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timthelder

100ED doublet lens

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Well I took it apart again, thought I would share a few pics of the grease pencil marks. The crown glass element has two marks, but the ED element only has one that I can make out, (I examined it with a magnifying glass.) I guess this boils down to several trial and error lens line ups...The lens spacers I made to replace the original felt spacers measure 0.007th of an inch. I have read that the originals measure 0.010, meaning spacing is closer by 0.003. Not sure if that much of a difference matters much or not.

IMG_2160_zpsbce78a92.jpg

IMG_2163_zps0274d669.jpg

IMG_2168_zps08ae0a0d.jpg

And my little homemade spanner wrench.

IMG_2170_zpsbe3a2e4d.jpg

I have given some thought to the 'pinching' of the optical lenses due to extreme temperature fluctuations. My scopes are all housed outside, therefore they are always at ambient outdoor temperature.

The only way I contribute to any temperature changes, is by turning on my dew heater an hour before I snap my first focusing image.

Tim.

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Tim It looks like you have the 2 elements lined up correctly in your photos. I would at this point make my own mark with a sharpie just for reference. The spacing I would think is fairly critical. The air space is actually acting as a lens element as air has a refractive index. Point 003 may not seem to be much at the element end, but if you look at the light cone, at the focal length of the scope, that .003 may have a larger effect on the image 700mm or whatever the focal length of your scope is, away. Especially so as light is measured in nanometers. Point 003 inches is like a football field to light. Another thing is the figure of the elements. Although one would think that the back of the crown lens would be symmetrical to the front of the FPL lens it may not be the case. The two lens colud have different figures that are just not visible to the eye. That .003" may be the difference of the center of the 2nd lens making contact with the back of the crown lens thus causing deformation and changing the figure in both elements.

To put the elements back in the cell and remain aligned. I went to my cupboard and found a small bowl that was lesser diameter than my lens elements. I got a piece of surgical hose and cut 4 sections about an inch long and split them length wise. Open the splits and place over the bowl edge at four places. Stack the lens elements with the crown front down on the rubber and then the spacers and then the FPL element. Carefully slide the lens cell over the elements and down over the bowl. Using thumbs and fingers grasp entire assembly even the bowl and flip it over. Wal la! Lens in cell without messing up alignment. When tightening the retaining ring back up care must be taken that the 2 elements remain aligned. The retaining ring may want to cause the crown element turn a bit as you tighten the ring.

Eric

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Correctly is subjective ATM I think. When I popped the lenses out I had the (1) line-up mark on the ED glass, above the left hand mark on the crown. Pictures shown are after rotation to the right-hand mark, (after much scrutiny.)

Finding a suitable thickness spacer material is my conundrum ATM. I have several brands of stuff lying around, but none are the correct thickness. I have a few ideas on some others. I will just have to carry my caliper with me whilst I go shopping...lol.

Per your example, I will use the same principle on the re-assemble. The lenses are resting on a 'duct tape core' which will allow me to gently set the aluminum housing back around the lens cells. With a little slide off of the workbench and a flip, I can put the washer and retainer ring back on. A little spritz of spray silicone, hopefully no slippage. Will have to clean my prints off too with some eyeglass cleaner before final assembly.

Your input is appreciated, makes me think...

Gotta luv it! (again)

Tim.

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Tim....my 120mm ED EON had a circular plastic shim that went around the entire circumference of the lenses. Here is a link to some shim stock sheets that you could cut out a circular shim. This would ensure proper alignment and none of the shims slipping or having to use any adhesives. It also will not compress like felt or cork shims over time.

Link to shim stock: MSC Shim Stock 0.010" 20"x20"

Enough there for mistakes!

Eric

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Thanks Eric, reasonably priced, I added that to my favorites.

I took a few measurements the threaded retainer ring's ID measured 99.98mm. The outer diameter of the lens cells measured 102.496mm. Basically this means the width of the spacer ring/shims can be no more than 1mm wide!

Easy to mark, not so easy to cut.

Tim.

(Ordered the shim stock...)

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Please keep us posted on this - very interested to see how you finally sort it out.

Greg

P.S. I agree with Eric that the shim thickness is critical.

1mm wide shim - now that is going to test your precision engineering skills :)

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Well it's married again, now I need a clear sky so I can do a star test. Visually, nothing protruding past the threaded retainer ring.

All that's left...

lensshim_zps5ea23418.jpg

Tim.

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Haven't taken any with it yet, or any other scope. Due to some unforeseen difficulties, I am having to relocate my pier. Will do a follow-up as soon as is feasible.

Tim.

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