Re:ATM 8" achromat (another update)

[Roger Ceragioli <rogerc@dakotacom.net>]

Hi, Dan.

My thicknesses are about .5"-.75" each for the elements.  Nowadays, the
commonly used crowns and flints are so transparent and uncolored, that even
elements which are several inches thick absorb minimal light.

In the old days, this wasn't so, and the old flint glasses were strongly
yellow in color.  It's surprising to see specimens of them.  But not now.
F-2 looks exactly like BK-7 and the transmission rates across the visual
spectrum for 1" thick pieces of both glasses are, according to Schott's
catalogue, over 99%.

I would imagine that the throughput of even a 40" objective nowadays would
equal a reflective system with the best coatings.

Take care,
Roger

At 12:17 AM 12/11/99 -0600, you wrote:
>Hi Roger,
>
>what is the thickness of the blanks you started with?
>It used to be, or so I've read, that glass bigger than 
>200mm got so thick that mirrors started to equal, if 
>not surpass the overall through-put of light for a given
>aperture. I imagine some of today's  glasses that you
>could blow your savings on are transmitting more 
>light/thickness than the traditional  stuff. Any comments?
>
>D Chaffee.
>
>