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Re: filmscanners: Stellar ghosts and Nikon Coolscan IVED (LS40)

I think I would clone them out.


----- Original Message -----
From: "Harry Lehto" <hlehto@oj287.astro.utu.fi>
To: <filmscanners@halftone.co.uk>
Sent: Friday, May 11, 2001 8:25 AM
Subject: Re: filmscanners: Stellar ghosts and Nikon Coolscan IVED (LS40)

> On Fri, 11 May 2001, Rob Geraghty wrote:
> > OK, it sounds like some sort of aberration in the scanner lens system.
> > Is there anyone near you with another film scanner you could send
> > a sample slide to in order to test it?  Maybe with a Polaroid scanner?
> >
> > Out of interest, does it make any difference if you insert the slide
> > the scanner the other way up?
> Yes, it makes a difference... I did some further testing last night.
> I turned the slide 90 degrees, and sure enough the ghost rotated 90
> degrees in repect to the stars (that is if you keep the orientation of
> the stars fixed) => so clearly due to the scanner and not the slide.
> Furthermore, I measured the effect on about 10 bright stars in the field,
> and noticed the following behavior. Along the line that is parallel with
> the long edge and goes through the center of the slide you seem to have no
> ghosts, but as you approach the longer edges of the slide you see these
> the ghosts emerging from the stellar image, the closer you are to one
> of the long edges of the slide the more pronounced the effect is. You can
> understand this as internal reflections (as was suggested by Art in
> the messages that arrived last night) if you consider that the scanner
> scans perpendicular to the long edge of the slide.  Clearly such a process
> is optimized to the centerline. Actually the ghost images are also there
> on the center line, but they are superimposed on the stellar images
> making a nearly unnoticeable halo.
> Yes, replying to several of you, I have a neighbour just down the road who
> has a scanner too.. It's not a polaroid, but a Canon FS 2710. So I ran the
> same picture on through his scanner - similar resolution. And boy that
> must have been one of those moments, when I was happy to see an optical
> distortion. A similar phenomenon was visible in the images scanned with
> the Canon scanner, but here the image looked like a small comet (similar
> to a coma distortion), but with two separate tails, a red one and a
> green one. So instead of one blue-green ghost spot, there were two more
> noticeable tails of different colors pointing in the same general
> direction as the spot is in the picture scanned with the Nikon. The size
> of the distortion was more or less similar to what I had in the Nikon
> scanner. It  appeared that the tails had a significantly higher level
> than the ghost spot I saw with the Nikon. It appears that I hit the (
> not so bad afterall) limit of the Coolscan IV scanner.
> It seems also that this phenomenon may be a common problem to desktop
> scanners. I think you should see it at any bright source (e.g streetlight)
> against a (nearly) pitch black background and here only on edge of the
> light that is closer to the edge of the slide. It should not effect
> significantly ordinary day time images.
> The web reference had one typo in it... So here they are again.
> http://www.astro.utu.fi/~hlehto/nikontest/crop0016.JPG
> http://www.astro.utu.fi/~hlehto/nikontest/crop0020.JPG
> Since it appears that I'll have to live with it, are there any remedies
> for removing this effect from the images? It appears that if I could scale
> the image by a few % in y direction only, skysubrtact and multiply
> the new image by a suitably small number, I would have a "mask" that I
> could subtract from the orginal image to get rid of the effect. Can
> this be done easily?
> Regards
> Harry


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