39 replies to this topic

[jragsdale]

Greetings, I'm trying to collimate an older AP oil spaced triplet (6" f/12.5). It has a collimatable cell, and I noticed that when I received it, there was a significantly larger gap on one side of the 2 piece cell than the other. Curious about this I checked the reflections in a cheshire and they were significantly off. Here's a photo captured with a ColliDream, the 2 crosses should coincide:

 

 

So I collimated it so they align and views were good. Seeing was average so I couldn't push it much past 300x. I checked out a star for seeing the in/out of focus pattern and noticed that it wasn't good at all. Quite a bit of astigmatism (elongated one direction inside of focus, the other direction outside of focus). The airy disk was not round, and the bottom half of the airy disk also flared out and was blurry. Stars in focus had noticeable flare to the bottom as well.

 

Here's a test I did this morning just doing a terrestrial artificial star (car parked about 100 yards away) showing the inside/outside focus. This was several turns away from focus, so the closer you get to focus the more pronounced it becomes, but this demonstrates that I'm not hallucinating: 

 

 

I thought it might be temperature related so I let it cool down without the dew shield for an extra hour. Scope had been outside for about 3 hours at this point. Still the same issue. Tried different diagonal and eyepieces, no change.

 

I checked how close the focuser was to being centered on the lens with a laser and the dot on the objective was within a 1/2" of the center. I don't see any adjustments to perfect this so may need to put some shims somewhere on the back end, but I don't believe a VERY slight focuser tilt would cause what I'm seeing, if anything, maybe field flatness with a camera might be noticeable upon close inspection.

 

So I returned the collimation back to close to where it was before, and I saw the star test returned to close to circular and more even, but couldn't assess it very well, the out of focus was quite a bit blurry compared to the inside focus. I wouldn't say I'm a very experienced star tester though, usually the cheshire method gets it bang on every time.

 

I made sure the lens retaining ring wasn't too tight, the lens has a very slight rattle when you jiggle it in the cell, but it doesn't have much room to move, fraction of a mm I'm guessing.

 

Any ideas on what I should try next or what the issue could be? Also it's worth noting that views were pretty good with both collimation points. But with an AP lens like this, I would expect a close to perfect star test with negligible astigmatism visible, right?

Edited by jragsdale, 05 April 2025 - 11:17 AM.

[ngc7319_20]

First a comment on the star test images... That looks suspiciously like a heat plume running up the middle of both images.  So there is at least some thermal stuff going on.  Also, even if the scope was out for 3 hours, there could still be thermal happening to deform the lens.  For example, if the daytime temperature was rising and the lens was not able to keep up.

 

I've seen two lenses before where the Cheshire and star test disagreed.  One was a D&G 6" F/12 and the other was a Unitron 60mm F/15.  Both had some star test weirdness that was repaired by tilting the lens. So it is at least possible.  Both performed well tilted, so I just used them tilted.  And at F/12 there is not much off-axis field curvature or weirdness in the edges of the field, so using it tilted is acceptable.

 

As the lens is oiled, one could speculate whether the oil might had migrated or became wedged.  Or perhaps the elements de-centered.  After that, one might wonder if the glass had anneal problems.  Maybe worth contacting AP.  Good luck!

Edited by ngc7319_20, 05 April 2025 - 12:30 PM.

[Psion]

Yes, this is what a badly collimated lens looks like, it's tilted to one side.

[jragsdale]

As the lens is oiled, one could speculate whether the oil might had migrated or became wedged.  Or perhaps the elements de-centered.  After that, one might wonder if the glass had anneal problems.  Maybe worth contacting AP.  Good luck!

Thank you for the advice. I'll reach out to AP once the NEAF excitement has died down. Don't want to catch them in a busy moment. Lens is from 1982 so quite old for AP.

[jragsdale]

Yes, this is what a badly collimated lens looks like, it's tilted to one side.

It's odd the cheshire reflections don't match, I've never noticed that before. Do you think it's possible the lens itself is wedged or not all aligned on center? Or just simple collimation?

