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Old 02-08-2020, 05:27 PM
Stefan Buda
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Grzybowski collimator

About 20 years ago I made a small alignment telescope with cross-hairs that can focus from a few inches to couple of metres. A few days ago I decided to turn it into a Grzybowski collimator, by adding the light sources and a 2" barrel to the front end.

Hopefully I will be able to use it to fix Andy01's TOA-130 problem.
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Old 02-08-2020, 06:04 PM
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Looks a bit like one of those Hubble Star things.
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Old 02-08-2020, 06:45 PM
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No, this is an auto-collimator. It plugs into the eyepiece holder and looks at retro reflections from lens surfaces.
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Old 02-08-2020, 07:24 PM
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Ah! So it’s kinda like a Tak collimating scope on steroids
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Old 03-08-2020, 07:45 AM
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Very interesting device. I tried to make a collimating scope with an old lense but I could not sort out a light source. Is the placement of the led critical in relation to each other and the centre ?
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Old 03-08-2020, 09:23 AM
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Quote:
Originally Posted by Atmos View Post
Ah! So itís kinda like a Tak collimating scope on steroids
The Tak one is not very useful for refactors. With this one you can see the retro reflection from each air/glass interface.

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Very interesting device. I tried to make a collimating scope with an old lense but I could not sort out a light source. Is the placement of the led critical in relation to each other and the centre ?
Yes, the placement of the light sources need to be accurate and also the optical/mechanical axis of the whole device.
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Old 03-08-2020, 04:20 PM
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Quote:
Originally Posted by Stefan Buda View Post
About 20 years ago I made a small alignment telescope with cross-hairs that can focus from a few inches to couple of metres. A few days ago I decided to turn it into a Grzybowski collimator, by adding the light sources and a 2" barrel to the front end.

Hopefully I will be able to use it to fix Andy01's TOA-130 problem.
Hello Stefan. I see youíre still at the forefront of optical-engineering excellence. Is there a thread link to Andyís TOA problems? I have a TOA 130 also so want to be abreast of inherent issues...I havenít been on here much since December 2019.
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Old 03-08-2020, 07:33 PM
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Hi Les,

Nothing is seriously wrong with Andy's TOA. The objective appears to have a bit of a tilt and that prevents the flattener/corrector to work properly.

The rest of the problems were not from Tak hardware. The field rotator could not be locked in a way that would guarantee squareness and the focuser had a bit of a tilt too. Focuser tilt can be adjusted but only in one plane, which is not enough to get it perfect.

Other than that, he inherited a bunch of adaptors/extenders with wedge errors.
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Old 03-08-2020, 08:09 PM
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Ok. It's always been a fear of mine that one day misadventure would require re collimation of my TOA. Would your "Grzybowski collimator" in the hands of a reasonably handy intelligent person enable self re collimation in situ?
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Old 04-08-2020, 08:47 AM
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This collimator is good for adjusting only the tilt of the lens groups, not the spacing. Andy's scope needs the whole objective to be adjusted, not the individual lens groups. At least that is what I hope, as I have not looked at it yet. My assumption is based on test images that Andy sent me.

If you need a full collimation, you must use the collimator in conjunction with Roddier testing that will tell you how much spherical aberration you have due to incorrect spacing. Not difficult but very time consuming unless you have a suitable optical bench where you can set up and artificial star.
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Old 05-08-2020, 07:13 AM
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TOA now delivered to the master for what we all hope will be the final step in a long saga of rectifying tilt issues.
Thanks to Stefan for all of his care and attention done on this
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Old 05-08-2020, 03:48 PM
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First look at Andy's TOA-130.

My estimate, based on defocused star images taken through a 3D printed mask, for the objective tilt, seems spot on. I was expecting about 3mm at the sensor.
However there is a bit of a problem because about one quarter of the error is coming from the focuser. As I rotate the focuser, the error varies from 50% to 100% at the best and worst orientations.
If I eliminate the objective tilt, the remaining focuser tilt might become insignificant though.
All four sets of reflections seem consistent so I'm happy that I won't need to touch the inter lens adjustments.
The attached image shows only three sets of reflections.
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Old 05-08-2020, 04:12 PM
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Interesting! Seems that the collimator is showing the error up well - the spacing between reflections appears to be inconsistent.
Not sure what options there are regarding the focuser - wait & see I guess.
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Old 05-08-2020, 04:46 PM
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You say it does the lens cell tilt and not individual element spacings, but I wonder if, given that you are seeing reflections from all lens surfaces, this would work for colimating the individual lens elements within a cell (thought not their spacings) so all lens elements are centered with respect to one another?


