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    Default Spin-casting a mirror



    It's been a dream of mine for a long time to make a very large dobson. Aside from the mirrior, the rest of the scope seems fairly straight-forward to fabricate.

    So I'm wondering, how involved is the process of spin-casting a mirror? If you wanted to go into the 40" range without mortgaging your house? Is it even possible to do at home?

    Let me say I have no immediate plans to build anything, but if I were, I'd start be planning the most difficult part - the mirror. So anyone know whats involved?

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    Default spin casting technolog

    It is a significant undertaking.

    The molds for the glass are generally aluminum silicate. Carved to shape. If you don't feel the urge to go cellular this would not be so hard.

    The furnace to melt the glass needs to reach and sustain a temperatrue of about 2100 degrees and then have a slow cool down capability. Depending on the mirror size from days to weeks.

    At about 1000 degrees the mirror blank must be cooled in a very controled manner to achieve a fine anneal.

    The glass used must be a low expansion glass like pyrex or E6 from Ohara glass in Japan. Glass without the low expansion character might compound the cool down problem beyound the ability of the blank to tolerate it.

    The furnace and mold must be spun at a constant speed during cool down until the blank is below the fine anneal temperture. I have the formulas for rpm VS focal length somewhere in my files I will send them to you if you are interested. As I recall it is an amazingly simple relationship.

    If the furnace is gas fired some sort of rotateing conection to the gas supply is required. Same if it is electrically heated.

    In short lots of challenges. However I have it on the back burner to do and electrically heated rotating furnace. I don't expect to get anything going for a couple of years. I am planning on running some tests on mold materials and annealing processes this winter.

    Al Powers

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  4. #3
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    Default

    Something I've always wanted to try is a "live" spinning mirror. Use a pool of mercury in the appropriate container, and spin it with a nice accurate turntable, and view away. Then when you're done, pour your mirror back into the bottle.
    It probably wouldn't actually work, but it's an interesting idea.

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    Default

    Well a Liquid Dob would be plausible but it would have to remain verticle, they are planning build one on the moon.
    A Plan to Build a Giant Liquid Telescope on the Moon

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    Default

    I have one practically in my back yard. I should visit it sometime.
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    Quote Originally Posted by tinkerer View Post
    It is a significant undertaking.

    The molds for the glass are generally aluminum silicate. Carved to shape. If you don't feel the urge to go cellular this would not be so hard.

    The furnace to melt the glass needs to reach and sustain a temperatrue of about 2100 degrees and then have a slow cool down capability. Depending on the mirror size from days to weeks.

    At about 1000 degrees the mirror blank must be cooled in a very controled manner to achieve a fine anneal.

    The glass used must be a low expansion glass like pyrex or E6 from Ohara glass in Japan. Glass without the low expansion character might compound the cool down problem beyound the ability of the blank to tolerate it.

    The furnace and mold must be spun at a constant speed during cool down until the blank is below the fine anneal temperture. I have the formulas for rpm VS focal length somewhere in my files I will send them to you if you are interested. As I recall it is an amazingly simple relationship.

    If the furnace is gas fired some sort of rotateing conection to the gas supply is required. Same if it is electrically heated.

    In short lots of challenges. However I have it on the back burner to do and electrically heated rotating furnace. I don't expect to get anything going for a couple of years. I am planning on running some tests on mold materials and annealing processes this winter.

    Al Powers
    Building the furnace would definately be expensive and challenging - but not 40,000 dollars expensive, which is the cost of large mirrors.

    Also, what do you mean by "the molds for the glass"? Why do you need anything more than a flat dish for the molten glass to form in, it should still form a parabola.

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    Quote Originally Posted by WWPierre View Post
    I have one practically in my back yard. I should visit it sometime.
    Wow, that one certainly seems to be working. Those are nice images !

    And in such a beautiful spot too...

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  10. #8
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    Default

    The mold I was refering to is the flat dish with side walls high enough to give you the appropriate blank thickness at the outer edge of the mirror. You would want some extra depth so there would be no chance of molten glass making it over the edge and gumming up the insides of the furnace. Also you want to allow for material to grind off the blank as it is finished into a final mirror. Take a look at the steward mirror lab web site for some great info on the big mirrors they produce by spin casting. I toured the lab last spring and while it was a bit lite on technical detaisl for my taste it was a blast to see an 8.4 meter mirror going thru the lab. The grinding, laping and optical testing was amazing.

    Al Powers

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  12. #9
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    Years ago in NASA Tech Briefs I read an article about a slip ring interface for a helicopter rotor data collector/power interface that might be beneficial in spin casting a mirror.

    Instead of actually using graphite or carbon slip rings to transmit parallel data (or power) to and from a rotating shaft, which is the normal practice, the researchers invented a rolling ring connector. A gang of what looks like inner ball-bearing races is on the shaft, surrounded by a matching set of outer bearing races. Actual ball-bearings can't deform and so would make intermittent, noisy electrical contact, so instead of using a ball the engineers used what looks like a wedding band made out of springy brass, rolling between the races like a little wheel going around a racetrack. A springy ring can deform as it rolls, so the races squeeze the ring into a slight ellipse which maintains constant contact between the bearing races as it rolls between them. They found that such rings could transmit very large currents, and that's why I bring this up.

    Transmitting power for heating elements (and receiving temperature probe data) through conventional slip rings is a large potential trouble spot in spin casting a mirror, and this rolling wedding band in ball bearing races idea might solve it.

  13. #10
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    This may be a stupid question, but is there a reason the mirror HAS to be made of glass? Can't you use a more easily formed material to apply your reflective coating to?

 

 
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