[Oldfracguy]

I had that type of situation before with an achromat, with a "star test" diffraction pattern indicating something akin to astigmatism as shown in the lower photo back in Post #1.  I eventually traced it to foil lens spacers that were not all the same thickness.  I made some new foil spacers literally out of tiny pieces of Reynolds wrap folded over and pressed together to obtain a total thinkness close to 0.10 mm which seemed to work best for that particular objective.

 

I don't have any familiarity with oil-spaced triplets, but uneven lens separation with one lens element tilted with respect to the others, could be the cause of what you are seeing.

[CHASLX200]

It's odd the cheshire reflections don't match, I've never noticed that before. Do you think it's possible the lens itself is wedged or not all aligned on center? Or just simple collimation?

Not sure if you took the cell off the tube. But it needs to go back the same way it came off.

[jragsdale]

Not sure if you took the cell off the tube. But it needs to go back the same way it came off.

This one just threads on, so no way to do it differently. 

 

Also of note I did the ColliDream and rotated the lens to see if the 2 reflection would make a big circle, and it didn't.

 

Here's a video, I had the scope horizonal so once I started unthreaded it tilted forward a bit, after a few rotations I noticed this then tried to keep it flush when rotating. Not sure if this is useful or not, but since I had the tool I thought it would be worth seeing objective rotation affects. I can also do this tonight on polaris: https://youtu.be/swT8kUZbzVQ

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Edited by jragsdale, 05 April 2025 - 06:05 PM.

[CHASLX200]

This one just threads on, so no way to do it differently. 

 

Also of note I did the ColliDream and rotated the lens to see if the 2 reflection would make a big circle, and it didn't.

 

Here's a video, I had the scope horizonal so once I started unthreaded it tilted forward a bit, after a few rotations I noticed this then tried to keep it flush when rotating. Not sure if this is useful or not, but since I had the tool I thought it would be worth seeing objective rotation affects. I can also do this tonight on polaris: https://youtu.be/swT8kUZbzVQ

Well that is another horse of a diff color vs how i messed up my AP 6" F/9 blue tude.

[RichA]

Greetings, I'm trying to collimate an older AP oil spaced triplet (6" f/12.5). It has a collimatable cell, and I noticed that when I received it, there was a significantly larger gap on one side of the 2 piece cell than the other. Curious about this I checked the reflections in a cheshire and they were significantly off. Here's a photo captured with a ColliDream, the 2 crosses should coincide:

 

20250405_080012.jpg

 

So I collimated it so they align and views were good. Seeing was average so I couldn't push it much past 300x. I checked out a star for seeing the in/out of focus pattern and noticed that it wasn't good at all. Quite a bit of astigmatism (elongated one direction inside of focus, the other direction outside of focus). The airy disk was not round, and the bottom half of the airy disk also flared out and was blurry. Stars in focus had noticeable flare to the bottom as well.

 

Here's a test I did this morning just doing a terrestrial artificial star (car parked about 100 yards away) showing the inside/outside focus. This was several turns away from focus, so the closer you get to focus the more pronounced it becomes, but this demonstrates that I'm not hallucinating: 

 

comp.jpg

 

I thought it might be temperature related so I let it cool down without the dew shield for an extra hour. Scope had been outside for about 3 hours at this point. Still the same issue. Tried different diagonal and eyepieces, no change.

 

I checked how close the focuser was to being centered on the lens with a laser and the dot on the objective was within a 1/2" of the center. I don't see any adjustments to perfect this so may need to put some shims somewhere on the back end, but I don't believe a VERY slight focuser tilt would cause what I'm seeing, if anything, maybe field flatness with a camera might be noticeable upon close inspection.

 

So I returned the collimation back to close to where it was before, and I saw the star test returned to close to circular and more even, but couldn't assess it very well, the out of focus was quite a bit blurry compared to the inside focus. I wouldn't say I'm a very experienced star tester though, usually the cheshire method gets it bang on every time.

