Special case planetary photography

Special case planetary photography

Since planetary photography often uses large focal lengths to show the very small planets large on the chip, seeing would always be dominant in long exposures and the image would be blurry. That is why lucky imaging is used in planetary photography. Here extremely short exposed images (or videos from which the single images are extracted) are created. The idea behind this is that exactly at a moment, when the seeing is not present or extremely weak, a photo can be taken. If enough images were taken in one session, the 10% images with the most suitable seeing are selected by a software (e.g. AutoStakkert) and then stacked. Due to the very short exposure times the single images have a bad signal-to-noise ratio, but the large amount of images negates this effect again.

Since seeing does not play a role for the successful single image, the theoretical resolving power of the telescope can be inserted into the formula for calculating the image scale β for the optimal telescope-camera-system. The resolving power of the telescope, i.e., the ability to still separately display two objects closely spaced at an angular distance, is now the limiting factor. However, a separate display of objects is only successful if there is at least one more pixel between them.

This means that if a telescope and camera are available, a focal length factor can be determined to adjust the focal length of the telescope so that 2 pixels sample the range of the theoretical resolving power. Or in other words, one pixel must be able to represent half the resolving power. It is valid:

If the formula is then converted according to the focal length, the optimum focal length can be determined with the available camera.

The required focal length factor then results from:

A suitable focal length factor can be determined via the calculation tool 'Solar sytem' in the menu point 'Tools'.