source of 'white' float glass

[davidc135]

Hello all, my first post here.

My atm project is to replace and figure the Schmidt corrector plate in an 80s Bausch and Lomb 4000 spotting scope. Although ordinary 'green' float glass would do at a pinch I'd prefer the better transmission of 'optiwhite' glass made by Pilkington's or equivalent. Does anyone know of a source that would be willing to cut out a few small circles?

More specifically I'm looking for 6mm thick glass as I wouldn't be using a vacuum technique. Any help appreciated.

David

[steppenwolf]

You may have some success with H V Skan.  I have recently had a long chat with one of the technical consultants there and found them to be very knowledgeable and keen to assist.

 

[davidc135]

Many thanks Steve, I'll get in touch with them.

David

[Astrobits]

If you do try "ordinary" float glass you will need to check it for internal striations with crossed polarising sheets. Any strain in the sheet will show up as will any variation in composition due to incomplete mixing of the glass melt.

Glass for optical applications has to be mixed very carefully and the casting needs to be annealed to remove strain. Glass for house windows and tabletop applications does not need such careful attention.

Nigel

[davidc135]

Yes, it would be worth checking before embarking on fine grinding and figuring. My assumption that it's likely to be OK is based partly on making a 7'' F3 Schmidt camera where the 10mm float disc seemed excellent when included in an optical train, Foucault tested. The '80s B&L 4000 rubbish corrector was made out of 2mm window glass which suffers from gross surface errors. I might be wrong but I think all mass produced scts from that period were made from similar material and I'd bet still are. Although a good idea to do the extra check as you mention.

Advantages of float are that I'd expect it to be free from wedge, also sufficiently flat to begin fine grinding with 400 grit or finer, and cheap, of course. Disadvantage of green float is I think it absorbs 1% light per mm. As for price optical glass in this size shouldn't cost too much but I like the idea of popping down to the local glazier.

David

For anyone interested: Schmidt write up in Cloudy Nights articles June '14

 

[Peter Drew]

Nothing wrong with green window float glass provided as Nigel says, it is free of striations, early Celestron C8's were certainly this material. I think they probably moved on to white glass when Meade entered the market with their 2080 8" as this would be a competing sales issue. I've used a few B&L 8" SCT's and have to agree that they were not particularly good, I never had one apart and I'm surprised that the corrector thickness was so small. In view of price and availability I would have thought it a plan to use a suitable green glass disc for the first attempt, the light loss would be marginal. 

[davidc135]

I'll probably use the ordinary green stuff. As you say, I'll only lose a little light; maybe 6%. I expect the 8'' scts had thicker plates.

The little B&L 4000 is nicely engineered mechanically and both primary and secondary mirrors I think are good. B&L and before them Criterion let themselves down with their correctors which are an amateurish botch, odd when care and attention to detail seem to have been put into everything else.

My example seems to have an error of 3/4 wave or so, maybe not too bad for low power spotting.

David

[davidc135]

Post script on corrector plate glass: I bought a number of discs of green float from my local branch of Roman glass, as it was available. 109mm by 6mm. It was scribed and cut by hand and could do with working to an exact circle. Some Dremel size diamond discs should manage that. Also a square of polarising film which, in combination with a monitor screen showed the glass to be fine. I bet all this Pilkington's glass, green and white, would be OK, up to 10mm at any rate.

Another time I would order the white stuff, the poor man's optical glass. They have a minimum order size of .25 square metre so 50 cm on a side and it's pretty cheap too.

Now I just have to buy the bits for a polishing machine and I'll be ready to make a start.

David

[Merlin]

Good luck with your project David. It's good to know that in an age of so much off-the-shelf stuff, there are still people with the patience to grind their own glass, etc.

[Peter Drew]

When grinding the edge of a thin disc with a Dremel make sure that an edge chamfer is maintained as it is easy to chip a flake of glass from the face.   

[davidc135]

Thanks for your best wishes, Merlin and for your point, Peter. The ultra cheap set of mini diamond tools were a waste  of time. Maybe not surprising as they were practically free. I could buy a decent bit of kit or just leave it, at least for now. There is only a .5mm variation in diameter as it is.

I used a spinning method in the successful making of a 7'' corrector plate 25 odd years ago. The degree of correction will be very similar in this 4'' disc (about 20 fringes) but of course the diameter is much less so I can't assume success. I'll be able to start in a couple of weeks.

David