 

I made sure the lens retaining ring wasn't too tight, the lens has a very slight rattle when you jiggle it in the cell, but it doesn't have much room to move, fraction of a mm I'm guessing.

 

Any ideas on what I should try next or what the issue could be? Also it's worth noting that views were pretty good with both collimation points. But with an AP lens like this, I would expect a close to perfect star test with negligible astigmatism visible, right?

Stand the telescope on its front (objective) end (but on the DEWSHIELD, not the objective) for a couple days and retest it.

[jragsdale]

Stand the telescope on its front (objective) end (but on the DEWSHIELD, not the objective) for a couple days and retest it.

I can just take the lens cell off the tube and leave it face down. Spread out the oil you think? This is on an alt-az mount so only knows gravity one way, no opportunity to spread it around.

[jragsdale]

Well that is another horse of a diff color vs how i messed up my AP 6" F/9 blue tude.

Yeah, with all the big refractors I have it's nice to have a collimation tool I can use from just one side. Setup laptop with USB cable to camera in the ColliDream then I can just watch the screen while adjusting the lens collimation. No back and forth from the lens to the back over and over.

[Jeff B]

That is a very early vintage AP objective.  Is it even coated?  Seems like an early one-off.

 

Your focuser is off by almost 1/2 degree, not bad but astigmatism is the dominant aberration off axis, so that might be a part of the astigmatism in the picture.

 

But there is most definitely a tube current at work in the inside of focus image and they can mimic astigmatism to a degree. The is a thermal plume in the outside of focus image too.  Tube currents will cause flairs. 

 

I'm not sure what magnifications you used for the star test images, but spherical correction seems quite good with no indication of a turned edge.

 

If I may suggest, repeat the tests as is but wrap the tube in a couple layers (yes, two layers) of Reflectix (from Home Depot, Lowes....) to insulate the tube.  It should help to thermally calm the interior.   The stuff is super easy to work with too.

 

Good luck.

 

Jeff

[jragsdale]

That is a very early vintage AP objective.  Is it even coated?  Seems like an early one-off.

 

Your focuser is off by almost 1/2 degree, not bad but astigmatism is the dominant aberration off axis, so that might be a part of the astigmatism in the picture.

 

But there is most definitely a tube current at work in the inside of focus image and they can mimic astigmatism to a degree. The is a thermal plume in the outside of focus image too.  Tube currents will cause flairs. 

 

I'm not sure what magnifications you used for the star test images, but spherical correction seems quite good with no indication of a turned edge.

 

If I may suggest, repeat the tests as is but wrap the tube in a couple layers (yes, two layers) of Reflectix (from Home Depot, Lowes....) to insulate the tube.  It should help to thermally calm the interior.   The stuff is super easy to work with too.

 

Good luck.

 

Jeff

Thanks for the tips Jeff. This is an all brass instrument, so might hold onto heat longer than aluminum. The star test pictured above was during the day without being acclimated. Mainly it was to show the astigmatism, but the view is similar to what I was seeing at night.

 

And yes, it's coated. Here's the original receipt.

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Edited by jragsdale, 06 April 2025 - 11:02 AM.

[jragsdale]

Photo of the scope for reference.

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[Psion]

Beautiful piece, those were the days when the owner had his name on the lens!

[Sol Robbins]

Thanks for the tips Jeff. This is an all brass instrument, so might hold onto heat longer than aluminum. The star test pictured above was during the day without being acclimated. Mainly it was to show the astigmatism, but the view is similar to what I was seeing at night.

 

And yes, it's coated. Here's the original receipt.

I don't really want to rain on your parade, but do you think it's a little bit untoward to post a guy's name & address on an internet blog?

[jragsdale]

I don't really want to rain on your parade, but do you think it's a little bit untoward to post a guy's name & address on an internet blog?

