a 400mm focal length, three inch diameter rich field achromatic refractor on altazimuth mount will be used with far greater frequency than a 24 inch dobson mounted newtonian. Small scopes give larger fields of view. Our 400mm focal length scope - using an eyepiece that admits light to the full 7mm of pupil size - can be used at 11x. At 11x, an eyepiece stopped to 60 degrees of apparent field, can pull some 5 angular degrees of sky into the retina at one time. Meanwhile, our 24 inch scope must bear a magnification of 87x to ensure that all the light collected by the 24 inch primary fits through that same 7mm eye pupil. Because of this, only 41 arc minutes or roughly 2/3rds of one degree of sky can be seen fully illuminated in most common eyepieces.
As more and more of the retina receives an image, flaws in the eye (astigmatisms) may become problematic. For some, below 10x per inch aperture, bright stars no longer appear perfect points. In fact they flare astigmatically while passing through focus. Thus to see perfect stars through a 24 inch scope, some 288x is needed. This reduces the field of view to 12.5 arc-minutes!
a certain minimum exit pupil is needed to show stars against a “dark” background sky. 0.5x per millimeter of aperture is the minimum magnification needed to experience this under average skies (where stars can be seen unaided down to magnitude 5.0 direct). Thus, our 24 inch scope must be used either under the darkest or at 300x plus under average skies to get a sense of skydark in the eyepiece. Meanwhile, the 3“ accomplishes this same aesthetic at a mere 40x and 6” at 70x.
Larger apertures also collect more atmospherically aberrated light and make this type aberrations more apparent in the eyepiece. Generally, the 3“ is able to experience 8/10 conditions where 6” experiences 7/10 skies. Thus, a rule of thumb might be that if a 3 inch scope has 9/10 seeing once every two weeks. A 6 inch scope may have 8/10 seeing that often. On that same night a high quality 12 inch scope might get 7/10 stability. While a 24 inch scope would, at best, have 6/10 seeing. At such an aperture, 8/10 seeing - even if the optics could support it - might be experienced only once or twice a year. Is it any wonder that the world's “great telescopes” were installed in locations where only the very finest seeing conditions prevail?
this is why scopists, are attracted to smaller apertures. High quality scopes in the 3 to 6 inch aperture range can be found relatively inexpensively and the kind of skies needed to enjoy them come with far greater frequency.
There is one area however, where a deepsky 3“ is valued - this lies in the realm of large open clusters, a few nearby galaxies, and certain over large, bright and obscuration nebulae. For here it is essential to get the “wide field” perspective - To gather as much of the night sky in the field of view as possible. And as we have seen, larger apertures and especially longer focal length scopes are disadvantaged when it comes to studies of this kind… It is in this area that the 3” is without peer, while 6“ must go wanting. For neither perfect collimation, nor absolute color correction is needed to enjoy the Pleiades, Beehive, The Two Great Nebulae nor The Cygnus Veil Complex.
8” of aperture provides the additional light grasp needed to enjoy every class of celestial studies. There are no exceptions here. Face on galaxies and extended bright nebulosity are all susceptible to a quality 8“ scope. But unfortunately, few 8” scopes possess the optical quality needed to give the kinds of views that a 6“ MCT gives of Moon, planets and disparate double stars. (These are doubles whose magnitudinal delta is 1.5 or greater and are found are at or near Dawes limit of resolution.) But such scopes do exist (or can be made possible by simply collimating the optical train to the highest degree of perfection possible). Even though 8” of aperture is ideal for all classes of studies, the difference between 8“ and 6” - even 10“ and 6” is not enough to justify acquiring a larger scope in this aperture range. Rare is the night sky when 12“ of aperture can be put to full advantage on Moon and planets.