I am not sure, (exposing my ignorance here), if Australia uses the metric or the the other system of measurement. So I will convert my notes to metric and if I am wrong someone let me know. I assue the US is the only hold out these days.
I am working on slumped ultrathin mirrors. How thin can we go?
The first project was a 406 mm f 3.5 that is 8 mm thick. That is the mirror in the photos from OSP. I have now begun a thinner project. It was a 457 mm disk but errors in the slump caused me to reduce it to a 394 mm disk. I try and post to the atm list but as there was interest here I thought I might post here also, if that is OK with every one. I have included my first two postings to ATM. There is some history on the 406 mm at ATM also.
www.atmlist.net
October 15, 2005
Something there is that loves a fool and a mirror grinder.
I had slumped a piece of 3/16ths, (5mm) glass over a bad form. What I got was an outer ring with a nasty case of ripply droop. I ground on it a bit and realized I was going to grind through before I got the ripple and the droop out. So, having never tried to cut glass before, and having this piece of now worthless glass, I decided to cut the outer 1.25 inches, (32mm) off.I will post pictures later, but I made a beam compass out of a scrap piece of fir stock. I did this by nailing a wheel cutter to the end of the scrap and then running a screw through it at the turn point. The screw acted as my point for the compass. I put a couple of layers of masking tape near the middle of the glass and then using a ruler I marked the middle of the circle on the masking tape. I etched/scribed a circle on the back of the curve and then using the ball on the end of the cutter I tapped from the inside of the curve following the etch line through the glass. It worked! I cannot believe I got away with that.
So now it is a 15.5 inch instead of an 18 inch.
The specs on this one:
15.5 inch diameter, 3/16th inches thick
3/16 inch of sagitta
80 inch +/- focal length based on r squared / sagitta
f5 more or less
394 mm diameter, 5 mm thick
5 mm sagitta
2032 mm +/- focal length based on r squared / sagitta
f5 more or less
Should be a fun project.
October 17, 2005
I finally got the camera out and took a shot of the project. This is the mirror with the pieces I cut off in front of it and the cutter I cobbled together to do it. In the middle of the mirror blank is an odd shaped white blob. That is the masking tape on the back that provided a place to hold the point of the cutter. On the lower left is a series of grind marks. Those were caused by trying to grind the ripple and droop, off of the edge of the original blank, using a bottle bottom. It didn't work.
If you look closely at the pieces I cut off you will see the ripple pattern that formed. I had not fully understood the softness of the glass as it slumped and so did not dress the edge of the form well enough. I have not made that mistake again.
David Davis
Toledo, Oregon
http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/15%20inch%20ul tra%20thin/15.5%20mirror.jpg