Re: [APML] Flat Fields for vignetting

["Jim Ives" <flyboy9990@earthlink.net>]

Matt,
    I found after using Sean's tenique I have a problem bringing the colors
back up in the nebula without it looking fake and over processed.  It does
remove weird shaped vignetts though. What would you do as a next step after
doing the vignett removal?  I know it's "operator erro" with me. I'm doning
something wrong but dont' know what.
    Should the screen method be used instead of subtract with Sean's method?
Jim
----- Original Message -----
From: "Matt BenDaniel" <matt@starmatt.com>
To: <astro-photo@seds.org>
Sent: Friday, November 15, 2002 1:12 PM
Subject: Re: [APML] Flat Fields for vignetting


> Frank,
>
> Steve's advice is good re: my friend Sean Walker's article in S&T Sep
2001. Sean originally taught me that technique in 2000. I have spent
hundreds of hours refining the technique, and my conclusions are as follows:
>
> 1. The screen combine function in Photoshop is the optimal way to
compensate for light falloff.
>
> 2. Photoshop layers are your friend.
>
> 3. For the best AV, each image needs its own custom hand-tweaked AV layer.
You may be able to save some time by using Sean's method or by re-using an
AV layer from another image, but, if you want to show the faint features
across an entire vignetted frame, you must tweak.
>
> My 5-inch refractor vignettes about one stop from center to corner on 6x7,
and my 4-inch refractor vignettes about half a stop. This is significant
vignetting (although some setups are even worse). Most of the images on my
web site are 100% full-frame. They have to be AV-ed in order to display
well. The article on AV on my site is a rather poor explanation of what is,
I'm afraid, a technique that is difficult to learn. However, the proof is in
the pudding, as they say.
>
> I believe the reasons we can't develop a generic flat field for film are
film's nonlinear response and reciprocity failure.
>
> Matt
>
> At 11:38 AM 11/15/2002 -0500, Frank Barrett wrote:
> >Hello list.  The last couple of rolls of film I've shot using the Meade
f/6.3 focal reducer.  It does a great job, but with my el cheapo OAG I
consistently get vignetting on longer exposures.  I've managed in photoshop
to remove this via Matt BenDaniels AV layer technique (thanks Matt) and via
cropping, but I was wondering if it were possible to apply flat field
techniques like the CCD guys do.  Anyone doing this successfully or is this
just another one of my trojan horse ideas?  What are the best ways to remove
a consistent vignette pattern?  Thanks in advance.
> >
> >
> >Carpe Noctem,  Frank
> >
> >Astropix:
<http://bellsouthpwp.net/f/r/frankb02/astropix.html>http://bellsouthpwp.net/
f/r/frankb02/astropix.html
>
> --
> Matt BenDaniel
> matt@starmatt.com
> http://starmatt.com
>
>
> --  APML Archives at <http://astro.umsystem.edu/apml/>  ---
>              Unsubscribe at <majordomo@seds.org>
>


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>