[APML] Stability of Acetate Film

["Steve Walters" <s-walters@comcast.net>]

I got a response from Kodak about acetate film stability for temp and
humidity. Here is their response:

===================

Here is the information on the dimensional stability for acetate films:

Acetate Dimensional Stability

Thermal Coefficient = 0.003% change in size per degree F
Humidity Coefficient = 0.005%-0.010% change in size per %RH

The film is packaged in a %RH of around 40 to 50%.

===================

Since my 6x9 film has a 108 mm diagonal, these numbers translate into:

3.24 microns per deg F
5.40 microns per %RH (using their "best case" value)

This is worse than I thought. I am aiming for a 6 micron worst case trail as
this works out to be 1 arcsecond in my system and the smallest stars I can
find are about 20 microns. That means I need very tight control on temp and
RH. Also notice that out of the bag, the film will be at 40-50% RH. I
suspect my nitrogen purge is drier than that.

I called the Kodak expert who responded and he suggested pre-conditioning
the film to the environment prior to using it. Just break the seal and let
it "soak". I may try this. I could place the film in my vac chamber, pump it
down and back fill with nitrogen. Then let it sit for a few days. Or maybe
forget the nitrogen and let it soak up that good old Pennsylvania humidity
for an hour before shooting.

The temp is no problem to control except that level of accuracy may be
difficult. I've already modified my dew controller so I can switch it
between "Relative" and "Absolute" modes. Relative mode has it tracking the
air temp at an offset I can control while Absolute is, well, absolute.

Steve...

Retirement: The job I was born for...

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