Re: [APML] Pleiades response to Processing Challenge: LMC

["Bert Katzung" <katzung1@attbi.com>]

Hi Juan:
Thanks for the detailed description. No wonder I'm not coming close to what
you routinely achieve---it does involve some pretty advanced procedures! It
would be great if you could  implement some of them in SGBNR or another
program. I have a 6x7 camera and love the wide FOV but I hate the vignetting
that I get with my 130 mm f6 / f4.5 refractor.
Bert

Bert Katzung
katzung1@attbi.com
www.astronomy-images.com

----- Original Message -----
From: "Juan Conejero" <skycad@ctv.es>
To: <astro-photo@seds.org>
Sent: Wednesday, May 22, 2002 4:03 PM
Subject: Re: [APML] Pleiades response to Processing Challenge: LMC


> Bert,
>
> Thank you. It's always a pleasure when one works with such a good raw
material.
>
> We use two methods for vignetting correction. Both methods are based on a
> quite simple principle: we try to isolate all the background pixels of the
> image to obtain a vignetting correction mask. Background pixels are those
> corresponding exclusively to the sky background. An ideal, vignetting-free
> image would have a perfectly uniform distribution (exception made of local
> variations due to random distribution) of background pixels over the whole
> image. Vignetting correction is difficult (and can be terribly difficult)
> because stellar and nonstellar objects hunt us background pixels. The
> correction techniques try to figure out what the sky background should
look
> like in the absence of objects.
>
> Our first method, which is also our preferred one, is what we call "the
> object suppression method". A fundamental property of background
> pixels is that they are locally (i.e., at small scale) randomly
> distributed. If this is true, then we can safely copy neighbor, nearly
> adjacent background pixels over stellar and nonstellar images without
> altering the background pixel distribution's properties, including both
its
> small-scale (random) as well as global (due to vignetting) properties. In
> Photoshop, we basically create a selection comprising all the stellar
> images (through Select>Color_Range channel by channel) and then use the
> clone stamp tool to copy adjacent background pixels over stars. Then we
use
> the vignetting simmetric properties to suppress nonstellar objects, again
> by copying adjacent background pixels. We use a special circular
concentric
> grid to help doing all that. This method is extremely accurate, but it
> requires a considerable amount of work and can be very difficult to
> implement when nonstellar objects are covering large areas of the image.
>
> The second method is similar to Jerry Lodriguss' vignetting correction
> method. We take a number of samples, for background pixels, to reconstruct
> the vignetting profile over each diagonal of the image. Then we set up
four
> gradients (one for each diagonal), usually with 10 to 20 sampled values,
> and apply them. This method is simpler (also more elegant from an
> algorithmic point of view) but less acurate than object suppression. We
use
> it when object suppression is causing us a headache.
>
> We have in mind, since some time ago, the possibility of automating these
> procedures (or at least most of them) with a vignetting correction
> software. Time permitting...
>
> For the Matt's LMC image, we have used object suppression for the entire
> image except its central area, which is too covered by the LMC. For these
> central regions we have used a gradient, and for the very central area we
> have had to guess a constant value, since no background pixels are
> available. As we have used both methods we talk of an "hybrid" technique.
>
> For chromatic balance, we make a number of careful selections of
background
> pixels with the lasso tool, then save this selection, and create a curves
> layer. With the Image>Histogram command, having the selection loaded, we
> can see the median values for each color channel. We adjust curves until
> the three median (*not* average!) values for the R, G and B channels are
> identical. This way we achieve a neutral sky background. This doesn't
> guarantee prefect color balance over the whole image, indeed, but is a
> necessary starting point for vignetting correction and image integration
> techniques.
>
> Hope this helps. Regards,
> ______________________________________
> Juan Conejero, Pleiades Astrophoto Lab
> skycad@ctv.es
> http://www.pleiades-astrophoto.com/en.html
>
> At 10:04 22/05/2002 -0700, you wrote:
> >A very impressive job of processing, Juan! Can you give us  more of a
> >tutorial on the first set of Photoshop techniques (sample background
zones,
> >hybrid object suppression, etc)? Those are new terms for me. And
> >congratulations to Matt for a fantastic raw image!
> >Bert
> >




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