Image Scale

There are a few astro bloggers that I think do a fantastic job of sharing their expertise in the field of astrophoto. Jerry Lodriguss is certainly one of them. His blog post "Catching the Light" is a fantastic source of info to understand more about how to take great pictures.

In a recent post on Image Scale, he describes how to calculate the image scale on your camera, that is, what field of view each pixel is representing.

So an approximation of the formula (converted in metric system) goes like:

P = (206 * S) / (FL)
Where:

P is the image scale per pixel in arc seconds
S is the size of the pixel in microns
FL is the focal length in millimeters

(the real formula is P=3600*arctan(S/FL), but since S is very small compared to FL, the simplification will be good enough for astro use)

Example: Calculate the image scale per pixel for a Canon 500D camera with 4.7 micron pixels when used on a Perl Vixen 130/720 telescope (720 of focal length).

P = (206 * S) / (FL)
P = (206 * 4.7) / (720)
P = 1.3 arc seconds per pixel

So each pixel of the 500D on the scope will cover 1.3" of the sky.

Jerry goes on with saying that this is very useful to estimate the smallest details you hope to record. You typically would like your image scale per pixel to be 2x to 3x smaller than these smallest details (eg 0.3" to 0.5" to record 1" details on Jupiter).

How do you do this? Increase your focal length with either a barlow, eyepiece projection or lens on your camera held up to the eyepiece of the scope.

Something else to consider as Jerry points out is the seeing. Let's say the seeing is 1". If you are doing deep sky astrophoto, then with the additional tracking errors and other atmospheric variations over the time of the picture, you're unlikely to reach the seeing true limit, so you're probably correctly sampled at 1.3"

But in planetary, where exposure times are very short, it's worth considering the 2x or 3x oversampling as the seeing is less of a real limit.