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Re: [APML] Film Creep
Hi Bobby,
As you say, I did use a Pentax 6x7 for a time and it did not show these
problems. I had a purge on it and I only used it for one winter when there
was less humidity. I'll try the suggestion about the bag, it's a good one
and I can do it I think. There is a shutter plate as well as the dark slide
that have to be accomodated but I think I can enclose both and still
manipulate them thru the bag. Have to try that before going to the field but
it's worth a shot.
I've also had some problems with the vacuum pump for a while but these are
solved I think. I put a vac gauge in-line with the vac pump and I could see
it drop from time to time. This usually let the film lift off a bit and one
region or another would be defocused. So I replaced the pump and watched the
gauge like an eagle for a while and this problem is gone now.
I'm pretty sure I'm down to this one problem now. The results are still very
good but after going this far down the 6x9 format folly, I may as well get
every nanoacre of it right.
Needless to say, I've put TechPan on hold until this is right.
Thanks for the suggestion!
Steve...
----- Original Message -----
From: "Bobby Middleton" <bobm@koyote.com>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Saturday, September 27, 2003 8:22 PM
Subject: Re: [APML] Film Creep
> 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
> > http://seds.org/mailman/listinfo/astro-photo
> >
>
>
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