263 replies to this topic

[MikeECha]

I figured you were using a compass-like thing like this.  That's what I mean by "not very precise".  I made a disk for exactly this application once by drilling a hole in a plastic sheet, roughing out the plastic circle around it, mounting the plastic on the drill bit I used for the center hole (now embedded in a block of wood), clamping the block to a disk sander table, and rotating the plastic against the disk sander around the drill bit.  I started with the hole and rounded the plastic around it, in other words.  It made a nice precise central hole and a disk which slipped into the telescope back like butter, but it resulted in a poor collimation.  I wasn't using the same method you describe though.

A 3D printed part might do the trick to improve precision. 

[MikeECha]

...or you experience the 'squinted collimation' David07 is describing in his post http://www.cloudynig...read/?p=9224176

 

His method with a cheap 58 mm cardboard disc works like a charm:

 

"In my experience, the collimation of the GSO RCs, that is the design where the focuser extension tube is attached to the primary mirror support, is extraordinarily difficult. Any method based on using a laser or Cheshire inserted into the focuser is immediately compromised because of the lack of a fixed reference in the telescope design. There are unknown alignment errors between the primary mirror and it’s holder and the mirror holder and the focuser. Moreover, moving the primary mirror, when collimating the scope, also changes the pointing direction of the focuser. I spent 18 months fiddling about with my RC8 until I read the Austria Teleskop blog and that gave me the idea for the collimation method I now use.

I began by taking the scope to an optician friend with an optical bench and artificial star.  The first discovery was that after I had spent weeks of adjusting the secondary to primary separation to get the specified focal length of 1624mm, a Ronchi grating test quickly revealed that the scope was overcorrected: the mirrors were too far apart. We adjusted the mirror separation until we got a properly corrected image. The focal length was 1660mm, as it still is. So problem number one, the actual focal length might not be as advertised.

 

To collimate the scope:

 

With the scope on a bench, I removed the focuser and extension tubes. I then measured, with a calliper, the inside diameter of the central hole in the primary mirror holder. I cut a piece a styrene about 2mm thick to exactly fit the hole and drilled a 1mm hole in its centre. Now gently fit the styrene disc into the central hole of the primary mirror holder.

 

Next, mark the position of the central screw holding the secondary mirror. A little dab of white acrylic paint, for example. Also, mark the side of the secondary boss and mirror holder. This will enable you to replace the secondary exactly in the correct place and not alter the scope focal length.

 

Carefully unscrew the secondary mirror central screw, holding the secondary mirror. Count the number of turns to release the screw and note the number. Don’t touch the three collimation screws.

 

Place a light behind the telescope and looking from the front of the scope, sight the hole in the centre of the styrene disc though the centre of the hole that the secondary securing screw came from. Now adjust the primary mirror so that the reflections of the secondary support vanes coincide with the support vanes themselves. You should see a tiny spot of light in the centre of the secondary screw hole and the reflections of the support vanes will be hidden behind the vanes themselves.

 

The primary mirror is now aligned with the secondary holder.

 

Replace the secondary mirror. Count the number of turns and align the paint marks.

 

Next, reach into the scope tube and unscrew the baffle tube. Let it rest on the inside of the tube.

 

Now bring the lamp to the front of the tube and set it to shine onto the styrene disc.

 

Go go to the back of the scope and look through the small hole on the centre of the styrene disc. You should see the central alignment ring on the secondary. Adjust the secondary until the reflection of the hole in the centre of the styrene disc is centred in the secondary alignment ring.

 

The secondary is now pointing at the centre of the primary mirror.

 

Replace the baffle tube. Replace the focuser etc. Sell your laser collimator.

 

Check the collimation on a star in the centre of the field of view. It should be really close.

 

On a night of good seeing I get HFD readings of 1.6 to 1.7 arc seconds at focus. Previously I couldn’t get below 2.5 arc seconds. I don’t use a tilt plate and I’m pleased with the images I get.

 

Hope this helps,

 

David"

 

Christer, Sweden

My method is in principle what you described. I create the reference to collimate the secondary by first resetting the primary all the way back with the collimation screws. I took the scope apart and measured the height of thee edge of the primary with the back plate/mirror assembly on a flat surface on the M91 thread edge to make sure the mirror in was actually parallel to that surface. That means the mirror (at reset point) will be looking straight down the tube when put back in. Now with a laser on the focuser that also passes through the hole of the secondary holder will stablish the reference I need.

