Re: [ATM] schmidt Correctors

["Bob May" <rmay@nethere.com>]

Both sides will be the same for readings on a dial indicator of
high accuracy.  Best to measure the surface with the edges of the
glass as the circle diameter as anything smaller will mean
smaller accuracy.
At F2, you really want to use some thin glass so be aware.

As to the grinding, just do the 5 micron grit and when the whole
surface is ground, that is the time to stop!  You won't need to
go to anything larger as the amount of glass removed by the whole
process is very small.  ON slower systems, you can almost just
get away with just polishing the surfacces to a good sphere.
I wouldn't use even  12 micron unless the surface started out
rough to begin with!
For the dial indicator, get a 0.0001" reading or equivalent
metric dial indicator for the job.  YOu can even get the short
travel one as the change between flat and curved isn't that big.
The big thing here is to get the glass and the vacuum pan and
just do it.  You'll find that the sag. will correspond to the
vacuum that you appply to the pan qu9ite nicelyl.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Steve Renden <stevey0303@yahoo.com>
To: Dominic-Luc Webb <dlwebb@canit.se>; ATM Superheros
<atm@atmlist.net>
Sent: Wednesday, March 12, 2008 10:46 AM
Subject: Re: [ATM] schmidt Correctors


> Hi Dominic,
>
> do you have thoughts on my questions? sorry to be so
persistant. but i have scoured the internet for how to grind, and
when to stop polishing, i need this to finish my scope!!! :)
> my questions for anyone are
> how do i measure the depth? by dial indicator? and if so, what
should the readings be for two sides?
> when do i stop? what should the readings be? for both sides. I
already have the pan, have the glass, have it all made but this
last step. i dont know what to do!
>
> Thanks!
> Steve
>
> ----- Original Message ----
> From: Dominic-Luc Webb <dlwebb@canit.se>
> To: ATM Superheros <atm@atmlist.net>
> Sent: Wednesday, March 12, 2008 2:21:57 PM
> Subject: Re: [ATM] schmidt Correctors
>
> On Wed, 12 Mar 2008, Steve Renden wrote:
>
> > Hi Thank you for the site!
> >
> > I will make an 8" F/2 schmidt newtonian, don't want to rush
to quickly into making a cassegrain, since im really not too sure
how to test those things yet.
> >
> > Thanks!
> > Steve
>
> Well, an F/2 is a pretty fast wide-field system. The Cass is
usually
> for getting high magnification. Two different animals, but I
think
> you made a great choice with a fast system. It seems like all
the
> other people always have high magnification systems. Not sure
what
> is possible in terms of eyepieces. I know Meade makes Schmidt
Newts,
> but I am pretty sure these are generally around F/4. You might
want
> to think on this point.
>
> Dominic
>
>
>
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