Re: [ATM] Tom's back - Parabolizing Plate Glass 12.5" x 0.73" f/5.5

["Richard Ozer" <rozer@pacbell.net>]

Hi Mel,

What do you mean by "heavy towards the center and lighter towards the edge." 
I'm having a hard time picturing what you are describing.

RO

----- Original Message ----- 
From: "Mel Bartels" <mbartels@bbastrodesigns.com>
To: "'ATMList'" <atm@atmlist.net>
Sent: Friday, July 14, 2006 4:19 PM
Subject: Re: [ATM] Tom's back - Parabolizing Plate Glass 12.5" x 0.73" f/5.5


>> In response to your later message about a specially carved
>> parabolizing lap, all I can say is it can definitely work, but you'd
>> be off on your own working out your own procedures, which is not a bad
>> thing, it's just not easy.  I've never used this type of lap on a
>> parabola, but I have used "formatted" laps or petal laps on Cassegrain
>> secondaries.
>
>
> It's best if people learn the standard techniques first.
>
> I've used parabolizing laps of 4 types.  Let me discuss 3 types at first. 
> I
> shape the pitch based on Ronchigrams, of which there are three variations:
> inside of radius of curvature such that the pitch is heavy towards the
> center and lighter towards the edge, outside of radius of curvature such
> that the pitch looks like a mirror image of the former, and finally at the
> radius of curvature where the pitch looks like a mirror image of a donut.
>
> The outside of RC in particular yielded beautiful surfaces because the 
> edge
> is under heavy work.  However, all three are rather difficult to control 
> in
> arriving at 100% parabolization across all zones at the same time.
>
> Hence, my fourth type of parabolizing laps which I've used in my mirror
> making classes.  This is actually a partial parabolizing lap built in the
> inside of RC style mentioned above with the aim to help the student
> parabolize more quickly.  This calls for a bit of trimming of the pitch
> squares nearer the edge.
>
> Mel Bartels
>
>
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