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Thread: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

  1. #11
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?



    Hi All,

    Several months later, I've finally got the (damaged) corrector plate and telescope in my hands. Telescope model is an older Meade LX 10" SCT, I believe it's an LX6. S/N 106696. Primary and secondary mirrors are in excellent condition, corrector plate is f***ed.

    The chemical damage to the corrector plate is truly cringe-worthy...I have no idea how the previous owner managed this but he mentioned using all *kinds* of solvents and cleaners. I couldn't get any information from him as to exactly how those chemicals were applied...e.g., soft cotton rag, sponge, etc. I'm really hoping he didn't screw this up with something abrasive because this OTA will quickly go in the dumpster next to my heart. There are a few small areas on the glass that are, somehow, in seemingly perfect condition and free of the optical coating.

    Your guess is as good as mine as to how those small clear spots got there, but I think these may indicate the good viability of the whole corrector plate. The glass doesn't look pitted in the clear areas...looks ok.



    Pauls72, thanks for your notes on using Ferric Chloride and HCL. I'm really curious to know how you arrived at the conclusion to use those two specific acids?

    Alignment/Collimation:

    Thanks again Pauls72 for your input. Indeed, I found a black mark on the inside of the OTA where the corrector plate sits, however I'm not seeing any alignment markings on the corrector plate itself. I do see text written on the outside edge of the glass: F-7-78-27.

    Am I correct about the following:

    1. Rotation of the secondary mirror relative to the corrector plate is critical, because these two optical surfaces are inherently imperfect in some way (surface roughness?), and therefore a "sweet spot" between them must be found to meet factory spec.

    2. Rotation of this assembly (secondary mirror + corrector plate) relative to the primary mirror is NON-critical, because the primary mirror is inherently optically "perfect", so rotation makes no difference.

    3. The better that the secondary mirror is centered on the corrector plate, the better the possibility for a perfect final collimation.
    Attached Images Attached Images

  2. #12
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    The writing on the outside edge of the corrector should be your orientation mark, I believe there should be something on the secondary so it can be pointed in the same direction as the corrector.
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    I've just done a bit of research, and I've found some extremely specific information on removing MgF optical coatings using strong acids. Apparently, the acid, Fluorine in the MgF coating, and free Water in the solution react together to form unwanted Hydrofluoric Acid (HF). The article expressly mentions that the formation of Hydrofluoric Acid (HF) will permanently etch the precision glass surfaces laying underneath an optical coating.

    ...But it seems easily correctable:

    The article says:

    Use the ratio of 50 grams of Boric Acid, to 1L (one liter) of concentrated Sulfuric Acid.
    Raise the solution to nearly the boiling point (300C). Evidence of this would be white fumes of Sulfur Trioxide (I've personally done this before, this is unbelievably scary).
    Remove heat source, let cool to prevent thermal shock to the optics.

    The Boric Acid is used as an inhibitor to prevent the formation of HF, and protect the underlying precision glass surface...

    Can anyone confirm?

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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Ferric Chloride is used to etch printed circuit boards. It is pretty weak and I know it eats copper and aluminum, so I figured it was worth a try. It did OK.
    HCL is a pretty strong acid and it is readily available in the form of muriatic acid. My thoughts there that this would clean off any remaining metal deposited on the glass pretty quickly. If I didn't leave it in too long, it should not hurt or etch the glass.

    Man, that looks really nasty. Almost looks like they used heavy sand paper on it. You might try using an assortment of fine sand papers on an orbital sander followed by some glass or metal polish. One of those automobile headlight restoration kits would have everything you need. Something like this:
    Mothers« Nulens(TM) Headlight Renewal Kit
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Hi All,

    - I found a ceramic-lined pan, which is about the only thing compatible with hot concentrated Sulfuric Acid. The pan was 12" in diameter, collector plate was 11", so the plate rests nicely on the edges of the pan, so no surfaces can be scratched

    - I sourced Sodium Orthoborate (Boric Acid), in the form of laundry detergent booster. The article mentioned that this compound would be ok.

    - I put the collector plate in the pan, then carefully added the two acids.

    - Raised to the point at which it evolved white fumes. This might have just been water vapor...I'm now beginning to question whether or not I got it hot enough to work.

    - I let it slowly cool to limit thermal shock to the collector plate. Took it out, rinsed with tap water, then distilled.

    Didn't change a damn thing. I believe that the previous owner removed 100% of the coating, and the white fogginess shown in the pictures is damaged base glass. It might be dumpster time : (

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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Sad to here that outcome, from the look of the picture it seems like it was scrubbed with a abrasive pad.
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Holy cow that's some real damage! Good luck with your efforts to clean it.
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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Thanks everyone. Yeah...this might end up a splinter in my mind because I really can't understand WTF happened with this thing, or how to proceed. The extreme edges of *both sides* are clear. Strange, no? Additionally, there are these inexplicable "thumbprint" looking spots that are clear, and there is one area that is a blotch of maybe a couple of square inches, that is clear. Then there are perfectly round little clear spots...it looks exactly some kind of chemical splash, but the chemical is exactly what the plate needs applied to it everywhere. The optical surface in the middle of the clear spots are *perfect*...which really kills me...such a tease...

    Thanks Pauls72 for the Cerium Oxide recommendation. Because I can't think of anything else.

    This was intended to be a project scope anyway

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    Default Re: Chemical-Stripping a Severely Cloudy 10" 1/6.3 SCT Corrector Plate?

    Hi. Now I am just a newbie here but I did spend some time in a DOD optical shop. From the picture, it almost looks like a layer of resistant coating that has been glazed by some form of harsh acid. I would try different types of acids or acetone on a 1" or so spot. keep it wet for a half an hour or so then rinse with water to see what effect it has. if no effect, try muriatic on another area, keeping it wet for half an hour to give it time to work. Some lens coatings are VERY resistant! We used to clean our lenses with grade Acetone with no damage to the coatings. Just a suggestion. Regards, Bill

 

 
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