Normally yes, but a few caveats:

 

1) He put his home address in all of his advertising in Sky & Telescope and Astronomy Magazine

2) He moved from Huntington Beach in 1994

3) He passed away in 1997

 

[Jon Isaacs]

Normally yes, but a few caveats:

 

1) He put his home address in all of his advertising in Sky & Telescope and Astronomy Magazine

2) He moved from Huntington Beach in 1994

3) He passed away in 1997

 

telrad.jpg

 

 

 

The first thing I did when I saw the first owner's name, I looked at the photo again. Yes, that magnificent brass refractor has a Telrad mounted to the OTA...

 

Did Steve build the scope?

 

Something else interesting:

 

1994 or before: Price $49.95

 

Today, Astronomics will sell you one for $46.

 

Jon

Jon

Edited by Jon Isaacs, 06 April 2025 - 03:33 PM.

[Eddgie]

For about 15 years now, I have recommended that the first step in collimating a refractor is to use a laser in the focuser and make sure the laser is hitting the center of the objective. If it is not, then you really should not be moving the lens until you have corrected this issue. 

 

The curve on these lenses is such that even a half inch will cause problems. You are tilting the light so that  the reflection point is not at the exact center of the rear lens curve. The light from the rear curve reflects back at a very slight angle. When the light enters the back lens, it changes angle because you are not hitting at the center of the curve. When the light reaches the back side of the front lens, once again, it is hitting at an angle, and goes back through the stack and gets refracted again when it exits the air glass boundary of the rear curve on the way back to the focuser. 

 

1/2" Does not sound like much, but look at the curves in the glass, and remember, these curves and the refraction of the light as it passes through the stack can be greatly altered if the Cheshire is not perfectly square to the center of the curves of the lenses. Don't underestimate the effect that these various curves and the refraction of the light entering and leaving the rear lens can have. 

 

Maybe it won't make any difference, but my experience is such that I personally would always start with the focuser collimation. 

Edited by Eddgie, 06 April 2025 - 03:30 PM.

[jragsdale]

Did Steve build the scope?

The scope was made by Broadhurst Clarkson & Fuller, it's their 6" Export Refractor. Purchased in 1980 (delivered 1981). Steve didn't like the lens so had it replaced by Roland at AP. The Alt-Az mount was made by Steve and it's a marvel of engineering. It even has encoders so can accommodate a push-to system like Argo Navis or others.

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[Jeff B]

That's soooo Clark-retro cool!

 

I recommend what Eddgie says, start alignment with the focuser, otherwise, you might end up chasing your tail a bit.

 

I went ahead and cropped the tailpiece assembly, expanded it and added circles where adjustments may be possible, especially if it is a bolted assembly. 

 

During my own ATM refractor work, I usually bolt the tailpiece on with three or six non-countersunk screws with a single, usually brass, washer.  During collimation of the focuser, the screws are firm but not tight and I tap the tailpiece/focuser assembly, with laser installed, until the beam exits the center hole of the paper mask over the objective.  I then carefully and sequentially, start to tighten the screws down, watching the laser beam to make sure it stays centered on the mask.  Once that's done....your done really, with focuser alignment, except for maybe running the focuser along its travel length to see how much the centering might vary with focuser travel.

 

Jeff

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[jragsdale]

I recommend what Eddgie says, start alignment with the focuser, otherwise, you might end up chasing your tail a bit.

I fiddled with it a bit today, loosened a few screws and a little rubber mallet tapping. I'm within a mm from dead center, that's probably good enough for now. I think I'd need to disassemble the focuser to get any better.

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[Jeff B]

STOP!

 

Remember, the dire enemy of really good is truly excellent.

 

You are within .1 degrees, and, if I may repeat myself....

 

STOP!

 

Now get the Reflectix stuff.

 

Jeff

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[jragsdale]

Yeah, I've got a spare roll of reflectix, use it on my Maks and SCTs. This tube is also 6" OD, for a 6" clear aperture, so the front of the light cone is dangerously close to the tube wall where the scary tube currents live.