Re: [APML] Film Creep

["Bobby Middleton" <bobm@koyote.com>]

Steve I've seen some of what you describe on my 6x9 BRC shots. It's pretty
inconsistant so I doubt if it's coma or edge abberations as Chuck mentions.
If it was I would expect that you'd get this on most (all) shots. I've had
two problems with my 6x9 BRC shots; some mis-collimation (we've spoken about
this) and some film movement or flatness issues.
I strongly suspect most of my abberations were film flatness (or creep)
issues. The reason I say this is because when I shot with a Pentax 67 I had
only minor colliomation issues. But when I started using the Toya 6x9 vacuum
camera (with paper-backed 120 film) I had much worse problems. 220 film was
better as well as 120 TP with paper removed. But the Pentax 67 had best
results with smallest stars. Of course it had a smaller field with
significant vignetting. I'd  suspect you are having some sort of film issue.
You are using film with paper removed I believe. You might try wrapping the
camera with a large plastic baggy of some sort; even put some dessicant
inside. Tape around the holes you make for purge lines, etc. Get it big
enough to leave slack to advance film and slide the shutter. This was
do-able with the Toya but somewhat of a pain. But I felt the extra humidity
protection was worth the trouble. It might not be so easy with your
Mitsubishi camera.
You once used a Pentax 67 right? Did you ever see this then?
Bobby Middleton

baloney detection:
http://www.koyote.com/users/bobm/Baloney%20Detection.htm
----- Original Message -----
From: "Steve Walters" <s-walters@comcast.net>
To: "List - APML" <Astro-Photo@seds.org>
Sent: Saturday, September 27, 2003 5:26 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
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>


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