 

The rest is a combination of cheshire and laser. I am now waiting for a chance to check how it came out.

 

I predict that is going to be a bit off. I think hole on the center of the secondary holder is big enough to allow the laser beam to pass at too much of an angle. But I have a solution for this. I will 3d print a part slightly bigger than the screw with a 1mm hole on the center and make laser go through it. 

 

It is still work in process but preliminary checks have shown decent collimation. 

[TinySpeck]

Feel free to start new threads on different collimation methods, folks.  Let's keep this one to the method I first described.

[Terry White]

And a mosaic of the image periphery:

 

mosaic.jpg

 

And here are the PixInsight contours:

 

contours.png

 

The FWHM isn't perfectly flat, but it is symmetrical and only increases about 0.6 pixel from the center to the corners.  This may be due to imperfect field flattening.  The threshold for visual detection of eccentricity is about 0.42 according to PixInsight, and the entire image falls under that.  The stars do look round in all tiles of the mosaic, too.

 

I'm pretty happy with this.  It only requires a Cheshire and no star testing.  You do have to be very careful to get everything as perfectly centered as you can, but that can be done in about 20 min, and during the daytime.  I'm experimenting with a pinhole video camera now to blow the Cheshire image up on the computer screen and overlay circular graticules.  That should take the guesswork out of some of the procedure.  Any ideas on how to capture the Hall of Mirrors images on the computer screen?

Do you have an in-focus image taken without the FF/FR that you can share? Some of the images on M29 taken with a smallish 8"-10" RC appear sharper than what you have posted. Some are worse than your images, but it's difficult to do an apples to apples comparison with a FF/FR.

[TinySpeck]

Do you have an in-focus image taken without the FF/FR that you can share? Some of the images on M29 taken with a smallish 8"-10" RC appear sharper than what you have posted. Some are worse than your images, but it's difficult to do an apples to apples comparison with a FF/FR.

No, unfortunately.  This method locks my focuser rotation and I need to add another spacer on my OTA back to operate at native focal length, so the focuser is no longer rotated to the correct alignment with the OTA.  I did the collimation with the spacer I use for reduced imaging.

 

Are you looking at raw subs or finished images?  People shrink stars in processing sometimes.  Even looking just at subs the exposure time can bloom your stars and make them look less sharp than a faster exposure.  There's a theoretical calculation for what you should expect for FWHM based on aperture, etc, that I think I have somewhere, I'll see if I can dig that up.

[Terry White]

Thanks, yes I was looking at the FWHM, shape and color of the processed stacks. A color sub would be nice. I presume you're seeing limited, correct?

[TinySpeck]

(EDIT: I'm not sure I was using the formula I had presented here correctly, so I'm withdrawing it.)

Edited by TinySpeck, 31 July 2021 - 11:05 AM.

[TinySpeck]

Thanks, yes I was looking at the FWHM, shape and color of the processed stacks. A color sub would be nice. I presume you're seeing limited, correct?

The sub I included above is debayered -- is that what you're looking for?  The color is pretty faint in just a single sub.

 

Yes, I assume I'm seeing-limited based on the analysis above.

Edited by TinySpeck, 30 July 2021 - 07:34 PM.

[dg401]

My stars look pretty bad.  There is obvious on-axis coma even though everything is bang on to the procedure.

 

It likely boils down to the physical versus the optical center of the secondary.  If they coincide, the procedure should work well.  My physical and optical center must be among the many that do not coincide.

[nebulachadnezzer]

My stars look pretty bad.  There is obvious on-axis coma even though everything is bang on to the procedure.

 

It likely boils down to the physical versus the optical center of the secondary.  If they coincide, the procedure should work well.  My physical and optical center must be among the many that do not coincide.

How good did your Hall of Mirrors effect look comparing all four vanes?

I'm still thinking that the HOM should give you a strong indication of how well aligned the two mirrors are to each other optically (rather than mechanically). It seems like if the secondary was tilted and the primary was tilted to match it you could only get symmetrical HOM effect on two out of four vanes.

The possibility exists, of course, that this measure simply isn't precise enough.

[dg401]

How good did your Hall of Mirrors effect look comparing all four vanes?

