 'scope with all the trimmings, you may rest assured I've been all over it. Do you see something in the three (or four) steps I described contradicted on that page? I must confess I don't.

 

Wanna eliminate the fourth step? Fine. Just seems simple after doing the alignment to defocus a star a few waves and look for concentricity, just like I feel better looking thru the collimation cap to see the centered image of the mirrors in the drawtube,  Just checkin'.


Edited by vhinze, 03 June 2018 - 01:06 AM.


#21 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 38009
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 03 June 2018 - 12:50 AM

Classical offset allows the center line of the tube and the optical axis to be coincident.

How it's achieved is with dropping the secondary down toward the primary and away from the focuser.

In this scenario, the optical axis will hit a point offset away from the center of the secondary.

 

If the secondary is raised from that point, and centered in the tube (from wall-to-wall), it will not be offset, but the centerline of the optical axis will coincide with

the centerline of the tube, just as with Classical offset.  The reflected image of the primary will appear offset toward the bottom of the secondary.

This is the one scenario in which a center dot on the secondary could be useful.

 

Typically, modern scopes will have the secondary centered in the tube and centered under the focuser.  This is what you described in step one.

The optical axis, during collimation, will tip toward the focuser and it will not, in fact cannot, coincide with the centerline of the optical tube.

In this scenario, the optical axis will hit a point offset away from the center of the secondary.

If the secondary is center-dotted, and this is used for collimating the secondary, it will result in a tilt error at the secondary and the reflected laser beam will not return to that spot.

If it did, it wouldn't hit the centerline of the focuser. Your step 3 wouldn't be possible without raising the secondary in the tube.

 

The point is that a center dot on a secondary mirror is incompatible with collimation if it is used as a collimation guide and the secondary is centered under the focuser.

It is a holdover from a bygone era when secondaries were not centered under the focusers.



#22 vhinze

vhinze

    Sputnik

  • -----
  • Posts: 27
  • Joined: 10 May 2009

Posted 03 June 2018 - 01:25 AM

Got it.

 

Obsession must have done just as you describe. When I follow up, the beam ends up on the center dot they applied. Bottom line is the three steps worked, and I ended up with a do-nut on the target and concentric images of the secondary, primary and focuser tube.

 

Thank you for setting me straight.



#23 jtsenghas

jtsenghas

    Gemini

  • *****
  • Posts: 3326
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio --Bloomdale

Posted 03 June 2018 - 08:47 AM

Note that with the classical offset properly achieved , the secondary APPEARS centered from the focal point both in the focuser and from the focal point at the scope top end. The furthest point on the secondary from the focal point is just outside of the light cone at a larger radius closer to the primary. 


Edited by jtsenghas, 03 June 2018 - 08:50 AM.

  • Starman1 likes this

#24 vhinze

vhinze

    Sputnik

  • -----
  • Posts: 27
  • Joined: 10 May 2009

Posted 03 June 2018 - 09:16 AM

Sure, I am on the page now. Here's a link wherein Niels Carlin Olaf makes it clear as day. Thanks to you guys for taking time to explain.

 

Using the given formula to calculate classic offset for my 3.1 inch secondary yields 3.97 mm

 

 

 

http://www.skyandtel...condary-mirror/


Edited by vhinze, 03 June 2018 - 09:20 AM.


#25 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 38009
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 03 June 2018 - 09:30 AM

And that is the distance a center dot on the secondary would be above (closer to the tube opening) the center of the secondary for a dot there to be useful.

In practice, no dot is necessary because slight rotational differences in secondary placement have little effect on collimation.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics