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60mm F/15 C.O.C. with an issue.

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#1 Jon Isaacs

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Posted 21 November 2020 - 01:36 PM

I recently rejoined the ranks of the Classic Telescope owner in earnest.  A Craigslist add for a 60mm F/15 for $50 and my wife wanting to go for a drive resulted in this 60mm F/15 Carton.  It's in reasonable shape, everything seems to work. It only came with straight through porro prism and one eyepiece but the draw tube seems to standard 36.4 mm Vixen and I found a visual back so everything was under control.  

 

Carton 60mm F15 1.jpg

 

Carton 60mm F15 2.jpg

 

 

I was able to get first light the first night. It was inconclusive, just a quick look.   The objective looked like it might have a mold issue between the lenses, it was certainly quite dirty and looked like there could be mold. I was able to get the objective apart with the aid of strap wrenches and some hot water under the kitchen sink spout. It actually cleaned up quite nicely so that was good.

 

When I disassembled it, two of the spacers stuck to one element, the third spacer to the other. This at least served as alignment marks so I knew which side was which and how they had been aligned.  

 

The issue:  Astigmatism  There's classic astigmatism, best focus is a cross rather than a point and passing through focus the image is a line that rotates 90 degrees. 

 

It didn't look like the lens had been disassembled so I am in a holding mode while I think about possibilities. It's possible the one lens is tilted somehow due to the spacer, it came off and was positioned again where it had been. 

 

Any thoughts would be appreciated.  I may try rotating the elements but I am not hopeful.

 

Jon 

 

 


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#2 eyeoftexas

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Posted 21 November 2020 - 02:06 PM

Best of luck.  It looks nice.


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#3 davidc135

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Posted 21 November 2020 - 02:06 PM

I suppose you might try varying the thickness of one of the three spacers to see if there is an effect. Similarly, the tilt of the whole objective. Also check if the objective isn't being supported only across one diameter and whether the retaining ring is too tight (but I know you know this). I doubt if it's the cause but the objective should be supported at three points corresponding to the spacers.

 

Probably those checks will draw a blank and so it could be that one or both of the objective lenses are faulty. The crown could be rotated relative to the flint. If nothing changes then it's likely the flint. If it gets worse or improves both lenses have a problem. If it is in either the crown or flint the air-space could be annulled with oil to show which surface is bad; just for interest's sake. 

 

For completeness there's always the diagonal and the eye at low power.  David


Edited by davidc135, 21 November 2020 - 02:37 PM.

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#4 PawPaw

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Posted 21 November 2020 - 02:09 PM

Jon..........Very nice Carton and it looks complete.  Here is a very informative thread on Vintage Lens and spacers etc:

 

https://www.cloudyni...?hl=lens spacer

 

Show us a picture of the lens.

 

All the best

 

Don


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#5 MisterDan

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Posted 21 November 2020 - 02:46 PM

Hi, Jon.

 

My Carton has the same issue.  While I have not confirmed via trial and error, I suspect axial tilt may be the primary problem.

 

David's notes are valid, of course.  I tried the element-rotation scheme years ago, but no clear results were seen - granted, it was a very tedious process, so if you do try it, you may wish to seriously consider a makeshift optical bench, rather than chugging through the removal-rotation-reinstallation method I tried with the objective and cell.

 

Another thing to check for is axial tilt - in either the objective or the focuser (or both).  A quick "eyeball" method will catch significant misalignments.  Mount the scope in a manner which allows for a long line of sight towards the front end of the objective (say, in your yard).  Using the clear aperture, the dew shield's edge and the focuser drawtube at the other end, your line of sight (perhaps 30-40 feet) should reveal any significant tilt, if it's there.  A similar check via line of sight towards the focuser can help confirm.  If you remove the visual back, the shiny chrome edge of the drawtube provides a nice annulus "target."

 

Sometimes, one end (or maybe both ends) of the optical tube is simply not-quite-square.  If you do find axial tilt, you may try a simple rotation of the focuser 120 degrees (to the "next screw hole") to see if tilt is lessened or worsened.  If there's no improvement via focuser rotation in either direction (clock- or counterclockwise), then trial-and-error shimming of the objective (ugh...shudder) would be my next (and hopefully final) check.  In this case, work will likely need to be done while the o.t.a. is assembled.  Make your shim/tilt adjustments at either end of the "astigmatic oval's" major axis.

 

I sincerely hope it's nothing more complicated than a simple case of focuser tilt.  Even in an f/15 refractor, it can be easy to "find" and see through simple visual alignment over a long-enough sight line.

 

Let me know if I can clarify any of my babble.

Best wishes and luck.

Dan


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#6 Kasmos

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Posted 21 November 2020 - 05:05 PM

Checking the objective for centered Newton rings should give you an idea if the lenses are equally spaced. Even when they are a little off, the images should be decent. That said, you may have another problem all together.

 

On one objective I have, I had to take it in and out of the cell many times to get it to work well and I never found out why most of the times it performed poorly (lots of CA and soft focus). I inspected and retraced every aspect of the lens and cell and it's fit on the tube. Once it went back to working very well, I've left it alone, but now I'm afraid of ever taking it apart again.