I'm still thinking that the HOM should give you a strong indication of how well aligned the two mirrors are to each other optically (rather than mechanically). It seems like if the secondary was tilted and the primary was tilted to match it you could only get symmetrical HOM effect on two out of four vanes.

The possibility exists, of course, that this measure simply isn't precise enough.

Hall of mirrors produces the same series of receding curved edges at all four vanes and anywhere else around the perimeter of the OTA.

 

I've been setting the focal length to 1370mm per GSO specification, but I may have to revisit the fact that when I received the RC6 new, it was set up at ~1410mm.  I was annoyed by this as 1410mm requires the 2" extension tube and both 1" extension tubes to find DSLR focus.  1370mm only needs one of the 1" extenders.  I have to consider the possibility that GSO set my scope to 1410mm for a reason.  I don't suspect GSO is going to start providing useful information about the quirks and inconsistencies of these RCs anytime soon, so it's all speculation.

 

It's not that my stars aren't round, they're not even ovals... they're v or even y shaped blobs.  I'm not over-tensioning the primary or secondary... or at least I don't think I am.  I'm starting to suspect that this will make a good closet scope.

[nebulachadnezzer]

Hall of mirrors produces the same series of receding curved edges at all four vanes and anywhere else around the perimeter of the OTA.

 

I've been setting the focal length to 1370mm per GSO specification, but I may have to revisit the fact that when I received the RC6 new, it was set up at ~1410mm.  I was annoyed by this as 1410mm requires the 2" extension tube and both 1" extension tubes to find DSLR focus.  1370mm only needs one of the 1" extenders.  I have to consider the possibility that GSO set my scope to 1410mm for a reason.  I don't suspect GSO is going to start providing useful information about the quirks and inconsistencies of these RCs anytime soon, so it's all speculation.

 

It's not that my stars aren't round, they're not even ovals... they're v or even y shaped blobs.  I'm not over-tensioning the primary or secondary... or at least I don't think I am.  I'm starting to suspect that this will make a good closet scope.

Hmmm... Sorry to hear that.

Y-shaped or trifoil-shaped stars usually mean pinched optics. I achieved that one of the first times I collimated my RC8. It was great the first night, but I had snugged the push screws up pretty tight after adjusting the mirror using the pull screws. The next day I had trifoil stars. I backed off all the push screws and let the mirror relax for a couple of days before attempting another collimation. Now when I get done collimating I just tighten the push screws to the point that they make contact without any real tension.

In my experience, inaccurate collimation produces stars that I think look more like the symbol for the Rebel Alliance, i.e. semicircles with a central spike: https://en.wikipedia.../Rebel_Alliance

I've thought about picking up an RC6 just as a platform to experiment with collimation. My RC10 and RC8 are in pretty solid shape at the moment. I have a (baseless) theory that collimation difficulty (or need for accuracy) increases with the square of the radius. However, the greatest ordeal I've seen for collimating one of these scopes is with the guy dealing with his club's 16" version.
 

[dg401]

Hmmm... Sorry to hear that.

Y-shaped or trifoil-shaped stars usually mean pinched optics. I achieved that one of the first times I collimated my RC8. It was great the first night, but I had snugged the push screws up pretty tight after adjusting the mirror using the pull screws. The next day I had trifoil stars. I backed off all the push screws and let the mirror relax for a couple of days before attempting another collimation. Now when I get done collimating I just tighten the push screws to the point that they make contact without any real tension.

In my experience, inaccurate collimation produces stars that I think look more like the symbol for the Rebel Alliance, i.e. semicircles with a central spike: https://en.wikipedia.../Rebel_Alliance

I've thought about picking up an RC6 just as a platform to experiment with collimation. My RC10 and RC8 are in pretty solid shape at the moment. I have a (baseless) theory that collimation difficulty (or need for accuracy) increases with the square of the radius. However, the greatest ordeal I've seen for collimating one of these scopes is with the guy dealing with his club's 16" version.
 

Yea, I'm pretty heavy-handed with the primary push screws.  I'll try backing off on the tension and see if I can send the Rebel Alliance packing.  Interesting insight on allowing time for the primary mirror to "relax" after being pinched.  Do you have an opinion on whether the secondary screw tension can similarly pinch the secondary mirror?

