I am using the formula: cube of the focal ratio * 0.022, to determine the coma free zone of a telescope.

On one site (think it was Astronomics) it suggest you cube the focal ratio to get a coma free zone in mm.

There are a few other formulas I came across but for the sake of the argument would like to ask that we stick to the one above unless it is grossly misleading.

The answer I will get is how large a diameter I will have in my image circle that has coma less than the size of the airy disk eg. no discernable coma.

I would like to work out the following:

- What would the formula be if I take my seeing into account rather than calculate it based on the airy disk size for the system?

- If I have a coma corrector added which yields a 2x improvement on the calculated results, can I just double the figure I get from the above?

- Is the formula above (using 0.022) calculating the diameter or the radius of the corrected image?

Any views?