If you are taking pictures of a CRT, then you will need relatively long exposures, at least as long as a single frame on the CRT, but more likely several frames would be better, so for example 1/25th Sec or slower if the CRT refresh is 25fps.
If we knew a bit more about the
CRT, that might help. If the CRT has a fairly long persistence phosphor then the timing is less critical. Photographing a CRT oscilloscope for example is usually fairly straight forward as the phosphor usually has a fairly high persistence (high persistence is not good for television pictures, but great for rock steady stills).
One other question. Is there a "video out" connector on the system, if so it may be possible to digitize the image directly.
EDIT: Having seen the device in action
here I think I will revise my shutter speed down to around the 1 second marker. More info and an even older SEM for those interested
here.
One other thing that might be of interest in obtaining the best possible resolution images, there is mention of "digital storage" in the various on line articles that mention the Philips 515, so it *might* be possible to access this "digitised" output directly. This would of course give you the best possible resolution available from the device (although it takes out the fun of using CHDK and a digital camera).
If you don't mind me asking, how did you come by this toy. I've tried asking Santa Clause for one, but I was told it wouldn't fit down the chimney.