[nebulachadnezzer]

Yea, I'm pretty heavy-handed with the primary push screws.  I'll try backing off on the tension and see if I can send the Rebel Alliance packing.  Interesting insight on allowing time for the primary mirror to "relax" after being pinched.  Do you have an opinion on whether the secondary screw tension can similarly pinch the secondary mirror?

I’m not sure on the secondary since the screws push the holder instead of the mirror directly. I haven’t looked at the construction of the secondary mount enough to decide whether that tension could flex the mirror. I will say I similarly attempt a fairly light touch with the secondary push screws. I release tension on the opposite screws before tightening one. I don’t force it.

 

i will note re: my previous comment that the Rebel Alliance can also become a Y shape if it’s far enough out of collimation. That is more apparent with out of focus stars. The trifoil stars characteristic of pinched optics are visible when in focus.

[dg401]

I’m not sure on the secondary since the screws push the holder instead of the mirror directly. I haven’t looked at the construction of the secondary mount enough to decide whether that tension could flex the mirror. I will say I similarly attempt a fairly light touch with the secondary push screws. I release tension on the opposite screws before tightening one. I don’t force it.

 

i will note re: my previous comment that the Rebel Alliance can also become a Y shape if it’s far enough out of collimation. That is more apparent with out of focus stars. The trifoil stars characteristic of pinched optics are visible when in focus.

I do have to challenge you to convince me that cranking down the primary push screws will pinch the primary.  If I remove the primary mirror from the cell, I can nominally set my pull screws (I use three turns), then I can drive the push screws hard into the plate.  This can't pinch the primary mirror because it isn't mounted yet.  So if I drive the push screws in hard, and then gently set the primary and it's plastic spacer onto the cell, and gently tighten the gasket ring, the primary is secured, and all of the tension is on the cell, not the mirror.   So how is the primary mirror being pinched?

 

It's probably a good idea to ask the OP if he thinks discussions such as this are drifting too far off topic.  This does illustrate a problem with any collimation procedure, namely, if the user is doing it wrong, the collimation will fail.  It's easy enough to dismiss the procedure when the real problem is with the user. 

[TinySpeck]

...

It's probably a good idea to ask the OP if he thinks discussions such as this are drifting too far off topic.  This does illustrate a problem with any collimation procedure, namely, if the user is doing it wrong, the collimation will fail.  It's easy enough to dismiss the procedure when the real problem is with the user. 

I'm interested in results using this method.  If it takes working through other effects then so be it, I'm fine with that.

[TinySpeck]

My stars look pretty bad.  There is obvious on-axis coma even though everything is bang on to the procedure.

 

It likely boils down to the physical versus the optical center of the secondary.  If they coincide, the procedure should work well.  My physical and optical center must be among the many that do not coincide.

I'm sorry to hear that, too.  You might try taking another look at the Cheshire images and make sure they're just as perfect as you can make them.  I found my first effort had maybe 1/20 turn of correction required on a couple adjustments.  It was a very tiny difference in the Cheshire image but it made a visible difference in my stars.  I think there's no getting around that RCs are very fussy to collimate, regardless of your procedure.

Edited by TinySpeck, 31 July 2021 - 10:45 AM.

[TinySpeck]

Trying again on this.  I re-read the original CN post here, this time followed it to a correction here, fixed an error in my worksheet, changed a couple assumptions, and I think I understand what the result means now.

 

 

Anyway, with the assumptions shown in the worksheet I should expect a FWHM of 2.5 pixels.  My measured central peak was 2.8 so that's not far off.  Seeing, tracking error, "Q", and things like moisture or haze in the air affect the FWHM of the image, so I think the stars in my test above were about as sharp as they should be.  Centrally anyway, but I think my image flatness could be improved.

[jerahian]

Changed the title to this thread upon request from the poster.

[nebulachadnezzer]

I do have to challenge you to convince me that cranking down the primary push screws will pinch the primary.  If I remove the primary mirror from the cell, I can nominally set my pull screws (I use three turns), then I can drive the push screws hard into the plate.  This can't pinch the primary mirror because it isn't mounted yet.  So if I drive the push screws in hard, and then gently set the primary and it's plastic spacer onto the cell, and gently tighten the gasket ring, the primary is secured, and all of the tension is on the cell, not the mirror.   So how is the primary mirror being pinched?

