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Astigmatism in an Achromatic Doublet

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#1 Larry Geary

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Posted 06 May 2013 - 10:44 PM

I'm having a problem with a 60mm f/15 Carton lens from a vintage (early 1960's) refractor. The initial star test showed strong astigmatism. I eliminated my eye as the culprit, and planned to take a closer look at the lens.

I removed the lens from its cell, cleaned it, and replaced the spacers with three new ones that brought the Newton ring pattern close to the center of the lens, where before it was off the edge of the lens. There was no index mark on the lens, but rotating the front element made no difference in the new location of the rings. I made sure the lens was loose in its cell, and tested again, without a diagonal and using a 7mm Nagler (130X, 54X/inch). The result was essentially the same.

One thing I should mention is that when I measured the original spacers, it seemed like two of them measured 0.002" and the third measured 0.003", but this may have been an error on my part. The three new spacers (two of the originals had fallen off by themselves) are all 0.002". I don't know if this possible difference of one spacer is significant.

As for collimation, using a Takahashi collimating scope all three reflections are in a tight - though not perfect - group in the center of the field. I can't check the focuser collimation because my laser collimator died and a new one won't come for a couple of weeks.

I've heard these Carton objectives have pretty good optics, so I'm surprised by this result. Is there something I'm doing wrong or have overlooked?

#2 bremms

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Posted 07 May 2013 - 05:47 AM

I would rotate the elements with respect to each other. Mark the current orientation then rotate. I've tuned a few older objectives like this. In one case I was able to eliminate some slight astig. In another it helped, one it did nothing. Worth a try in any case.

#3 Mark Harry

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Posted 07 May 2013 - 06:19 AM

Sheldon F. is selling the Carton 60mm F/15 achros for about $12 + spg;
That is, if you can't get the problem remedied. Look in the classifieds- ATM parts.
M.

#4 Pinbout

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Posted 07 May 2013 - 06:59 AM

you can eliminate your eye by doing a ke test on a star. if it has stig the ke will clock around. I used white artist tape layed acrossed the diagnol for my ke.

here's my jaegers. it's suppose to be a good lens also. suppose to be.

#5 Mike I. Jones

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Posted 07 May 2013 - 07:48 AM

Wedge tolerance in that airspace is very tight. Be sure to measure the shim thicknesses and get them the same to <0.001", and <0.0005" would be even better.
Mike

#6 Larry Geary

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Posted 07 May 2013 - 09:34 AM

Mike, I'm able to see the Newton rings using a fluorescent light. Would a centered ring pattern mean that the shims are all uniform? Is there any other way to identify a wedge error?

#7 Larry Geary

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Posted 07 May 2013 - 09:55 AM

Sheldon F. is selling the Carton 60mm F/15 achros for about $12 + spg;
That is, if you can't get the problem remedied. Look in the classifieds- ATM parts.
M.

The ones I see are Chinese, not Cartons.

#8 Ed Jones

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Posted 07 May 2013 - 10:19 AM

Here are some possible causes:
1 The individual lenses have wedge. Check out this video but use plastic tips on a polished lens. Mark the thickest point on each lens and assemble with the thick edge of on lens with the thin edge of the second.
2 Wedge between the lenses. Check the spacer thickness.
3 Surfaces are astigmatic. Thin lenses can warp in polishing from a poor holder. Hard to test without test plates however.
4 De-centered from too much slop in the lens cell.
5 Bad collimation. I'm assuming you've checked this.

#9 GlennLeDrew

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Posted 07 May 2013 - 11:35 AM

A centered fringe pattern only tells you that the air gap is uniform, and hence the spacers are of equal thickness. If differential rotation of the elements does not materially reduce the astigmatism, then one element has most or all of the wedge.

This presumes, of course, a squared-on cell and reasonably co-axial focuser...

#10 Larry Geary

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Posted 07 May 2013 - 11:46 AM

A centered fringe pattern only tells you that the air gap is uniform, and hence the spacers are of equal thickness. If differential rotation of the elements does not materially reduce the astigmatism, then one element has most or all of the wedge.

This presumes, of course, a squared-on cell and reasonably co-axial focuser...


So are you saying that, although rotating the upper element shows no change in the ring position, I could see changes in the star test?

Here is a picture of the rings. The offset from center is real.

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#11 bremms

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Posted 07 May 2013 - 11:54 AM

As long as your spacers are the same thickness, the only thing to do is rotate the elements collimation is good. Measuring is all well and good, but there are not a lot of variables that you can change. Rotate elements and look at the images at 50x per inch or so. If that doesn't work, the lens has astig.

#12 Larry Geary

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Posted 07 May 2013 - 12:03 PM

As long as your spacers are the same thickness, the only thing to do is rotate the elements collimation is good. Measuring is all well and good, but there are not a lot of variables that you can change. Rotate elements and look at the images at 50x per inch or so. If that doesn't work, the lens has astig.


The spacers are the same thickness as they were cut from the same piece. HOWEVER, I notice that not all are exactly at the edge of the lens. There is also some residual glue from the original spacers that may be varying their height. I'll correct these things. Unfortunately it looks like I'm in for two nights of rain.

#13 GlennLeDrew

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Posted 07 May 2013 - 12:05 PM

Larry,
Rotating an element will not cause the location of the fringe pattern to change, because the air gap is fixed by the spacers.

I wonder if, when wedge is dominant in one element, astigmatism can be lessened/eliminated by varying the air gap so that the lens as a unit has uniform edge thickness...

#14 bremms

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Posted 07 May 2013 - 02:19 PM

The picture was posted while I was posting and I didn't see that one... Yea, spacing is not even. I had assumed the newton's rings were centered now. My bad. Yes, rotating will not help in that case.
Those newton rings are way off. Clean all the old spacers off and use 2 or 4 layers of aluminum foil folded into a strip pressed flat. that gives a very accurate spacer (no glue) If that looks good you can use a TINY bit of adhesive to hold them in place.

#15 Larry Geary

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Posted 07 May 2013 - 02:25 PM

Do you think that is the cause of the astigmatism?






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