Re: [ATM] Schmidt corrector tools

[George Anderson <tillerman1@videotron.ca>]

Hi Dominic-Luc

Good point! I'll dig through and find my calculations (I did them two
years ago, this is a long project) and I suspect that I will probably
have to start at 600 or even 1200.
I haven't yet calculated the f ratio for the system, I still have to
finalize the design of the film holders. The mirror is a full thickness
6" f/2 from Surplus Shed that cost less than a 6" blank.
Thomas Janstrom came up with excellent numbers when he measured a
similar mirror that he bought from them.
I will mainly shoot hypered B&W film (Tech Pan and Agfa ST9 sound
recording film) with an H-Alpha filter due to local light pollution. The
few times that I get to a dark site I'll probably shoot E-200, unless we
get a nice comet and I'll try shooting that locally.
Do you have a mailing address for while you are at Berkley? I still have
some metal "samples" (I'll have to find them again) for one of your
other experiments. Contact me off list
George Anderson
Montreal Canada

Dominic-Luc Webb wrote:
> 
> On Tue, 24 Jan 2006, George Anderson wrote:
> 
> > I am about to finally start on my schmidt corrector for the 6" f/2
> > camera that I am building.
> 
> Exciting project! System or mirror is F/2?
> 
> > The glass I have for the tools is 1/4" plate, is this too flexible?
> > Would bonding two pieces together with either pitch or "crazy glue" to
> > for 1/2" thick pieces make sense?
> 
> As a rule I do not bond two pieces for this. There is plenty of thicker
> glass around if you need it, but I suspect you do not. If you calculate,
> and maybe you already have, the corrector curve is not very deep. For
> mine, I could almost get it with polishing (primary is 310 mm F/1.55,
> corrector is 6 mm thick). The Schmidt curve I settled on is as follows:
> 
> http://www.canit.se/~dlwebb/catadioptric/310schmidt/schcorr.jpg).
> 
> > thinking of 220 grit for "hogging" (
> 
> This seems extreme. The corrector is not very thick and you are at
> least a little bit dependent on the thickness to arrive at your
> final configuration. I would check this glass to see that it is
> reasonably flat on the two sides and decide if there is any reason
> to hog out so much glass. I do not remember exactly how I decided
> on this, but I am quite sure I ended up using 600 SiC (not larger
> than 400) grit from the start. As you can see from the link above,
> I remove 80 microns at the deepest point. This you can identify even
> with a bad sherometer. If you already have glass that is reasonably
> flat and does not contain deep pits and scratches, etc (probably not
> if it comes from a scanner) I recommend you try considerably finer
> grit than 220.
> 
> Dominic-Luc Webb
> 
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