[ATM] Focal Ratio for a Fast 24"
gfbrandenburg at yahoo.com
Thu Mar 17 02:19:41 JST 2011
To answer the question, I would suggest graphing the curve of the sphere and of the parabola on the same axes --and it's ok to exaggerate the vertical scale by a factor of 10 or so while keeping the horizontal scale at 1:1 (ie, 1 inch represents one inch).
If you can actually see any daylight whatsoever between the two graphs on anything smaller and slower than a six-foot f/3 mirror, I will be amazed.
Sent from my iPhone
On Mar 14, 2011, at 9:56 PM, "Mel Bartels" <mbartels at bbastrodesigns.com> wrote:
> At what point would you consider working parabolization into the mirror
> during fine grinding instead of waiting for polishing? Do you have any
> ways for a crude test of a mirror figure before it is polished out? There
> had been talk of infrared, but perhaps there are other ways...
> Why would you want to work parabolization into fine grinding? Because it
> naively reduces amount of glass to remove during parabolization? But
> pre-parabolizing adds time and is unnecessary. Further, time to figure is
> best reduced by small certain steps, even if they add up to more overall
> glass removed than the theoretical minimum. It seems to me that
> pre-parabolizing is a major gamble for an amateur mirror maker, not in
> keeping with lowering risk and getting done expeditiously.
> Mel Bartels
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