Dave, if it wasn’t that my eyes have a moderate astigmatism, I would not even wear glasses when viewing as otherwise I am only farsighted. That is my only concern with the eye relief. I do have a few basic eyepieces, but I really would like and intend to upgrade to something better without breaking the bank. I am comparing the Celestron X-cel LX, AT Paradigm Dual ED ( or Agena Starguider Dual ED), and the AT UWAs. I also currently have an Orion high power 2x 4 element Barlow. In a nutshell, this is what I am trying to decide on. Other than getting a optical finder scope ( likely a RACI), to replace the red dot which is impossible to use with it’s location on my scope and no available space to move it to because of the phone cradle on the top of the tube for the StarSense, I am all set to go! As far as choosing sizes, if I understand correctly, after factoring the focal length of the scope, I choose pieces based on the magnification that I want and the AFOV that I prefer. The only other thing that I don’t quite understand is what the difference is between a fast or slow scope? Not which ratios are which but rather what is the difference with the viewing? Wow, that was a mouthful, but it summarizes the only concerns or questions that I have at this point. Sorry for being so windy and hope the long post isn’t bad etiquette.
I was addressing the astigmatism that is present in wide-field eyepiece designs that don't have additional lens elements to correct for it, not any astigmatism present in your vision. Eyepieces like Erfles work fine in slow telescopes but not so great in fast ones.
Disregarding issues like coma, central obstruction, and chromatic aberration in refractors, images at the same magnification should appear about the same in fast and slow telescopes of the same aperture and optical quality.
While an eyepiece's focal length determines the optical system's magnification, the f/number is of little importance visually. A "fast" telescope implies a short focal length and a large field. Fast, however, is a term borrowed from photography (an f/5 telescope can take a photograph with one-fourth the exposure time of an f/10 instrument).
Visually, well-made fast and slow telescopes of the same aperture have no difference in image brightness or resolution. I find that photographers have the most difficulty understanding this concept, because their experience that a faster f/number means brighter images on film and in the viewfinder is so ingrained.
Myth #3: Faster telescopes show brighter images.
This is a misconception carried over from photographic use, where the fast f/ratios do mean brighter images and shorter exposures for extended objects. Telescopes with equal apertures and equal magnifications have the same visual image brightness, regardless of the objective's f/number.
Myth #4: Long-focal-ratio telescopes give higher-contrast images.
In general, refractors offer the potential for higher contrast because mirror coatings, by their nature, tend to scatter more light. But when comparing well-made, highly corrected refractors, there is no gain in contrast with instruments of long focal ratio.
Reflectors too, if well made and having the same size of secondary obstruction, will have the same contrast at the same magnification regardless of the f/ratio.