This is a good question
1. The larger the aperture the more photons you collect per unit time
2. The shorter the focal length the larger fov of a fixed pixel size in the imaging camera
3. The larger the aperture the smaller the diffraction limit of the scope which governs the smallest resolution. Small aperture of 80mm has about 1.7” resolution limit. A 127mm has about 1.1”. A 152mm has .92”. Some image with a 51mm which as a resolution limit of 2.7”. Note seeing limit is somewhere between 1” and 2” most places, seldom as low as 1”
4. We want to match the effective pixel size to the overall seeing of the atmosphere. Also pixel size (in arc seconds) should be matched to your guiding performance. In general we presume guiding has small errors and thus adjust our imaging train to optimize the pixel size to our atmosphere seeing. There is no need to sample the image more than twice as good as the seeing allows. Note I am assuming we have a scope for which the diffraction limit is about the same or smaller than the atmospheric seeing
5. Sooooo. We use a big aperture to shorten our exposures times. This relieves pressure for long guiding performance. It is also important if you are using narrowband filters to reduce those exposure times. We use a focal reducer to change the focal length to adjust the image scale of a camera pixel to better match it to the seeing and diffraction limit of the scope. In practice if you have about a 120mm or larger aperture your diffraction limit is not a concern. ES 127 is perfect.
6. If the scope has a long focal length, the pixel scale is small. This is a waste since your image resolution is limited by the seeing. It also limits your overall field of view unnecessarily. Thus for long focal length scopes we use focal reducers to optimize both our field of view and pixel scale to seeing limits.
7. Field flateners don’t do much to the resolution, but rather it corrects distortion at the edges of the lens field of view. Often flateners are included in the optics of focal reducers.
So these points address your question and add up to a set of parameters you have at your disposal to tune your image train to achieve your goals. Understand these considerations are relative to imaging deep space objects. Planetary imaging has a different set of considerations which would make another email.
In general I feel the sweet spot for imaging is about 900 mm fl. For DSLR cameras this yields pixel scales at about 1”. There is no reason to go to longer focal lengths. At 127mm you get enough photons to be able to expose at relatively low gain (iso 320) for 3 or 4 minutes for your subs and take nice images for many many DSO. If you can get a larger aperture with a focal length in this range (with or without a focal reducer) and it is light enough for your mount then larger is good. The exos2 ends up being able to handle a wide range of apertures as evidenced by the forum equipment lists.
For visual, aperture is king. Your eye needs photons.
Wes, Southport NC
EXos2-GT PMC-8, iExos 100
ES ED 127, 10" LX200GPS+wedge, Astro-Tech 8" Newt, ETX-90, 60mm no-name guide scope ~ 260mm FL
Polemaster, Orion ST-80 and SAG, ZWO 290MM, D5300 astro modified
Nina, Bootcamped Mac Mini control computer, RDP to iMAC
110 amp hour lead acid deep discharge battery for field power
Electrical Engineer, Retired