It produces a video signal for a video monitor. We can record that NTSC video into a video editing program using a card like a Dazzle card and create an AVI file to get images from. This can work well for images like planets and other images as well. We can stack that video and get an image of a planet or the moon, or some other astronomy object and see more details, basically improve seeing by using hundreds of exposures. We do this with programs like Registax. These programs will analyze and select only the in focus frames of an AVI movie or some of the higher quality frame images and stack them or put them together.
The results can be nice, but there can be some problems. One problem is the recording software or Registax may not account for the NTSC video pixel aspect ratio. That is there may be stretched pixel conversions that can appear. This is due to the television and video standards of early television using Non-square pixels, that is pixels that are stretched. Here is what would typically happen.
Round images which use square pixels will be displayed on an NTSC monitor which is not using round pixels. Since the pixels in video are not round the resulting graphic would be stretched and would not look round. People doing graphics on a computer trying to display those graphics would have to use Computer Graphics programs that could calculate and stretch the graphics to display on a stretched pixel format, making the image appear correctly on the NTSC monitor which is not using square pixels.
Now consider what would happen if you took a video camera and made it sensitive to low light for astronomy and wanted it to display it's pixels and images on a tv monitor, that is using an old school monitor and you wanted the image to be correct and display correctly on a video monitor. You would have to add a function to convert and stretch the ccd sensor pixels to NTSC or elongated pixels, so they would display correctly. You would stretch the pixels so they would look correct on video.
The top chart below snows this. Because the tv monitor is not displaying square pixels we stretched them, in the hardware so they will look correct, and the round circle for example the shape of a planet or the moon would look round on the tv monitor.
But wait a minute. What happens when you try to digitize that video signal using something like the dazzle board. The ntsc signal going out the cable is stretched for the long pixels on a video monitor. That video if its not interpreted correctly will end up looking like longer pixels and this will mean that pixels that are supposed to be 1 dot wide will be 1.33 dots wide for the dazzle board to see them.
The software that captures the video may not have aspect ratio settings that can account for the non-square pixels. It will end up showing a planet that is not round. It looks round on an ntsc monitor, but when its digitized to an avi file, the software may not rescale it.
What youll end up with is a planet that looks oblong.
As you can see below, the programs down stream that use this AVI file will generated a stretched image.
(notice Jupiter looks nice, with good black and white details, but it's stretched a little bit and is out of round.).
The solution is to rescale and reformat the image later on to change it's width back and get rid of the stretch. I'm not showing that step here.
Registax is not expecting non square video sourced files for it's images, it's expecting video from a square sensor, like an astrophotography ccd camera.
How could a webcam work? The webcam uses video and displays and scales it for a normal computer monitor which is using square pixels not NTSC stretched pixels, so the regular webcam video or a camera like the Meade DSI camera would create a video clip with square pixels to begin with.
To make things a little more confusion, Jupiter is actually stretched a little bit in real life, because it's spinning at such a high rate of speed. Liquid and clouds literally fly out around the equator and cause the planet to bulge a little bit. So I wouldn't want to bring it back to a total circle with a perspective correction.
For more information on pixel aspect ratios see:
http://en.wikipedia.org/wiki/Pixel_aspect_ratio
But be aware it's only talking about square pixels being moved to NTSC or other video non square ratios. . . Its not talking about moving from NTSC to digital for digital processing that remains in the computer display realm.
Posted using BlogPress from my iPad
I have to make a few gram ER and spelling corrections to this post, sorry.
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