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Polishing optical acrylic lens element in a Lunt s

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#1 snorkler

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Posted 11 September 2013 - 06:05 PM

An optical acrylic lens element in the prism of my Lunt solar scope developed mineral deposits in storage. I removed the prism.

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Remove the prism by loosening the set screws. Then unscrew the tube. You can see the fogged element inside.
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Remove the retaining ring with a spanner wrench. For lack of better tools in Bryce Canyon National Park, we used needle nose pliers.
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#2 snorkler

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Posted 11 September 2013 - 07:02 PM

Photo shows the removed lens element. Until we removed it, I thought the element was a multi-element lens fogged internally. After removal, I saw a single element, which was coated on both sides with mineral deposits. Washing did not remove the deposits completely.

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After searching the WWW for polishing compounds for optical acrylic plastic, I found three recommended compounds: Meguiar's PlastX, Novus plastic clean and shine, and Brasso. We drove 80 miles to the nearest city, Cedar City, because Meguiar's is sold in Walmarts.

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I polished the lens element (or filter) with the Meguiar's using cotton underwear. The results were amazing.

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Views through my solar scope are as good as they have ever been.

#3 Sean Cunneen

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Posted 11 September 2013 - 08:09 PM

Nice!

#4 GlennLeDrew

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Posted 11 September 2013 - 10:42 PM

Good work! Many would be rather too trepidatious.

#5 Crayfordjon

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Posted 12 September 2013 - 12:57 AM

I once experimemterd making acrylic lenses, I polished them with plasticene laps and rouge, the finish was brilliantly clear.

#6 Footbag

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Posted 12 September 2013 - 01:00 PM

Is this the same issue as the "rusting"? Seems like a very simple solution for a worrisome problem.

#7 richard7

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Posted 12 September 2013 - 01:54 PM

This is something that neither I nor Cloudy Nights would recommend. You're dealing with something that could cause permanent eye damage.
If it looks bad, send it back to the manufacturer.

#8 RikkiHocking

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Posted 12 September 2013 - 04:24 PM

I am sorry to see you are having this issue. We have come across it on a few blocking filters. Sometimes there is a film that develops on this piece of glass. There is no degrading of the filter. There are no coatings on this piece. You can feel free to either clean the glass as you have seen here or contact Lunt Solar and we can send you a new piece.
Thank you,
Rikki Hocking
Lunt Solar Systems
(520)344-7348
r.hocking@luntsolarsystems.com

#9 richard7

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Posted 12 September 2013 - 09:44 PM

Thank you Rikki.

#10 snorkler

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Posted 13 September 2013 - 04:45 PM

I hadn't considered the element could have been coated, or glass. I would have backed off if I had seen any optical change, but I agree I should have contacted Lunt before proceeding as I did. You get used to doing your own repairs in park isolation.

Since my problem is fixed, I won't request replacement of the blocking filter. Thanks Rikki and Richard for your feedback.

#11 ion

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Posted 13 September 2013 - 05:20 PM

The Lunt website refers to this phenomenon as "premature failure" of the Induced Transmission Filter (ITF):

http://www.luntsolar...al-manuals.html

How can simply removing the damage or "film" be an acceptable repair? What is this "film" composed of if the part is not coated? Where exactly did all these "mineral deposits" come from and why don't they affect any other internal lenses in the telescope?

http://www.optcorp.c...ab33525d08d6...

#12 RikkiHocking

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Posted 13 September 2013 - 05:54 PM

Hi Ion,
We do not use ITF's in our systems anymore. The poster image you have is very old. This is BG glass. There are no coatings on it. The glass itself is the filter. We have not had any film on any other element.
Thanks,
Rikki Hocking
Lunt Solar Systems

#13 marktownley

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Posted 12 October 2013 - 01:57 PM

What a revealing thread. Very interesting :)

#14 BYoesle

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Posted 12 October 2013 - 03:34 PM

Makes sense with the use of a hot-mirror ERF coating with the front filters or on the collimator lens of an internal etalon scope.

See the BG glass filter curves in this document: http://www.us.schott...r/us/index.html

#15 marktownley

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Posted 12 October 2013 - 05:18 PM

Tee hee hee - I was looking at the same webpage Bob ;)

#16 BYoesle

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Posted 12 October 2013 - 07:29 PM

:waytogo:

#17 BYoesle

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Posted 20 October 2013 - 12:33 AM

BG38 looks like a likely candidate for the filter used, with decent transmission @ 656 nm ~ 50% :

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