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Thread: Stone or Granite blanks for newtonian mirror

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    Being a stone mason for 25 years and a granite countertop fabricator for 10+ years I can honestly say that stone will not make a good mirror.I have polished stone to a mirror like finish but,not even close to the reflective qualities of a actual mirror.Yes,stone is a very porous material which will absorb liquids like a sponge.A penetrant is used to seal the surface to act as a barrier for food stuffs.Wax is a common tool used to make a surface appear more glassy but,it also needs to be reapplied about every year.
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    It was just my thought that since Granite is so porous with the various inclusions, that it would be closed cell enough to work. Glass, or a mixture of it is predominately used in Mirrors for scopes. I found references to Flint Glass, but nothing regarding Granite or other types of rock.
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    Granite will give a good reflection at a angle but,looking straight on to the polished surface won't give the reflective qualities needed imho.I will say however,the harder and more dense the stone is the better the shine.The grit for polishing stone ranges from 40-5000.Their is also a grit that comes in powder form called tin oxide which is upwards of 8000-10,000 and is generally used as the final step in polishing.I do believe their are certain marbles that might be contenders for mirrors?!.
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    Has anyone found a reference to someone using this type of media? Even with the ultra fine grits, the natural imperfections and various inclusions within the rock would make difficult or impossible to reach the fine level needed for a mirror. It comes down to Geologic formation of the rock. It may well be possible to find a piece suitable, but the chances would be, well Astronomical. That just the way I perceive it IMHO.
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    Steve Yahn is just thinking outside the box and interested in using cheap available materials available to him. If stone due its properties is unsuitable is it possible onther non traditional materials could be employed? What could be used that might be lighter than glass, can be laminated or sprayed? Could we consider ceramics or plastics as new materials for mirrors? These mateials might end up being lighter and lend themselves to mass production.
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    Quartz if kind of the premier material from which to make our mirrors. But they make quartz glass from fused silica rather than from naturally occurring rocks that they cut up. So it's still a glass.

    Ceramics are not yet ready for use as mirror blanks. This may change in the future as a former member of our forum has obtained considerable expertise in ceramics and believes he can make that work. Others with considerable knowledge do not believe ceramics can be made suitable. We'll see how it sorts out.

    I don't think plastics can/will be made into suitably ground mirrors.

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    My Skywatcher Dob was made in China, and the mirror is glass.
    I believe if there was a cheaper material to make mirrors, the Chinese telescope industry would probably have found it by now.

    I'm no expert, but that's my thought on the subject.
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    Glass lens have always been the predominant material reasons, not to mention the ability to grind and buff it to a specific shape, but also the fact that the Silver Coating on the back gives incredible reflective properties. That's why the coating must be applies with as few flaws as possible. Making a flat surface, and then coating it, would only result in more work trying to make the surface as aberration free as possible. To me that would be a daunting task and a bit like trying to re-invent the wheel. I applaud any ones effort think out of the box and create something new. I love being shown I'm wrong. The question I ask now is, "Don't you think that others have thought of this also? The reason every one, the Chinese, Japanese, or any other Optic's maker of the world make their Optic's out of glass, because it is the medium that works the best. Optical glass is not made the same way as standard glass, and isn't cheap. There are various grades of purity, which again relates to cost and the better Quality of the Optic. Of course, a great piece of Optical glass can be made useless by poor grinding practices and a marginal piece of glass can be made great by a professional mirror grinder. Experiment, by all means, but do it intelligently. Explore the subject completely and then go forward with a real plan, not hope that something will work. You need to understand the basic theory behind the making of Optical Mirrors at this stage of their evolution before you can make something else work by chance. I'm behind you one hundred percent, not trying to say your crazy. You need to approach the problem intelligently and systematically to make it work. An old teacher once told me not to argue that something can work, but to PROVE it.
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    Default Re: Stone or Granite blanks for newtonian mirror

    If granite can be ground to such accurate surface plates
    Why cant it be ground to a telescope mirror???

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    Default Re: Stone or Granite blanks for newtonian mirror

    Hi and welcome to the forums X13.
    Please take note of the date on the threads that you post to, this one is from 2011. We discourage this practice, it is contained in our forum rules and terms of service(see link in my signature) under Necro posting.

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