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

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#1 astrogeek64

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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

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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

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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.



#4 macdonjh

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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.


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#5 Vic Menard

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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?


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#6 havasman

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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

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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


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#8 astrogeek64

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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

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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

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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

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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

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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.

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#13 Jacob.Redshift

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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

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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


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