Re: [APML] Film Creep

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

Hi Bert,

That's right, I don't know. I've sent a note to Kodak asking about this as
well. The E200 spec sez it has an acetate base so maybe it's not Estar. but
I think Estar is just a trademark (rhymes with "Eastern Kodak") so perhaps
Estar is an acetate. I'm not sure. We'll see what Kodak sez on this.

The Tech Pub I mention is the only one I could find that gives any form of
information on deformation due to humidity.

Steve...

----- Original Message ----- 
From: "Bert Katzung" <katzung1@comcast.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Saturday, September 27, 2003 8:48 PM
Subject: Re: [APML] Film Creep


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