Hi Steve,
It is not possible to completely separate the issues of focal ratio and aperture, since they are interdependent. There is is great deal of confusion and sometimes controversy surrounding discussions of aperture vs. focal ratio for astrophotography, but it often ends up that all parties understand the issues equally well and agree on the principles, but can't see eye to eye on the details.
In that other post I pointed out that it is the aperture that determines how bright the image of a "point source" such as a star becomes in a given exposure time, because more photons from the star reach the sensor when the aperture is larger. Now consider that an extended object like a galaxy or nebula is really just a collection of adjacent points - the light that reaches earth from an emission nebula consists of photons emitted by individual atoms within the nebula, so each photon emanates from a "point source". It is still true that more photons from each point in an extended object reach the sensor when the aperture is larger.
What does change, though, when the aperture is held constant while the focal length increases (i.e., the focal ratio is increased), is the total number of photons that reach the sensor from the entire field of view in a given time. Since the field of view decreases with increasing focal length, fewer photons enter an aperture of a given size. When the total number of photons reaching the sensor decreases while the size of the sensor and the number of pixels remains constant, each pixel receives fewer photons on the average and the image is darker. Images of stars are just as bright for a fixed aperture regardless of the focal length, at least until the size of the star image exceeds the size of one pixel. But the photons from interstellar sources such as glowing nebulae (and light pollution) are spread across more pixels due to the narrower field of view, resulting in each pixel receiving less illumination.
IMHO the aperture is of primary importance for astrophotography for reasons not related to focal ratio. The theoretical limits of resolution, which can be expressed as the Dawes limit or Rayleigh limit, increase with aperture, unaffected by focal ratio or focal length. Also, the photographic limiting magnitude, which determines the faintest stellar object that can be recorded photographically, depends on the aperture alone, with the condition that exposure time is held constant.
What you don't want to do when purchasing a telescope for deep space astrophotography, in my opinion, is limit the field of view with an excessively long focal length - this is the primary reason I think for not recommending cassegrains to beginning deep space astrophotographers. Yes, you can use a focal reducer on a cassegrain, but this introduces additional optical distortion and expense. And focal reducers are not practical for refractors and newtonian reflectors. With today's digital cameras, there are plenty of pixels available to allow cropping of images when photographing smaller objects, but photographing objects that don't fit in the field of view is a much more complex subject. It can be done for sure, as Hubble demonstrates well, but it adds a great deal of time and layers of complexity to the process.
My recommendation would be to go for a focal length in the range of 500mm to 1000mm, and as much aperture as you can handle. Approaching the task in this manner removes focal ratio from the discussion, but of course the focal ratio is still right there at the heart of the matter.
Don
