|Recently I purchased some equipment for timing occultations of
stars by the moon. It consists of a very sensitive Watec 902H video camera, a
KIWI-OSD box from PFD
Systems, and a Garmin 18LVC GPS unit. The KIWI-OSD is an so-called
Video-Time-Inserter (VTI). Details of the box can be found at the site of its
The picture below shows how this works. The Watec camera, e.g. attached to a telescope, sends an analog video signal to the box. The Garmin GPS unit provides the location, but also a very precise time signal. This time signal is displayed on the video signal. The analog video-out feeds in to a video recorder, in this case a Sony digital 8 camcorder with a analog video-in port, and is recorded on a digital 8 tape.
After the recording has finished, the camcorder is connected to a PC with a FireWire (IEEE 1394) cable. The videostream is captured with VirtualDub as an *.avi file.
What are fields, and why do I want to see them?
Video is recorded at 25 images (frames) per second. This is true for PAL, the US NTSC system records 29.97 frames/second, but the principle is the same. A video frame is made up out of two fields. One fields contains the odd lines, and the other field the even lines of the full image. A VTI like the KIWI-OSD put a timestamp on individual fields. If the fields can be viewed instead of full frames the timing accuracy is doubled.
The picture below shows two individual fields of a video frame. Field 438 was taken from 9:14:04.738 until 9:14:04.758 and field 439 from 9:14:04.758 until 9:14:04.778. Thus each field has an exposure time of 0.02 seconds. The time is displayed in UT. As I needed a dark background for this test, I used the box of the Garmin.
Please note: the process describes above works with equipment and software as mentioned. One should be aware that some *.avi files cannot be read by Limovie. This is the case for small video taken with a digital photo camera (Canon Ixus 30) and also when I used Microsoft Moviemaker to import a video from my TRV345E camcorder, and save the result as *.avi.