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help me decide on a build


crashtestdummy
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Im currently planning a dob build where i will build everything including the mirror and i cant decide on size.i currently have a 12" so it will have to be 16"+.i have never grided a mirror before but have always had a mechanical job so im pretty handingand buoyed up by the 360mm first time grind thread im going to give it a go.

So i cant decide on a mirror.should i start with a 16" in plate glass for cheapness if i dont get it right or is it not any harder just going for an 18",20" or 22"?im assuming i would struggle with plate glass above 16"?im giving myself plenty of time to finish with an aim of next April.doing a smaller mirror to practice is a bit dead money for me hence why straight into the 16"+.

Any decent build links also appreciated because every mirror grind site gives different meathods!!

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Personally I would go for the 16" in 1" plate. The larger the dia with 1" plate the trickier it is to get it right. It is amazing how much extra glass needs to be removed ( in coarse grinding AND parabolising ) just by going up to 18" unless you are contemplating a long focus mirror and using step ladders :smiley:. By the time you have made the structure surrounding the mirror ( cell, mirror box etc. ) a typical 16" dob is near the limit of going through normal doors if you need to do that. When working out the width for normal doors don't forget your knuckles will add to the width of the build if you choose the wheelbarrow position :grin:

Have you considered the mirror making class that Glasspusher has suggested?

As for the basic dob designs there are plenty here on SGL for you to copy/get ideas from.

Nigel

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My dob wont be strored in a house anyway so thats not a problem.if i do a 16" it will be an f4.2 max as i wont use steps and an 18" would be f3.9.im only 5'7" and steps in the dark doesnt inspire me with confidence.

I would definately consider a mirror making. class,its a great idea.if i went for 18"+ i would go for 40mm thick suprex/borofloat.

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A 16" f4.2 has a sagitta of 5.95mm. An 18" f3.9 is 7.2mm so you are correct in going for 18" 40mm thick glass as taking that much out of 1" glass leaves it very thin in the middle. With the permanent set-up, doors are not an issue either.

In view of that, go for the 18" x 40mm. If you feel that you don't want to finish the mirror you will be able to sell the profiled blank for pretty much the cost as there will be plenty of thickness left for anyone else to make a mirror of their choice. This would not be the case for a profiled 1" thick mirror as there might be insufficient glass left to make a mirror of different focal length.

Glasspusher posted a thread on here fairly recently about his suggestion of a mirror making meeting. I'll try and find it.

Nigel

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All the mirrors that I have done larger than 12" have been done with a sub-diameter tool on top ( for all grinding, polishing and figuring ). For mirrors between 16" and 20" I have used tools between 10" and 13" dia. If you go for the 40mm thick glass then an 18" dia block is very heavy to shove to and fro ( 18" dia x 1" thick is no lightweight either ). With a fast mirror, as you are contemplating, you might need a smaller tool for the final figuring stages, say 6"-8' dia. The problem with figuring fast mirrors is that you have to remove significantly different amounts of glass from adjacent areas of the mirror which becomes more difficult the larger the polishing tool. However, using a small tool can give rise to zones so they need to be used with care and mixing it with larger polishing tools to smooth out the surface.

Nigel

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