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Problem with 80mm WA Objective

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#1 brianc99

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Posted 01 December 2012 - 02:13 PM

A few weeks ago, I dug my old Celestron 80mm WA scope out of storage, and it was not in good shape - rusted screws, etc. As part of reconditioning, I decided to attempt some of the popular mods for this scope, including blackening the edges of the objective.

Unfortunately, the spacers between the elements have been problematic. One of them separated into 2 pieces, or layers. All of them seem to have some sort of gunk on them. That gunk has seeped out and spread along the edges of the elements, and doesn't want to come off. Last night, I got some of the worst of it off with isopropyl alcohol, but there remains some still, about 1-2mm along the edge of the elements.

So, my questions are: any suggestions on cleaning whatever that mess is? Also, does anyone have any idea whether those spacers can be replaced?

I was very surprised by this turn of events, so if anyone has any similar experience, I'd like to here about it.

At this point, the lens cell is re-installed, the spacers are in place, and there remains some residue along the edge of the elements, but I'm wondering if I should just leave well enough alone.

Thanks,

Brian

#2 sg6

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Posted 01 December 2012 - 03:24 PM

Next thing up would be acetone, found that what IPA didn't remove Acetone did.

Concerning spacers they should be replaced with alternative spacers of the same thickness. Although I suspect that the criteria may be a little loose, as the lens set up and used will I suspect be OK if a new spacer were +/- 1/2 mm on the original. Optically I suspect you will not really tell the difference.

Sort of thinking along the lines that if the ideal were calculated to be 1.15mm then someone will just have put 1mm spacers in anyway.

#3 Jon Isaacs

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Posted 03 December 2012 - 03:03 AM

Next thing up would be acetone, found that what IPA didn't remove Acetone did.

Concerning spacers they should be replaced with alternative spacers of the same thickness. Although I suspect that the criteria may be a little loose, as the lens set up and used will I suspect be OK if a new spacer were +/- 1/2 mm on the original. Optically I suspect you will not really tell the difference.

Sort of thinking along the lines that if the ideal were calculated to be 1.15mm then someone will just have put 1mm spacers in anyway.


I suspect that the spacers are much thinner than 1mm, probably closer to 0.1mm...

Jon

#4 brianc99

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Posted 03 December 2012 - 07:28 PM

Hello again, Jon. Yes, the spacers are very tiny. There are actually two layers, a clear layer and a black layer. And copious amounts of adhesive, apparently. I got the lenses much cleaner on my 2nd attempt with the isopropyl, and put them back into place, and I can see what is happening: I'm tightening the retaining ring too much, and the outer lens slides just a bit, and smears glue all over the place when it does.

I got some good ideas on how to proceed from the ATM forum. I need to get some feeler gauges to figure out how wide the air gap is supposed to be. Once I know that value, I'll find some suitable material to replace the old, delaminating shims. I'm not in a rush, because right now the lenses are 99% clean surface, with a tiny smudge of gook along the edge in a couple places. I was able to tell exactly how the two elements were aligned, because one of the layers of one of the spacers stuck to the front element, while everything else stayed on the rear, with nice neat glue outlines of the original position. So, I placed everything back in its original orientation, and made a pencil mark along the edge of the elements for future reference.

It might be a long time before I decide to address this issue.

Brian

#5 Sky Muse

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Posted 03 December 2012 - 08:22 PM

If you were using 70% isopropyl, 91% is much better for removing stubborn adhesives.

#6 brianc99

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Posted 03 December 2012 - 10:33 PM

Thanks, Sky. I did find some 91%, and it was much more effective.

#7 GlennLeDrew

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Posted 03 December 2012 - 10:53 PM

Instead of using feeler gauges, how about directly measuring the thickness of a spacer with a caliper that resolves to 0.001" or 0.01mm?

#8 Jon Isaacs

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Posted 04 December 2012 - 06:44 AM

Instead of using feeler gauges, how about directly measuring the thickness of a spacer with a caliper that resolves to 0.001" or 0.01mm?


:waytogo:

A caliper or better yet, a micrometer... Good quality digital calipers as well as standard micrometers are available at Harbor Freight for around $20...

Jon

#9 dan_h

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Posted 04 December 2012 - 06:50 AM

Just use some self adhesive postage stamp material. You can cut up a stamp or use the extra that comes around the edges of the stamps when you buy a sheet. It may not be perfect but it will be a very good starting point. Can be easily blackened with a marker.

dan

#10 brianc99

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Posted 04 December 2012 - 10:18 PM

I may need to go find a pair of dial calipers. Wish my dad was still around, he was an expert machinist, close tolerances were his bread and butter. I remember once when I was kid, he and my brother were at the kitchen table, picking up various small objects and trying to estimate both the size of the item in thousandths, as well as the durometer (hardness). Dad had all the measuring tools out, and his estimates were incredibly close.

#11 brianc99

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Posted 28 December 2012 - 08:54 PM

I finally tackled this issue last night, and it looks like it's pretty well resolved. I found a deal a a pair of calipers at Rockler, and it turns out that the postage stamp was just about right to replace the black spacer. The clear spacer remains, it's stuck to the glass with very strong glue. A layer of postage stamp was just the right replacement, and the 91% alcohol removed all the old glue - which was very visible when looking at the moon. The scope's probably working as well as it possible can, at this point. Thanks to everyone for all the advice.

Brian






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