RE: [APML] Matt, Jeff, Chris, Jerry and stuff like dat dere

["Chris Schur" <cschur@cybertrails.com>]

Warren,

When you combine images digitally from a film source, it is not the same as
just adding two CCD images.  Because film has a different shape in its
dynamic range, you "stack" the image pairs differently.  The main concern is
saturation of the image to pure white.  If I stack two slides that have m42
burned to the core in each, the result will be twice as burned, and no
detail will be gained in that area.  Negatives therefore are scanned at a
low contrast, or even as a raw, unadjusted 16 bit image.  You then take both
low contrast positives and adust each such that the skyfog is removed, by
moving the lower histogram slider up till the background is not quite black.
To stack, convert each to a negative again and multiply.  You dont want to
sum, since it is too intense of an operation.  Of course the images have to
be registered beforehand with some program such as PW pro or registar.  The
other option is to simpy average the positives together.  Although this will
not boost faint neblosity, it will cut grain by the square root of two, and
for bright objects, will avoid saturation of one of the RGB channels.  A
good guideline is for bright objects, like M51 or M42, stack by averaging.
For extremly dim objects such as faint nebula, stack by multiplying
negatives.

I hope this helps.

Chris

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Warren A. Keller
Sent: Saturday, December 07, 2002 1:37 PM
To: apml
Subject: [APML] Matt, Jeff, Chris, Jerry and stuff like dat dere


-Matt thanks for the reply. Agreed on manual curves, I'm still not where you
are though. I'd always placed pivot points on the curve, but never
manipulated the corner anchors before studying the M31 curves file you'd
sent me. I'd like to ask you, and you can reply off-list unless you think
it's omni-beneficial- The Auriga raw scan is very 'fogged' and I use the
term loosely. Is it actual skyfog, or a result of the scan? The histogram
looks much different than my scans from the humble, HP PS, which doesn't
really offer adjustment. My histos look more typically stretched like what
you and Chuck Vaughn illustrate on your sites, while Auriga's data is in
that very tight midrange notch.

-Jeff, et al- How many of us think that even the best color emulsions are
recording significantly after 90-120 min? Also, when I tried to slap my
color HH/Flame on your FANTASTIC ccd image, the color diminished to very
pale despite changing blend methods, opacity and percentages. What's an
appropriate way to blend an L w/ RGB in PS and PW?

-If the slide film is working so well, why are we still hypering, storing
and purging print? Slide does have the disadvantage of being sent out from a
lot of labs, trusting the cutting and mounting to someone if you don't have
a mounter (My scanner requires this or it will read as negative), is a more
expensive film, but I am making the switch.

-Chris Schur, why should we convert to negative when stacking? PW too? Why
don't you let PW stack them after registration? Could you explain a bit?

-Jerry, Thanks for the PS info! This dope has been Canceling out of modes,
not knowing about Alt/Reset. Good tip on the 'Auto' adjusts too.

-Agree with Gene, let's please stop the HTML, it only takes one second to
change. Also, it only takes a second to snip out what's needed on the
message you're replying to, and would make the posts much shorter and
readable.

-Lydia? Lydia? There are no women allowed on this list! Just kidding, bring
'em on!  ; > )

Warren A. Keller
Billions and Billions- Astrophotos
www.billionsandbillions.com


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