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Re: [APML] LMC & SMC
Hi Ray,
Of course I am also very interested in algorithmic approaches to the
vignetting/gradient correction problem, for obvious reasons :)
A reasonably accurate modelization of vignetting can be done quite easily:
just calculate the relative surface of the elliptic aperture of the
objective, as seen from the focal plane, at different radial distances from
the center of the FOV. This should give good results if vignetting is purely
"natural", assuming a flat focal plane. In presence of additional obstacles
blocking some light, however, this simple approach will fail. For example,
any aperture stop located after the objective (T-rings, poorly-dimensioned
secondary mirrors, etc.) should be taken into account. Vignetting models can
be complicated, but in theory they can be built if you are
algorithmically-inclined :). A more practical approach can be using some
flat-field images taken under well controlled conditions; Wei-Hao has a lot
of experience in this field.
Sky gradients are a completely different story. The problem is that, to my
knowledge, there is no plausible algorithm to automatically extract true
samples of the sky background from a real-world astrophoto, except perhaps
in the trivial cases (e.g., pure stellar fields). This is one of these cases
where the human brain/vision pair form the ideal system. Only a sophisticate
expert system (trainable, with lots of a priori knowledge) could be able to
emulate this to identify pixels pertaining to the sky, differentiating them
from surrounding nonstellar objects. For this reason, our approach is based
on a highly interactive system. We let the user define a set of samples over
the image; he/she must decide where to place them to build an accurate model
of the sky background for the whole image. Our implementation includes many
resources to assist the user in this task. They include the ability to
exclude small bright features (as stars) from background samples
automatically, the automatic generation and distribution of samples, and the
possibility to impose a variety of symmetrycal properties to the background
model, which helps building models even under very difficult situations
(e.g., when there are just a few free sky areas covering a small portion of
the image). A 2-D interpolating/smoothing spline and a 4-way tree structure
to distribute spline coefficients efficiently do the rest.
Hopefully we'll be able to show all that in the next release of PixInsight,
along with lots of interesting stuff. Before Summer, perhaps.
Regards,
Juan C.
----- Original Message -----
From: "Ray Butler" <ray.butler@nuigalway.ie>
To: "Wei-Hao Wang" <whwang@gmail.com>; "Discussion of Film Astrophotography"
<astro-photo@seds.org>
Sent: Saturday, March 05, 2005 3:43 AM
Subject: Re: [APML] LMC & SMC
>
> Hi Wei-Hao,
>
> Stunning widefield pics with the P67!
>
> Could you tell us whether/how this DBE deals with the combination of
> multiplicative (vignetting) and additive (sky gradient) effects, while
> steering clear of solutions which tend to locally harm the astronomical
> objects themselves? I'm very interested in the algorithmic approach taken.
>
> Ray "recently trying to do exactly that in IRAF, and not entirely happy
> with it yet" Butler
>
>>Hi,
>>
>>The function to remove sky gradient or vignetting pattern is called
>>"dynamic background extraction," which is part of PixInsight's standard
>>version. The current downloadable version of PixInsight
>>is the Limited Edition, which doesn't have that DBE function. The
>>standard version will be a commercial one while the LE version will
>>remain free forever. I don't know when the standard version will be
>>launched, perhaps after half year or even longer?
>>
>>Cheers,
>>
>>Wei-Hao
>>
>>
>
>
> --
> --
> Dr. Ray Butler
> Lecturer, Physics & Astronomy | Physics Department | Computational
> Astrophysics Laboratory
> National University of Ireland - Galway, University Road, Galway, Ireland
> | www.nuigalway.ie
> ray.butler-AT-nuigalway.ie | Tel: +353-91-524411 ext 3788 | FAX:
> +353-91-750584
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