Image scale in pixels

Unlike astronomical CCD camera users, DSLR astrophotographers do not often deal with single pixels because there are too many of them. Even a pinpoint star image usually occupies six or eight pixels on the sensor. This is a good thing because it means that star images are much larger than hot pixels.

Still, it can be useful to know the image scale in pixels. To find it, first determine the size of each pixel in millimeters. For instance, the Canon XTi (400D) sensor measures 14.8 x 22.2 mm, according to Canon's specifications, and has 2592 x 3888 pixels. That means the pixel size is

14.8

-= 0.00571 mm vertically

2592 y

5.4 Vignetting and edge-of-field quality and also 22.2

-= 0.00571 mm horizontally

3888

Now find the field of view of a single pixel; that is, pretend your sensor is 0.00571 mm square. Suppose the telescope is a common 20-cm (8-inch) f /10 Schmidt-Cassegrain with a focal length of 2000 mm. Then:

Field of one pixel = 57.3° x °-00571 mm = 0.000016359° = 0.59"

2000 mm

As with astronomical CCD cameras, the pixel size works out to be close to the resolution limit of the telescope.

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