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Unmanned Spaceflight.com _ Image Processing Techniques _ Question about reseau marks.

Posted by: J.J. Apr 18 2010, 03:50 PM

In a lot of old pictures--Voyager and earlier--these markings are prominent in raw imagery. From Galileo onward, though I've not seen them; what has changed in imaging systems to render them unnecessary?

Thanks in advance!

Posted by: djellison Apr 18 2010, 03:55 PM

The shift form Vidicon tubes to CCD's.

Posted by: tasp Apr 18 2010, 06:24 PM

To expand on that a bit,

in CCD sensors, the pixels are physically fixed in size and position. With the old style vacuum tube technology, the scale of the image would vary somewhat with the brightness of the image. If you have noticed on picture tube TV sets vertical lines bending in their proximity to other bright and dark areas in the picture, a similar effect occurs in the vidicon tube. The brighter a region of brightness, the more charge is 'read' by the scanning electron beam that records the picture, but the beam is also slightly deflected by that charge too. The electron beam in the vidicon is rastered across the image by externally applied magnetic fields, the local variations in charge (due to brightness) on the image can also deflect the beam slightly. If you want to precisely measure the relationship of various features to each other in a vidicon, the reseau marks provide a fixed grid for this purpose as they are physically present on the vidicon sensor and are immune to shifts in position due to the vagaries of the brightness and resulting charge in the picture.

A recent development in picture tube TV technology shortly before LCD, DLP, plasma, etc took over was something called scan velocity modulation. This is essentially the converse of the vidicon tube problem. Here though, the electron beam is slowed as it 'writes' the bright areas of the picture and accelerated as it does the darker portions. (and here slowed and accelerated refer to the transverse velocity of the beam, not the speed of the electrons moving down the beam) This makes the bright areas brighter, and the dark areas darker, thus improving the contrast (and hopefully the realism) of the televised scene. Turning a 'problem' into a 'feature' is one of the main drivers in the advancement of technology that allows for ever more capable spacecraft to be designed and launched, and for the supporting technology we use here to study the results.

Posted by: J.J. Apr 18 2010, 10:36 PM

Thanks much, folks!

Posted by: JohnVV Apr 20 2010, 06:03 AM

a good tool for removing them ( besides ISIS)
is the Gimp add on tool G'mic
the "hot pixel filter" dose a very good job