Edited by Kasmos, 21 November 2020 - 07:48 PM.

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#7 MisterDan

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Posted 21 November 2020 - 07:20 PM

I should have thought of this before:

 

Ensure the scope is solidly mounted, and loosen the objective cell (from the tube) by about a half turn.  Mark a "baseline"  with two small strips of masking tape.

 

Now, view a distant "point" source and defocus to induce a few-to-several wavelengths of astigmatism (you don't need a true point source, so long as it's "point" enough and distant enough).  Note the orientation of its major axis (ala, "2 o'clock/8 o'clock").

 

Rotate the entire cell a quarter turn (mark your stopping point) and check the orientation of the astigmatic major axis, again.  Did the axis rotate within the eyepiece view?  Did the major axis elongate? shrink?  Was there no apparent change, whatsoever?

 

Rotate the cell back to its "starting point" and make sure your defocus and original "o'clock" orientation are re-established.  Now rotate the cell a quarter turn in the other direction and check your new results (i.e. astigmatism axis rotation, extent, et.al).

 

If there's no change in the orientation of the astigmatic pattern, then the objective's alignment and the elements' relative orientations are almost certainly fine.  What's left is to check the focuser.

 

All of this can be done during daytime.

 

Best wishes.

Dan


Edited by MisterDan, 21 November 2020 - 07:26 PM.

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#8 Jon Isaacs

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Posted 21 November 2020 - 07:47 PM

Dan:

 

I checked the alignment with my Howie Glatter laser collimator, it was dead on, right in the center.

 

I disassembled the lens cell, did a final cleaning and carefully assembled it. I'm going to give a try to night.. if it's still astigmatic, I'll probably put it on the back burner for a while.

 

Thanks for your help and encouragement.

 

Jon



#9 Pete W

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Posted 21 November 2020 - 09:02 PM

Jon, another thing to check are the tube baffles.  I’ve had one tilt crimping the light cone.  


Edited by Pete W, 21 November 2020 - 09:03 PM.

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#10 Jon Isaacs

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Posted 22 November 2020 - 09:50 AM

An quick update:

 

It was clear enough last night to do some testing.  There's plenty of astigmatism to go around, whoever wants some, there is plenty.  It clearly focuses to a relatively large cross, worst I have even seen. 

 

I did try rotating the front element, no change in the amount of astigmatism.  Very disappointing.  

 

At this point, I am not sure what to do.  I may try to find a good objective for it or I may try to find someone who wants it.  For me, a 60mm refractor like this is for some casual fun, an easy scope that just performs.  I have already paid for Chucks 60mm x 800 mm Asahi-Pentax which is a known top performer.  

 

Jon



#11 MisterDan

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Posted 22 November 2020 - 12:17 PM

I'm sorry to hear that, Jon.

 

I suspect I'll be in the same boat, once I re-test and evaluate my own C.O.C.  I expect to find that the focuser is aligned and fine, and I expect no change in the aberration's major axis (beyond orientation) when rotating the cell.  If, indeed, that's where I end up, I will likely start from scratch and check the spacers for wedge and the cell "seat" for obliquity.

 

If it's objective tilt, I will find it and cure it.

 

If it's "in the glass," I'll try one or two systematic intra-element rotations and check for changes.  (If I get to this point, I don't expect a cure.)

 

If, after all that, I look down and see the same darn boat, I'll curse a few times and try for a replacement objective.

 

Cheers and best wishes.

Dan  



#12 memento

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Posted 22 November 2020 - 04:57 PM

Hello Jon, sorry to hear that your long 60mm refractor is underperforming like that.

 

 

My Dai-Ichi Kogaku 60/910's lens cell is visibly tilted when mounted to the scope. I would say that if you follow the center line of the dew shield (which is in line with the lens itself) until the rear of the tube where the focuser sits, that dew shield / lens cell's center line will be maybe 5 centimetres off! Now that I know what to look for, I can see that tilt of the lens cell / dew shield actually just by looking at the scope.

 

Nonetheless I don't see any significant problem in a star test. It focuses to a nice airy pattern. Surely there are better 60mm scopes out there but my scope's optics are absolutely good and don't show any visible astigmatism as you describe. Maybe your lens does have a problem in the spacers? Or the lens cell is too tight or something like that?

 

Thomas


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#13 Kasmos

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Posted 22 November 2020 - 05:20 PM

You really don't hear about bad Cartons, but I suppose every brand has sold a few dogs.



#14 clamchip

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Posted 22 November 2020 - 09:06 PM

Astigmatism is a interesting aberration.

I witnessed it once when I mounted a 4 inch Brandon lens in a piece of PVC tube.

I didn't have the lens square with the optical axis. My fault, trying to cut corners.

Try unscrewing the lens cell 90 deg and see if the aberration follows.

If it does there's something wrong with the objective and or it's cell.

If it doesn't then it's after the objective lens.

Of course I'm sure you are not using a star diagonal during this evaluation.

 

Robert 


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