This is only true if the seeing conditions limit the size of a larger refractor to not reach is potential.
Seeing conditions are by far the most important factor in the quality of any refractor or any telescopes performance, not how great it's optics are. Again, as I always do, I turn to Jupiter, the most difficult object to scrutinize in searching for optical performance.
On nights of near perfect to perfect seeing conditions, bigger always performs better. One look at Jupiter with a 4" vs a 6" APO clearly shows why bigger is better. The dark colors are darker, the light colors are lighter, much more details become obvious on the surface details. Colors become more obvious as apertures increase. Salmon and blue colors come through nicely.
It will no longer be about sharpness, or magnification, it will be about "details."
Little 4" refractors are excellent for back yard fun. They are more forgiving in less then Ideal seeing conditons, and of course excellent in excellent conditions.
Under very good to excellent seeing conditions, a large APO will give a crushing blow to any 4" refractor, regardless of the name on the label.
Try spending a full night observing the GRS transit across the entire Jovian surface. It takes a few hrs. Using a 6" APO for example, in the course of those two hrs, you can watch the entire surface dance and move, not just rotate. You can watch festoons change shape. You can't see this visually in a 4" refractor, there's not enough resolution to see these types of details. In a 4" refractor, you hit a point were more magnification just becomes a bigger number, but the details seen don't increase as magnification increases. You may top out at 300x and hold onto the details, but if the seeing conditions are excellent, you can push the magnification to 400x and above, with amazing details on the larger scopes. These are definitely rare days, but they happen every year. On days like this, you wake up your family to see what you see also. You stay up all night and take the day off if you can, since they are fun times.
Everything where more aperture wins, a larger refractor will win, when the playing field is equalized, when the seeing conditions allow the larger scope to perform to its size.
By the middle of winter, we can usually convince ourselves that the small scopes we can still use in winter are really all we need. They are untouchable.
Once the weather warms up and we start getting out more, the big scopes start to come out and with a sigh of relieve, we remember why we bought them and continue to own them and want them.
On predicted nights of excellent seeing conditions, a large quality reflector will also give the death blow to any 6" APO also on almost everything you can look at.
When the playing field is equal and the seeing conditions are there, when it maters, the small scopes can try, but they cant keep up, and its really not that close also.
Despite its apparent limited resolution, a good 4" refractor can often keep pace with larger aperture scopes because of a quality that refractors tend to excell at, and that is definition. Resolution and contrast are important but definition is paramount in a planetary scope, and optical quality is a determining factor. Resolution is effectively set by the aperture, but with planetary detail the resolution limit goes well beyond that set for stellar point sources. As regards contrast - well that can be increased with the use of increased magnification - most obviously noticed when observing deep sky. The use of good eyepieces play an important roll in getting the best out of all three qualities.
As regards longer focal lengths being best for planetary, well I think this stems from the era of the the achromats, where the longer scope controlled the colour spread better and so gave the superior definition. Today there's no real need for long focal length refractors as colour spread can be matched or even superseded using modern ED glasses. Using an F7 refractor with a 2X or 3X barlow will allow a longer focal length eyepiece to be used for high magnification views, retaining the comfort and eye relief of the eyepiece.
For a time I used a Takahashi 1.6X extender Q amplifier with my Tak FC100DC, giving me an F11.8 focal ratio. The views were simply outstanding in that the Q was invisible in the light path and provided a perfect view. On one evening when I was observing the Moon with the Q attached, and at the same time using a 2X barlow in my binoviewer with paired 16.8mm orthoscopics, I swept across the appenine mountains. As I crossed the cliff edge of the mountains in 3D, my stomach rolled and I gave out a slight cry, until I realised I wasn't actually going to fall. However, when removing the Q, the textbook star images of the FC100DC and its superlative planetary definition remaind the same. The only real advantage for me was that the Q allowed the use of very higher powers, especially in conjunction with a barlow. I now only use my scope at its native F7.4, but with a 2X barlow in my binoviewer. At F7.4 I get outstanding planetary views, yet i have the added advantage of having a beautiful rich field refractor for sweeping the milkyway.
Interestingly, I also have a 10" F6.3 Dob which gives great planetary views, but I still prefer using my refractor for planetary observation.
Edited by aa6ww, 13 January 2020 - 04:46 PM.