4/3 Pixel Processing

The Milky Way Galaxy

At first the 4/3 shape of the CCD's pixels doesn't seem like a big deal. The pixel count of a RAW image is 508 by 488, almost square. However, the CCD pixel shape is rectangular, with it's width being about 4 units for every 3 units high - just like a TV - oh yeah, this is a TV CCD chip. The Uncombined images are 648 by 488 pixels, which accounts for the pixel shape.

So, other than the objects may appear stockier or stretched, what is really the problem with 4/3 images?

The problem arises when the images are rotated during the tracking process from image to image. When selecting two objects to do drizzle tracking, the objects have a constant distance. But with a 4/3 image, the object could be 3x pixels away, but as the objects rotate 90° from their original orientation, the object could be 4x pixels away. This could cause tracking issues and would definitely produce inappropriate composite images.

The easiest way to compensate for this is to add an additional pixel horizontally after every 3rd pixel. This pixel is usually an average of the pixel on either side of the new pixel. Better algorithms stretch 3 pixels into 4 by using a corresponding proportion of each pixel as it contributes to the 4 new pixel.

Another method that does not average or otherwise modify data measured consists of replicating each pixel in an original image to be a block of 4 by 3 pixels of the same value in a new larger image. The resultant image will be 508 times 4 wide by 488 times 3 high (2032 x 1464 pixels). This method would work well for a drizzle approach that could be performed at the same time. I am unsure of how Meade performs these image stretches.

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