"Glenn Mulno" wrote in message
...
I'm just curious what in the process of making a hydrogen alpha filter -
makes them so expensive? Why would a plain solar glass filter be about
$100 - but the same size piece of glass for hydrogen alpha is a few
thousand
bucks?

Or is this really more of a supply and demand type of thing? Is it just
that so few companies make these that they can get away with charging a
fortune for them? I could see charging more for them but by a factor of
20 - 40?

Just curious. And jealous of those who can afford one of these. :-)

Glenn
It is because of just how narrow the notch has to be. In 'audio' terms, this
would be a filter, that would not just seperate two 'notes', but even a tiny
tuning error on an instrument, probably inaudible to the ear, would take the
note outside the passband. It is probably one of the narrowest band filters
created for any application. To get the band this narrow requires the optics
to be made to the same sort of accuracies.
There are two basic 'types' of filter involved. The first is a simple 'dyed
glass' filter. These are the cheapest filters to make, and are the basis of
most simple 'colour' filters. The second is an interference filter. This is
much more expensive, (with the cost rising with accuracy). The most
expensive and accurate form of interference filter, is the Fabry-Perot
Etalon, which is the system used in the Ha filter assemblies. Unfortunately
to make this work at the right frequency, requires two optical surfaces, to
be ground with a spacing accurate to a tiny fraction of the wavelength of
light, and flat to the same sort of levels. It represents probably the most
accurate piece of optical equipment in 'mass production'. The normal solar
'filter', is simply a partially reflective surface (rather than being a
'bandpass' filter at all), but with the base material selected to also block
UV & IR to some extent.
Small Ha filters, are not that silly in price, and can give truly remarkable
solar images. Though the small size will limit the ultimate detail, an
enormous amount can be seen through these devices.

Best Wishes