Vignetting, discussion about methods of resolution Options

[um3k]

Another problem is that Voyager images exhibit a large amount of geometric distortion. Correcting this would definitely help with your stitching.

[MichaelT]

Anyway, a mosiac is improved but not perfect:

I couldn't get it any better either. I think what edstrick wrote pretty much explains why: There are just too many other effects that cause the brightness differences in the images. I'm sorry that I can't help you there.

Btw.: Have you got access to Photoshop? If you use the filter that removes dust and scratches with a radius of 3 and a threshold value of 10, it removes all the dark dots without visible loss of detail (except the moons shadow, probably).

Michael

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[um3k]

jrdahlman, these websites may come in handy for you:
http://members.optusnet.com.au/serpentis/ - For viewing and processing Voyager images
http://pds-rings.seti.org/catalog/vgriss.html - Nearly complete catalog of Voyager images, including calibration images.
Good luck!

[jrdahlman]

Oh, I know that you can download all the images. I just have my hands full with one CD, focussing on one planet! (That CD's been sitting here for years--I should do something with it.)
I'm really just playing around. I don't expect to find anything "new."

Downloaded yet another program. If it doesn't gasp and die in my little Libretto, I'll try it out.

By the way, HOW is it geometrically distorted? Fisheye-lens curving?

[um3k]

By the way, HOW is it geometrically distorted? Fisheye-lens curving?

Vidicon electron beam scanning distortion. Thus, it varies somewhat from image to image, and is not evenly distributed within the image. That's what the black dots (called reseau marks) all over the image are for. The software I linked to does a fairly reasonable job of removing the distortion, and is capable of removing the reseau marks, although not in a very asthetically pleasing sort of way.

Also, the reason I gave you the link to the site with the image files is that you can download dark frame images there, which the software I linked to can use to remove the "anti-vignetting." That's the best way to do it.

As a side note, vidicons are lousy for scientific imaging.

[edstrick]

Playing with Voyager 1 wide angle images, I found I could make dark-field images that essentially perfectly canceled out dark-exposure structure and shading in images that had the same parameters.

I found to my distinct horror that there are image sets with apparently identical "relevant" parameters (readout rate, etc) that have different dark-exposure structure and needed separate calibration files.

Playing with Titan Voyager 1 Wide angle frames, I found that on a distinctly overexposed Titan image (exposed for limb or terminator structure), as you approached the moon's disk, there were image displacement fringes on the reseau marks (black dots) as the electrical charge loss in the area of the overexposed moon's image DEFLECTED the image readout electron beam during image readout!

Decalibrating vidicon images is a Siphysus-level task.

[jrdahlman]

I would like to point out that the program um3k's first link above:

http://pds-rings.seti.org/catalog/vgriss.html

is the program for getting rid of reseau and for correcting the geometric distortion. I've played with it a few days and it works fine even in my little Libretto computer. It has special settings for Voyager or Viking orbiter images, and seems to have almost as many complicated controls as Photoshop! (I imagine.)

The only thing is that it doesn't automatically do the flat-fielding--special flat-field images must be loaded into it. But is has separate settings for "flat-field" and "dark current"--didn't even know there was a difference.

[edstrick]

Dark current images are those taken with zero exposure. They actually are of two types.. Ones taken with zero exposure, and ones taken with time exposure of a zero-brightness scene, then read out at slower than the maximum possible speed. As a camera's detector "sits" before readout, it accumulates (or a vidicon looses) charge, and the blank image changes with exposure time and readout settings.

A flat-field image is one taken of a uniformly illuminated scene, to calibrate for camera and lens response variations AFTER a perfectly matched dark image is subtracted. You then divide data images by the flat-field to get hopefully clean decalibrated images.