What you're describing with the mirror out of its cell doesn't really translate, as the collimation of the primary is relative to the primary cell. With the tube-mounted RCs it's also moving the focuser with it. (The GSO truss RCs have independent focuser flanges with their own collimation screws.)

However, in a mounted mirror, if you don't loosen all of the push screws before adjusting the pull screws, and by that I mean well out of the way, you will almost certainly torque the mirror if it encounters a push screw. That's why I advise that you should back out the push screws significantly, make all of your adjustments with the pull screws, and then just tighten the push screws meet the mirror.

In my case I was heavy-handed with my first primary collimation on my RC8, and I didn't ensure the push screws were well out of the way first. The effect was obvious, as was the remedy. Since the mirror is an amorphous solid I backed everything out and let it sit at room temp for a couple of days before messing with it again. That pause may have had zero effect, but I decided it wouldn't hurt.

[dg401]

What you're describing with the mirror out of its cell doesn't really translate, as the collimation of the primary is relative to the primary cell. With the tube-mounted RCs it's also moving the focuser with it. (The GSO truss RCs have independent focuser flanges with their own collimation screws.)

However, in a mounted mirror, if you don't loosen all of the push screws before adjusting the pull screws, and by that I mean well out of the way, you will almost certainly torque the mirror if it encounters a push screw. That's why I advise that you should back out the push screws significantly, make all of your adjustments with the pull screws, and then just tighten the push screws meet the mirror.

In my case I was heavy-handed with my first primary collimation on my RC8, and I didn't ensure the push screws were well out of the way first. The effect was obvious, as was the remedy. Since the mirror is an amorphous solid I backed everything out and let it sit at room temp for a couple of days before messing with it again. That pause may have had zero effect, but I decided it wouldn't hurt.
 

I'm not following.  On the basic GSO RC scopes, the push screws do not engage directly with the mirror.   The mirror sits on a plastic gasket which slips around a steel tube that defines the central opening.  A tightening ring with a rubber gasket (52 OD/48 ID on an RC6 ) threads into the central opening and the gasket engages the top of the primary mirror.  It takes maybe a quarter turn after the gasket grabs on the mirror to secure the mirror.  A couple small  set screws can be gently secured for a bit more grab.

 

However, just because I'm not following doesn't mean you're not correct... so I'm giving it a shot per your tightening recommendation.

[nebulachadnezzer]

I'm not following.  On the basic GSO RC scopes, the push screws do not engage directly with the mirror.   The mirror sits on a plastic gasket which slips around a steel tube that defines the central opening.  A tightening ring with a rubber gasket (52 OD/48 ID on an RC6 ) threads into the central opening and the gasket engages the top of the primary mirror.  It takes maybe a quarter turn after the gasket grabs on the mirror to secure the mirror.  A couple small  set screws can be gently secured for a bit more grab.

 

However, just because I'm not following doesn't mean you're not correct... so I'm giving it a shot per your tightening recommendation.

I just looked at the CAD drawings for the RC8 tube and RC10 truss and upon review I believe you are correct about the tube models. I stand corrected because there's no apparent direct coupling of the collimation screws to the mirror. This appears to be different from the truss tube models. The screws on the truss models still engage a holding plate, but that plate is bolted to the mirror.

I don't know what to say, then. The experience I described was on my carbon tube RC8 so it can't have occurred the way I described it. I went from achieving decent collimation one day (DSI method) to trifoil stars (when in focus) the next day without touching the screws. Folks on the GSO RC Facebook group who generally know what they're talking about said my primary mirror had to be pinched due to torque and I didn't think to question that.

I then did as I described and backed everything off, re-doing the primary collimation from scratch a couple of days later and my stars have been round ever since. I've gotten a lot of great results out of that scope since then.

[dg401]

I just looked at the CAD drawings for the RC8 tube and RC10 truss and upon review I believe you are correct about the tube models. I stand corrected because there's no apparent direct coupling of the collimation screws to the mirror. This appears to be different from the truss tube models. The screws on the truss models still engage a holding plate, but that plate is bolted to the mirror.

I don't know what to say, then. The experience I described was on my carbon tube RC8 so it can't have occurred the way I described it. I went from achieving decent collimation one day (DSI method) to trifoil stars (when in focus) the next day without touching the screws. Folks on the GSO RC Facebook group who generally know what they're talking about said my primary mirror had to be pinched due to torque and I didn't think to question that.

