Good stuff Wavytone ( Nick )
Below is Mel Bartels :
Magnification roll-off: Another qualitative method, though it does demonstrate the MTF (modulation transfer function). A low contrast object such as Jupiter is selected, and a series of magnifications are run through. At some point, the image will 'roll-off' beginning to lose it sharpness. Dividing this magnification by the aperture gives a rating, for example, a mirror is good to 50x per inch of aperture.
Here is my scale:
Mirrors that can sustain 35x to 50x per inch of aperture I rate as excellent.
Mirrors that sustain 25x to 35x per inch of aperture I rate as good.
Mirrors that sustain 25x per inch of aperture are acceptable.
Mirrors that sustain 15x to 25x per inch of aperture are poor, usable only at lower powers.
Mirrors that fail at 12x per inch of aperture are plain just not finished.
For smaller slower mirrors, push these numbers higher, for large fast mirrors, push the numbers lower.
Bobby
