I've always wondered the following question :
We assume to have several objectives with the same focal length, same materials (ED or not), each one polished with the same optical quality but with different diameters.
For instance f=1200mm and diameters vary from 80mm, 90mm, 100mm up to 250mm.
The experimenter is looking at a planet or the moon and swap the different objectives.
Increasing the diameter, increase the resolving power but increase also the chromatic aberration.
Until what diameter (given by the CA ratio) CA become so strong that it reduces the gain in resolving details and the experimenter will see less ?
The power (eyepiece) can be chosen at his convenience (and so changed when swaping the objective) to get the best result.
If I know that a 4” at f12 will show more planetary details than a 80mm at f15 (so both with a focal length equal to 1200mm), can I still say this for a 6”f8 vs a 5”f9 or for a 8”f6 vs the 6”f8 ?
Increasing the diameter will allow an image brighter (at the same magnification) with a greater exit pupil.
This theorical experiment is what many owners of big achromatic refractors do by putting an aperture mask when looking at the planets
So the question could also be formulated like this : what aperture mask should I use with my refractor to get the best of it when looking at the planet ? In order to not depend on the lens characteristics (D and f), the answer should be given with the CA ratio.
The second question I have is this : can I get a perfect apo by reducing the diameter of the objective (example 6”f8 with a 2” aperture mask) ?
I only want to understand how optics work, so please do not tell me that using a 6” with a 2” aperture mask has no sense, or a 10” at f5 doesn’t exist (because it does, see the cosmo physics refractor thread ).
Edited by delorehal, 26 June 2020 - 07:33 AM.