SPC900NC Amp-off mod

Posted by XTSee on 3rd November , 2011

Prompted by a comment a couple of weeks back (Philip A Cruden at AstroGab) I decided to expand on the Steve Chambers long exposure SC1 LX-mod I have done previously to my SPC900NC webcam, by making the necessary change to allow the Amp-off mod (known as the SC1.5 mod).

The webcam electronics normally use an “amplifier” to increase the captured light data at fast frame rates when the camera is used in normal video mode. But when the camera has been modified to allow long exposures exceeding a couple of minutes for astrophotography purposes, and the images captured are post-processed to increase levels, a blue-white “glow” becomes apparent as a bright patch in the top left of the images, which gradually diminishes towards the bottom right.

The Amp-OFF mod allows a laptop to switch off the amplifier circuit for the duration of each long exposure, so removing this glow from the data, and this greatly improves images of nebulae and galaxies that require more extreme increases in Levels/Brightness/Curves, etc in image processing software.

This time around I have taken the circuit boards out of the original casing (last time I managed to cram it back into the original webcam body which was a tight squeeze with the extra wires and circuit board), instead installing everything into a much larger project box from Maplin Electronics, which makes it far easier to work with.

At the same time, I removed the circuitry I had previously used to implement the serial interface for my laptop to control the long exposure switching (based around a 4066 CMOS chip), and replaced it with a much simpler circuit comprised of a few transistors, diodes, resistors. This circuit board uses the USB to serial adaptor connection RTS and DTR signals to control both the LX-Mod and the Amp-Off mod.

Another advantage to using the bigger project box, is that a miniature computer cooling fan can be mounted on the lid to force cooled air over the webcam circuit board where the CCD chip is located. This helps keep the components cool, so reducing the amount of electronic noise and hot-pixels that can appear in long exposures (some people go so far as to use a peltier semiconductor chip cooling device to act as a heatsink to take the CCD chip temperature down to freezing, but this can be prone to causing condensation forming and freezing on the circuitry).

Once the construction was completed, I tested that everything works correctly, and it does! Making the Amp-off mod was fairly easy to do, and I can’t think why I didn’t do both mods at the same time and put them into a larger box the first time around.

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