39 replies to this topic

[Don W]

What an amazing thread!

 

Thanks!

[jragsdale]

What an amazing thread!

 

Thanks!

Thank you! I plan to do a big post in the classics forum, wanted to keep this post a little low key focused on the lens issue so the history doesn't overwhelm the goal at the moment.

[jragsdale]

I did a DPAC test with the scope collimated to where the cheshire reflections are centered and here is the result:

 

[RichA]

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.

Yes, if that's an issue, it may even it out.  But it would still have to be serviced unless there is a way to store the scope/lens in the vertical position.

[jragsdale]

Yes, if that's an issue, it may even it out.  But it would still have to be serviced unless there is a way to store the scope/lens in the vertical position.

Looking at the lens closer, there is a faint residue on the lens, I assumed it was on the outside surface as it looked like dew, but after gently cleaning the front and rear surfaces, I can confirm the residue is internal. So perhaps some or all of the oil has leaked out, causing both the residue and the wedge?

[RichA]

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. 

I always wondered how these eyepiece-mounted devices were supposed to really work.  Just locking the thing in-place in some focusers can induce tilt.  How  that impacts the collimation, I do not know, but intuitively, the focuser should be squared-on with the objective-lens and how they are mounted in the tube is a seconary issue.  In other words, both could be tilted by a small amount, relative to the OTA and it would not matter as long as they are parallel with each other.

[Oldfracguy]

I always wondered how these eyepiece-mounted devices were supposed to really work.  Just locking the thing in-place in some focusers can induce tilt.  How  that impacts the collimation, I do not know, but intuitively, the focuser should be squared-on with the objective-lens and how they are mounted in the tube is a seconary issue.  In other words, both could be tilted by a small amount, relative to the OTA and it would not matter as long as they are parallel with each other.

Exactly right.  That's the first thing I check: focuser alignment.  You might recognize this scope undergoing a focuser alignment check:

 

 

 

 

Then I loosen those three Phillips screws that secure the focuser to the tube and move the focuser until I see this:

 

 

 

If the scope does not have external "tilt adjustment" push-pull screws around the edge of the objective, or some other means by which to adjust the objective separate from the focuser such as the objective cell itself being attached by screws to the tube, I substitute a Cheshire for Refractors and rack the focuser outward to about where it would be with my diagonal and an eyepiece in focus, and adjust the position of the focuser on the tube until I see this:

 

Edited by Oldfracguy, 12 April 2025 - 04:20 PM.

[manbot13]

How do you guys get such nice images of a cheshire? Any pics of the setup? I'm trying to collimate a refractor but gave up on the cheshire and used an artifical star because of how small the image was.

[jragsdale]

How do you guys get such nice images of a cheshire? Any pics of the setup? I'm trying to collimate a refractor but gave up on the cheshire and used an artifical star because of how small the image was.

I use this guy, so you can insert a high power eyepiece or planetary cam and they give a very magnified view.

 

https://astro-gadget...optical-systems

Edited by jragsdale, 14 April 2025 - 10:44 AM.

[Oldfracguy]

How do you guys get such nice images of a cheshire? Any pics of the setup? I'm trying to collimate a refractor but gave up on the cheshire and used an artifical star because of how small the image was.

The two lower photos in Post #32 were taken with a little Kodak digital camera held up as steadiliy as possible to the Cheshire eyehole.  In the case of the lower photo, an artificial star is not used for this check.  What is done is to keep the cover that goes on the front of the dew shield in place, or hanging over the objective if the dew shield has been removed as in the case of the Astro-Tech and similar KUO-made ED scopes, and orient the Cheshire for Refractors:

 

https://agenaastro.c...refractors.html

 

so that the opening on the side is illuminated by a bright light source (not the Sun, of course).  With scopes that do not have black metal dew shield covers, like the orange ones on those Askar APO refractors, use a piece of black flocking material on the inside of the dew shield cover.  Looking in the eyehole of the Cheshire you will see two (or three for a triplet) small greenish circles.  They appear greenish due to the multicoatings on the glass lens surfaces.

 

For these types of refractors an artificlal star can be used to assess coma at extremely high magnification, provided you have enough room between the scope and the artificial star.  Even if the scope is collimated, and a subsequent real star test shows a coma tail extending out from one side of the outermost diffraction ring, that can be remedied by making tiny adjustments to the lateral position of one lens element relative to the other(s).  I have only had to do that one one scope, a KUO-made 80mm f/7 FPL-53 Doublet, by making very small adjustments to these radial "lens centering" screws located underneath the sliding dew shield at 90° positions around the edge of each lens element.  I only made adjustments to the rear element position:

 

Edited by Oldfracguy, 14 April 2025 - 05:46 PM.

[manbot13]

That's what I spent 6-7 hours doing last night with my LOMO 650/102. It's been a process and I'm going to start a thread on it. Coma was significant and I couldn't image it stacking barlows with an ASI 662MC so I gave up and I used an AP BARADV and a televue 3-6 zoom and had to be just out of focus (and needed quite a few extensions) I'll test it under the sky tonight.

But I wanted to start with a basic cheshire and the image was too small.

[Joe Bergeron]

Kopernik Observatory near me has a 6” f/12 AP scope that developed astigmatism. They sent to the lens to Roland and the diagnosis was the oil had solidified and was stressing the glass, as I recall. Talk to AP. 

[jragsdale]

Kopernik Observatory near me has a 6” f/12 AP scope that developed astigmatism. They sent to the lens to Roland and the diagnosis was the oil had solidified and was stressing the glass, as I recall. Talk to AP. 

Awesome, thanks for the confirmation. I sent them an email, waiting to hear back. I'm sure they're inundated right now after NEAF and all the new product releases. I'm a patient man though.

[jragsdale]

Here's an update on this scope, I also noticed there was a slight residue on the lens that looks like dried dew marks. I carefully cleaned both surfaces and noticed that the residue was INTERNAL to the lens, not on surface 1 or 6. Which would be impossible in an oil spaced lens unless the oil had leaked out. I sent all the information above, and the residue pics to AP and they agreed to service the lens. In fact, they already received it and did the work, they were so fast! Waiting to hear back on what they did officially, but according to George at AP; yesterday Roland inspected, reoiled and sealed the lens. So I expect they should be returning it to me fairly soon and I can repeat all the above tests and post the results. Woohoo! AP rocks! Servicing a 43 years old lens; amazing.

[jragsdale]

Update #2, I received this email from RC at AP;

 

Hello Jordan,

I did some preliminary work on the lens, cleaned the internal surfaces, re-oiled the lens and precision collimated it. The lens shows some defects in the rear element which shows up as astigmatism. The lens was sold without a lens cell - the customer machined a cell and placed the elements into that cell. Being over 43 years old, the rear element may have been subject to stresses for a period of time which could be the reason for the astigmatic error. 

 

The only way to fix this is to re-do this element. This will take some time since i have to develop tools for the project. I cannot guess what amount of time it will take, but if you're ok with this I can start in the coming days. The way the lens is now, it will not form a proper star image.

 

Roland Christen

 

So this is disappointing to hear, I was hoping the re-oiling would fix all the problems. I gave them the go ahead to fix that element, so when it comes back, I expect it to be an amazing performer once again!