[APML] Film Creep

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

I have a question about film creep.

I'm shooting medium format (6x9) film and am certain that I have this
problem. Stars in the corners are trailed and pointing towards the center of
the frame. The center of the frame is perfect. Stars along all four sides
are slightly trailed, also pointing towards the center. This could not be
field rotation, there are no arcs and also I know exactly where the
autoguider is sitting. On some frames, the stars in one corner or another
are perfect. This means the optics are not at fault (or you'd never see
perfectly round stars). We are talking about small trails, not big huge
ones. Because all the trails are pointing towards the center, it's not
guiding or tracking errors. I am already purging at high flow rates but
operate in very very high humidity, sometimes near 95%.  Being a dummy, I
just recently noticed that the camera is sopping wet, at least on the
outside, from dew condensation. I plan to stop that next outing with a dew
heater. Of course it's possible that the purge is keeping the inside dry but
I suspect water is literally running down the insides of the camera. Dumb
dumb dumb....Lastly, this is unhypered film, E200 in 220 format, exposed for
1 hour and there is a vacuum system holding the film flat. I get this result
very very consistently. Sadly I have dozens of images exhibiting this
behavior.

Anyway, here is my question.

Kodak tech pub TI-2598 gives the coefficient of expansion due to humidity of
ESTAR polyester base as 0.0008% / %RH. Since this is a positive number, it
seems that the film expands slightly due to humdity. That makes sense a bit,
just like a log soaking up water will expand. But I've always heard (I
think) people on this list say the film shrinks in high humidity. So which
is it? I'm not sure if ESTAR is the based used in E200 but

Now for a little arithmetic. My film is 6x9 cm so its diagonal is 108.2 mm.
Taking 0.0008% of that gives 0.87 um / %RH. I've sent a note to Kodak asking
them what %RH the film has when it is removed from the sealed bag but for
our quick calculation, let's say it starts at 60% and moves up to 90% RH
during a 1 hr exposure. That would mean a 26 um expansion across the
diagonal. This would be evenly divided about the center, I think, so it
would yield 13 um of trailing at the far corners. That's about what I'm
seeing. My smallest stars are about 20 um in diameter and they show anywhere
from 10 to 20 um trails.

Am I figuring this out right? Any comments? Am I missing something?

Any suggestions on solving this problem would be very welcome.

Steve...

Retirement: The job I was born for...

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