Re: [APML] Film Creep

["Bert Katzung" <katzung1@comcast.net>]

I thought Estar was the base used for Techpan and not for Ektachrome or any
of Kodak's other films.
Bert

Bert Katzung
katzung1@comcast.net
www.astronomy-images.com

----- Original Message ----- 
From: "Steve Walters" <s-walters@comcast.net>
To: "List - APML" <Astro-Photo@seds.org>
Sent: Saturday, September 27, 2003 3:26 PM
Subject: [APML] Film Creep


> 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...
>
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo
>


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo