Using the Mewlon 210 F11.5 with a Tak x1.6 Extender I managed to get a reasonable image of Jupiter on 14th August despite the less than optimal seeing conditions, as clouds kept rolling in throughout the session, disturbing the imaging acquisition. Theoretically this should have been taken at 3864mm at F 18.4.
I used the on-line calculator at:
http://www.wilmslowastro.com/software/formulae.htm
to calculate the actual focal length of my system with the x1.6 Extender , which turned out be 3687mm at F17.56, which gives me a multiplication factor of x6 i.e. 17.56 / 2.9. (FL / pixels size in microns)
Cheers
Dennis