-- "C" == Chris1011 writes:

C ..... The level of accuracy required to make etalon plates makes them
C very expensive.

What about an ATM approach ? I know etalon filters are expensive, but
our club already owns a 0.7 Å H-alpha filter which is basically made
with an ERF (IR-cut, heat-proof filter + W25 filter + negative lens to
obtain a longer f/ratio and to extract focus) followed by a cascade of
etalons in number variable depending on the desired bandwidth: about
0.8-1 Å with two filters, 0.7 Å with three, 0.5 Å with five; if I
remember well, each filter has a 2 Å bandwidth. "Tuning" is achieved by
tilting each filter in its seat.

This image:

http://www.gest.unipd.it/~jake/astro...un20032510.jpg

shows what I was able to do with the above filter (not that it looks
very well in front of other H-alpha imagers like Paul Hyndman, Thierry
Legault and Andy Chatman...).

I was looking for sources of other etalon plates, but it looks like I'm
not quite "in the market" (i.e.: I was not able to find anyone) for this
kind of supplies. Could you (or other SAA subscribers) point me to some
etalon makers ?

Regards,
Luca Polo.
--
Luca.Polo @ gest.unipd.it
Associazione Astrofili del Basso Vicentino "Edmund Halley"
http://halley.astrofili.org -- http://digilander.iol.it/aabv
e-mail: