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Re: [APML] Couple of Wide-Angle shots on Konica Centuria 400 (andscanner noise suggestion)
>Bit depth is indeed really important. I got so much better results (with
the same scanner, an Epson 3200 Photo flatbed) at 48bit (16bit/channel)
than at 24bit that I quickly stopped using the 24bit mode. In between,
at 36bit, might be enough.
Wich is the native bit deph of your scanner? 16 per channel?
>Film like E200 definitely has way more than
>256 brightness levels per colour channel; whether 4096 is enough I don't >know; but 65536 levels at 16bit/channel certainly doesn't seem too
>excessive when I examine the stellar profiles and background statistics.
And the image weights just the same (as 12bits) =)
>> BTW, I think that we may use a non exposed frame to
>> perform a bias "film" substraction. It might substract
>> the film base and any residual bias from the scanner.
>When I scan, I leave a large perimeter of the film "rebate" around each
>image. My scripts then subtract the median of this from the whole >image.
Ok. Just like a "overscan" ;-)
For the ones that don't have the possibility to write theyr own programs, here is what you have to do in PixInsight LE to perform the same operation:
1.- Open the image
2.- Create a preview in a non-exposed area of the film
3.- Right-click on the preview (it must be selected as the current view) and select View>Statistics.
4.- Read the Median value of each channel.
5.- You might repeat the 2-4 steps for several previews and then average the median of the samples.
6.- Show the histograms of the image view. For each channel, perform a shadows clipping of the value of the median calculated before (in the normaliced range)
7.- Extend the dinamical range in the highlights by the same amount of the clipping.
Steps 6 and 7 will perform the substraction of the film base without rescaling the result.
In PixInsight Standard the steps 6 and 7 can be achieved by simply substracting the image with a constant value (the median calculated before).
>This is the film equivalent of bias subtraction (not to be confused with
>bias arising in the scanner, which we are still uncertain about!).
Yes, of course. But, if we fully scan a non-exposed pic, we have the "same" scanner bias as in the previously normal pic. Then, perhaps the substraction of it shoots both birds at the same time <g>
>If
>you are taking a flatfielding (divisive) approach to the problem of
>vignetting correction, you absolutely _must_ subtract the film base
>level first.
Yes, I know that it is the correct procedure. But, if the film base is significaly small in the dinamical range, and we apply the division in a non-linear way, I think that we can forget it without much problems. Of course, I might be totally wrong.
Ray "who found that auto-cropping off of the rebate area when it is no
longer needed is a real boon" Butler
> =====
> Regards,
>
> Carlos Milovic F.
> -------------------------
> Astro & Photo - CMF
> http://www.astrophoto.vze.com
> -------------------------
> Visita "AstroFoto", el foro de astrofotografía en español
> http://espanol.groups.yahoo.com/group/astrofoto
>
>
>
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--
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
Na
tional
University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788 FAX: +353-91-525700
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Regards,
Carlos Milovic F.
-------------------------
Astro & Photo - CMF
http://www.astrophoto.vze.com
-------------------------
Visita "AstroFoto", el foro de astrofotografía en español
http://espanol.groups.yahoo.com/group/astrofoto
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