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# Found Mirror At Yardsale

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I just got a mirror along with a few other telescope bits from a local guy that was cleaning out his attic. The mirror is 200mm in diameter, and has the figures R of C = 85.9" and S = .044 written on the back of it, and I'm wondering what those numbers mean, I was thinking that if it is a spherical mirror R of C could mean radius of circle, but I'm nearly certain I'm wrong.

If any one could give me some more info that would be great, if the mirror's any good it could be a fun project to build a scope around it. Thanks

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R of C (i would have thought) = radius of curvature = 2x the focal length

S (i would have thought) = sagitta (the depth of the centre of the curve), but that would make a mirror with twice the focal length (!?).

http://www.atmsite.o...rewitt/sagitta/

Edited by Demonperformer

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DP is right the figures don't add up.

RoC is (should) be the Radius of Curvature, and is 2x the focal length.

If S = Sagitta then 0.044 means one of the values is incorrect..

If the Sagitta is 0.044 and assuming 8" dia (4" radius) you get from the Sagitta and mirror radius an RoC of 181"

This is close to 2x the implied value, which makes me wonder if the original person determined the focal length and mixed up what they wrote as a focal length of about 90" gives an f/11 mirror

I therefore wonder (suspect) that RoC of 85.9 is actually a focal length of 85.9

Edited by ronin

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I did a quick and crude measurement of the sagitta with some feeler gauges I use for valve adjustments, and it seems that the .044" measurement is accurate. Which leads me to believe that both of you are correct in thinking the figure labelled RoC is actually the focal length, which as stated would mean this is an f/11 mirror. I guess that answers one question for me, now I just need to figure out what to do with the thing, a 89.5" focal length would make for a simply huge telescope.

Is there an advantage to an f/11 8" mirror, it just seems like the resulting scope would be ungainly?

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That's one long scope! I'd need a step!

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Would be awesome for planetary tho!

Sent from my HTC One X using Tapatalk 2

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if it is in good condition why not make an off axis folded telescope

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If as you suspect the mirror is spherical it will be that long to reduce the aberations caused and so produce a reasonable image.

Sounds like it will be an interesting project whatever you do with it.

I wonder if one the ultra-transportable designs would be worthwhile, the mirror sits very low down in those and you could place the secondary a bit lower down and adjust the focuser position out a bit to accomodate it all. Still going to be questionable to use in the vertical however how often do you need to do that ?

Have fun.

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is ROC not Republic of China? everything seems to be made there! it does sound like a great planetary scope.

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is ROC not Republic of China? everything seems to be made there! it does sound like a great planetary scope.

I think China may be a little bit bigger than 89.5" ...

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Could the mirror be from one of those Newts that uses a focal reducer system near the focuser to get an optically shorter scope out of a long focal ratio spherical mirror? That way they reduce the aberations in the relatively easy to make mirror and still have a manageable sized scope.

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Thanks for all the info, I'm going to bring it to work today (mechanic) and see if I can measure the sagitta more accurately and get better idea of what this mirror is. Whatever the f/# I think it will make an excellent project for the summer. Thanks.

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A bit late on this, but you could simply have focussed the sun onto a wall, and measured the distance from the mirror surface to the focussed image on to a wall, or a sheet of white card between your mirror and the sun.

Since the sun is infinitely far away, the resultant focal length would have be quite accurate.

Ron.

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