Asteroid and Comet Magnitudes – Formula
No amount of googling told me what the (generic) formula for the magnitude of a comet / minor planet / asteroid is, in terms of the H and G parameters (Absolute Magnitude and Slope Parameter). The trouble was that the formula I got elsewhere on the net used the terminology ‘n’ for the slope parameter and didn’t care to call it so.
In summary, the formula is:
m = H + 5log( d ) +2.5*G*log( r );
Where the parameters are as defined here: http://www.ayton.id.au/gary/Science/Astronomy/Ast_comets.htm
It took me reading through Cartes du Ciel’s source code to find out that n == G.