Quote:
Originally Posted by bojan
Only for nebulae (extended objects)!
For stars, it will be the same.
Actually a bit worse... because the central obstruction (secondary) must be slightly wider for f4 system
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Hi Bojan. I think that the effect of FNo on star images depends on sampling. If stars are heavily undersampled, you are right and there is no difference. If sampling near Nyquist or oversampled, star angular size will be the same for both optics and determined by atmospheric seeing with the 8 inch aperture, so the energy of a star in the longer fl system will be spread out over a larger area (and more pixels). stars will thus be bigger and dimmer in the higher FNo system, just like an extended object (galaxy, nebula etc). If you want to be convinced that star brightness changes with focal length, try imaging with a Barlow in the optics - this will change only the focal length (not the aperture), but you will certainly see a major drop in star brightness.
The other effect of FNo for the same aperture and camera is that the higher FNo system will be able to resolve finer detail in some circumstances - again depending on sampling and atmospheric seeing.
Lewis, if this is to be used with your SX25, the f4 will be much faster, but the f5 will give noticeably more detail in good seeing. Either would be suitable, but I reckon I would go with the f5 to get better detail when it's available.