When I went to the Kelling Heath star party 2 weeks ago, I had a good go at using my SPC900NC webcam with my scope to so some astrophotography. Even though I had modded the camera several months ago, I’ve not tried using it for real yet! My bad. Anyway the star party gave me the kick up the butt I needed.
But I experienced an odd problem which took me a little while to figure out. I use K3CCDTools (or WxAstroCapture), with my Steve Chambers SC1 mod for long exposure, and it was successfully reading the image data back to my laptop for 2 or 3 images in a row, but then the next image would be blank/black (as if the LX mode had been turned off, i.e. it had gone back to a dim image where you could only just make out the stars/object being photographed), and this would occur for maybe 1 or 2 images, but then it was going back to a properly long exposed image again. This continued in a seemingly intermittent fashion switching between long exposures, then dark images.
This was quite frustrating with imaging runs because out of say 20 or 30 consecutive images, half of them would be useless.
I was thinking it must be some sort of timing issue with the USB/Serial port, and experimented with a setting in K3CCDTools for the post-image read delay, but to no avail. I was also beginning to wonder if my DIY effort at the SC mod was bad in some way, perhaps I had not wired things correctly or used the wrong components.
The next day I did some simple testing with the webcam connected in LX-mode and continuously running a simple 2-second exposure in K3CCDTools, and I also use WcCtrl which is a neat utility that allows viewing and changing the camera settings at the same time as K3CCDTools is capturing the images, e.g. adjustment of Brightness, Contrast, Gamma, Gain, White Balance, FPS, Colour or B/W, Mirror-image and so on, plus the ability to save settings as presets for loading in future.
What I discovered is that between K3CCDTools and wcCtrl they had differing opinions of what the frame rate was supposed to be set at. When switching LX-mode OFF in K3CCDTools (i.e. closing the Long Exposure control window), this resets the camera to Normal or video-mode, and occasionally turns the frame rate up to say 10fps or 15 fps, but when going back into LX-mode (by opening the Long Exposure window again), the Frames Per Second setting was remaining at 10 or 15fps.
The SPC900NC webcam does not seem to like LX-mode in conjunction with a higher FPS setting. So by simply using wcCtrl to change the frame rate back to 5fps, this put the webcam into a stable consistent state whereby the Long Exposures would return proper images every time, no matter how many consecutive captures you set it to do, and no matter how long they were - 5 seconds or 120 seconds, I was now able to get accurate images every time.
I’m unsure of the technicalities, but my guess is that if 10fps or higher is used, 1/10th of a second or less is insufficient time for image data to be read back from the camera before the next frame is reset, and so it messes up the image capture.
So I thought I would write about this little problem in case it helps people trying to use the SPC900NC with the LX mod. Remember to set 5FPS when using LX-mode.