I then did as I described and backed everything off, re-doing the primary collimation from scratch a couple of days later and my stars have been round ever since. I've gotten a lot of great results out of that scope since then.

You don't get trifoil stars from any of the regular aberrations, so something is pinched.  I'm thinking that maybe I cranked the rubber gasket down too tightly along with the set screws on the metal ring that holds down the gasket.  I've got everything put together just tightly enough that the whole thing doesn't fly apart.   Time for a star test, right?  Nope.  It started off clear enough around sunset, then clouds and rain rolled through in what I can only interpret as a big middle finger from Mother Nature.

 

This collimation procedure is very straightforward and I've memorized it without really trying.  Centering the Cheshire view with the secondary is easy albeit exacting and the rough primary centering is easy as well.

 

My Moonlite lines up with the primary without any adjustment, so I can get away with skipping that step.

 

The hall of mirrors takes a bit more effort.  Having the scope on the mount with the primary hanging straight downward helps.  I can leave the push screws loose and gravity keeps everything in line until I'm ready to tighten down the push screws.  I've found that putting a brightly lit white card on floor underneath the open focuser tube helps to better see the effect.  Aside from the receding "scallops", I look at the secondary for the bright and easily seen first reflection of the white card.  If everything is lined up, you should just be able to see a small crescent-shaped sliver of the 2nd reflection of the white card if you look at just the right angle.  If you get the visible amount of this 2nd reflection sliver of the white card looking identical down all four vanes as you rock your head slightly back and forth to check maximum extent and symmetry of the crescent, you're lined up.  I proceed from vane to vane and if I imagine there's any slight difference, there probably is so as the OP/author recommends, be fussy and aim for perfection.  Not offering this as the way to do hall of mirrors... just another option and one that works for me on an RC6.

Edited by dg401, 01 August 2021 - 08:31 AM.

[MikeECha]

I'm not following.  On the basic GSO RC scopes, the push screws do not engage directly with the mirror.   The mirror sits on a plastic gasket which slips around a steel tube that defines the central opening.  A tightening ring with a rubber gasket (52 OD/48 ID on an RC6 ) threads into the central opening and the gasket engages the top of the primary mirror.  It takes maybe a quarter turn after the gasket grabs on the mirror to secure the mirror.  A couple small  set screws can be gently secured for a bit more grab.

 

However, just because I'm not following doesn't mean you're not correct... so I'm giving it a shot per your tightening recommendation.

dg401

 

I think you are correct. The collimation screws cannot be the cause of "pinched" mirror on those scopes (on either mirror). There is no direct contact or path for stress to propagate in any way between the screws and the mirror. Only the tightening ring with the rubber o-ring can cause stress on the primary mirror if you tighten it too much. 

 

BTW, the o-ring goes on the outer edge of the tightening ring. There is a groove at the base that looks like the o-ring goes there but it does not. Just in case it helps someone who took it apart and did not pay attention (like me ).

[MikeECha]

I'm not following.  On the basic GSO RC scopes, the push screws do not engage directly with the mirror.   The mirror sits on a plastic gasket which slips around a steel tube that defines the central opening.  A tightening ring with a rubber gasket (52 OD/48 ID on an RC6 ) threads into the central opening and the gasket engages the top of the primary mirror.  It takes maybe a quarter turn after the gasket grabs on the mirror to secure the mirror.  A couple small  set screws can be gently secured for a bit more grab.

 

However, just because I'm not following doesn't mean you're not correct... so I'm giving it a shot per your tightening recommendation.

dg401

 

I think you are correct. The collimation screws cannot be the cause of "pinched" mirror on those scopes (on either mirror). There is no direct contact or path for stress to propagate in any way between the screws and the mirror. Only the tightening ring with the rubber o-ring can cause stress on the primary mirror if you tighten it too much. 

 

BTW, the o-ring goes on the outer edge of the tightening ring. There is a groove at the base that looks like the o-ring goes there but it does not. Just in case it helps someone who took it apart and did not pay attention (like me ).

 

EDIT: Oops! sorry for double post. 

Edited by MikeECha, 01 August 2021 - 10:35 AM.