Harry Dalek wrote:That's a good project a mechanical SSTV camera ,Why not a PMT Steve ?
Well, there's two reasons why I'm not going the PMT route...actually three...
1) If you recall my experiments with them some years ago I found them to be really noisy, to the extent of being unusable - perhaps I got a batch of dud ones (not from e-bay and the like, a trusted tube supplier).
2) As you want to only use part of the sensitive area of the photocathode that will make noise even worse - you only want a very small area of the projected lens image, in my case about 0.25mm square. The PMTs I used have a huge (and unused) area that simply contributes to the noise.
3) Large pulses, usually a few seconds apart from the odd Cosmic Ray that makes it past the Earth's magnetic field and atmosphere. I think that's the reasoning behind it, no matter how dark the PMT was, there were always these seemingly random pulses.
So PMTs for me at this stage are out.
So my plan (when I get to it) is to use a lens with a focal length of 100-150mm or so with an aperture of around 50mm, f2/f3 or thereabouts. Shallow depth of field, yes, but I'll live with that.
The method I used in the past was to select a jpg, then...
1) Crop/resize it to 256x256 pixels and reduce to an 8-bit monochrome TIFF file. Using Irfanview.
2) Strip off (or ignore) the TIFF header info.
3) Send that raw image data to a micro that scaled it to 128 lines, added SSTV syncs then converted it into a baseband analogue SSTV video signal.
4) Feed the whole thing into a DDS SSTV modulator and record it as a 8kHz 8-bit mono WAV file. Done.
Steve A.