Re: [APML] Couple of Wide-Angle shots on Konica Centuria 400 (andscanner noise suggestion)

[Ray Butler <ray.butler@nuigalway.ie>]

Hi Carlos,

Carlos Milovic F. wrote:
> Hi all
> 
> I have the feeling that at least the SS4000 does a
> dark frame substraction, so, if the bias is
> independent of time, and it is not bias corrected, you
> have both processes at the same time. Anyway, I think
> that we don't need to worry too much about them. The
> noise from the scanner is charactericed by a higher
> frecuency than the film's, so, using a
> multirresolution analisys like wavelets you can
> isolate it and reduce it, without affecting the
> information in a significative way. 

Yes, wavelets are amazing for that sort of thing.

> I'd better take
> care of taking true raw scans, save them at the higher
> bit deph as possible, multiscan the originals and
> average them, and later work with them with the best
> software we can.

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. 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.

> 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. 
This is the film equivalent of bias subtraction (not to be confused with 
bias arising in the scanner, which we are still uncertain about!). If 
you are taking a flatfielding (divisive) approach to the problem of 
vignetting correction, you absolutely _must_ subtract the film base 
level first.

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
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> Visita "AstroFoto", el foro de astrofotografía en español
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> 
> 
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-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700


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