Re: [Leica] Hypersensitized Film

["Dan Post" <dwpost@email.msn.com>]

DC or Dan'-
The chilling of the film in astronophotography was to diminish reciprocity
failure which denotes the breakdown of the rule that doubling the time of
exposure doubles the exposure. at extremely long exposures, the photons are
so few and far between that for every two photons that hits a silver atom
and dislodges an electron, thus exposing it, there is a silver atom where
the displace electron has gone back to the lower energy state, and that
silver atom is no longer exposed- a law of diminishing returns, if you will.
Chilling the film keeps the atom from re-grabbing the displace electron so
there is enough total exposure to make an image- some of those plates are
exposed for hours in holders chilled with either dry ice or liquid nitrogen
at the prime focus of large mirrors- Personally, I'd hate to have to hold
'the birdie' that long! I always was fascinated with astrophotography, but
never had the patience!
Dan"
dwpost@msn.com