From: Greg Harp (gregharp_at_antispamdomain.not)
Date: Tue Nov 25 2003 - 23:49:17 EST
--- In digital_astro_.at._yahoogroups.com, "Dennis Persyk" <dpersyk_.at._a...>
wrote:
> That's an very nice image of the veil.
Thanks... I guess I am comparing what I got to some of the real
experts here... and mine doesn't compare favorably. :)
Certainly it's my best image of the Veil, being my first. :)
> Your problem lies not so much in processing but more in
acquisition.
> At f/2.8 the lens has a bit of chromatic aberration, enough to
make
> the stars fat. If you stop down to f/5.6 you will have tighter
> stars. The factor-of-four loss in sensitivity hurts, but sometimes
> that is the only way out. The veil is pretty dim against the rich
> star field. That's why many image the luminance in H-alpha to dim
> down the bright stars.
Hmmm... Regarding the CA, I should have known this one. I'm still
learning to apply my terrestrial photography knowledge to
astronomy. Thanks for pointing it out to me. I suppose I could
aperture-mask the lens to help with this without causing the halos
around the brighter stars to assume the shape of the aperture
blades. I already have step-down rings to allow me to put a 48mm
filter in front of this lens, although I didn't use them this time
around.
I also like the idea of taking this RGB data and adding a luminance
channel based upon H-alpha. I may have to try that.
> Digital Development tends to increase contrast of stars, so you
might
> try just levels and curves. I like DD for instant, one-stop-
> processing, but sometimes it is not the best operator.
I'm with you on the levels and curves. As mentioned, I feel that
there is a lot more I could do with this image, and there probably
is, but I'm kind of hoping for some wisdom from the group to help me
avoid too many dead ends.
Is there somewhere that I can read up on exactly how DD operates on
the image? I think I could use it better if I knew the algorithm
behind it. I don't like the "move the slider -- ok, does that look
better or worse?" approach (actually, I usually type in the numbers,
but I digress). I dug through the help in ImagesPlus, but that's no
help. :)
--Greg
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