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Old 05-08-2020, 08:44 PM
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There are probably a lot of Tak owners anxious to see the result. I know my focuser is off the centre of the lense cell by a millimetre. Could collimation help? Very interesting to find out. I see Grzybowski collimaters can be purchased for not too many hundred euros so I am surprised they are not more common considering their effectiveness.

Last edited by Sunfish; 05-08-2020 at 10:53 PM.
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Old 06-08-2020, 08:21 AM
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Quote:
Originally Posted by Andy01 View Post
Interesting! Seems that the collimator is showing the error up well - the spacing between reflections appears to be inconsistent.
Not sure what options there are regarding the focuser - wait & see I guess.
You are misinterpreting the reflections - they are consistent but it is not possible to tell from the photo.

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You say it does the lens cell tilt and not individual element spacings, but I wonder if, given that you are seeing reflections from all lens surfaces, this would work for colimating the individual lens elements within a cell (thought not their spacings) so all lens elements are centered with respect to one another?
Cheers
Markus
Yes, you are correct. The problem is that it is not possible to change the tilt of individual elements without affecting the spacing as well.

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There are probably a lot of Tak owners anxious to see the result. I know my focuser is off the centre of the lense cell by a millimetre. Could collimation help? Very interesting to find out. I see Grzybowski collimaters can be purchased for not too many hundred euros so I am surprised they are not more common considering their effectiveness.
I'm not sure, but maybe those collimators come without the required cross-hair reticule eyepiece. If you need to supply your own, then it will add to the cost.

I will post the result of the collimation I did last night once I have the images prepared.
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Old 06-08-2020, 10:52 AM
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Ok, here's the result of about an hour of fiddling with the cell adjustment screws last night.

There is a mismatch of about 1mm between the mechanical and the optical axis. This, I think, is due to a number of errors such as focuser tilt, probably a bit of lens cell and rotator eccentricity, as well as cross-hair eccentricity.
The cross-hair reticule came from my junk collection and I never tested it.

Anyway I adjusted the the lens cell so that the position of the dots don't change relative to the cross-hair when I rotate the focuser. That should produce consistent images regardless of camera orientation. Otherwise if I cancel the offset in one particular position, I get twice as much error when change orientation by 180 degrees.

The two attached images were taken at 180 degree rotation of the focuser, with fixed, tripod mounted camera looking through the eyepiece of the collimator.

And yes, Andy, there is a slight misalignment of the two lens groups as well, but I would not worry about that, as it is very minimal
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Old 06-08-2020, 08:57 PM
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Good to know that the focuser offset does not get in the way of better collimation.

Many seem to think it is too hard to do without centre of the focuser-on the objective . So it seems the Tak designers have thought of that and minor focuser misalignments over time are inevitable and are not critical to good collimation. Good tolerances in the system.?

Would changing the focuser make a difference?

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Originally Posted by Stefan Buda View Post
Ok, here's the result of about an hour of fiddling with the cell adjustment screws last night.

There is a mismatch of about 1mm between the mechanical and the optical axis. This, I think, is due to a number of errors such as focuser tilt, probably a bit of lens cell and rotator eccentricity, as well as cross-hair eccentricity.
The cross-hair reticule came from my junk collection and I never tested it.

Anyway I adjusted the the lens cell so that the position of the dots don't change relative to the cross-hair when I rotate the focuser. That should produce consistent images regardless of camera orientation. Otherwise if I cancel the offset in one particular position, I get twice as much error when change orientation by 180 degrees.

The two attached images were taken at 180 degree rotation of the focuser, with fixed, tripod mounted camera looking through the eyepiece of the collimator.

And yes, Andy, there is a slight misalignment of the two lens groups as well, but I would not worry about that, as it is very minimal
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Old 06-08-2020, 09:57 PM
Stefan Buda
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I'm not blaming the focuser for the offset. I think it has a contribution too, as perfection only exists in mathematics, not in engineering.

For visual use, without a field corrector or reducer, such an offset would not matter, but when used as an astrograph the optical axis must go through the center of the corrector group.

Anyway, I played a bit more with the collimator and rotator and came to the conclusion that most of the offset is coming from my collimator's telescope.
This little scope was put together from junk optics and the objective doublet must have a bit of wedge error. One day I'll make a good sighting scope for this, if I find a need for it.
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Old 06-08-2020, 10:46 PM
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I'm sure you guys must have thought of this, but you couldnt use one of those focuser tilt rings you get for RC scopes, could you?


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