For example, the TeleVue 5x powermate will increase its power by 1x for every 35mm of extra distance introduced between the top of the powermate barrel and the focal plane of the camera. (reference: Televue Powermates).
In practice, this and other variables in the optical system make it very difficult to accurately calculate the effective focal length of the instrument.
The solution is to work backwards from a captured image and calculate the focal length directly. All you need to know the pixel size of your camera (in microns) and then use the following formula:
F = 206.265 x P x U --------------- O P is the size of the object in pixels as captured U is the pixel size of the camera in microns O is the true size of the object in arc-seconds F will be the focal length as calculated from this formula, in mm.
So my effective focal length was:
F = 206.265 x 342 x 7.4 ------------------- 39.7 = 13,149mmThe diameter of my primary mirror is 331mm, so this means I was using a focal ratio of 13149/331 = f/39.7.
So there you go, no more head scratching required to work out what focal length was in use in your planetary images.
2nd April 2007
Birds Astro Site