I will finish by restating what I have written above:
1) A medium focal length, ED refractor has considerable advantages over a long focal length one. It is possible to either use a lighter mount, or a larger size on a given mount. The larger size will show more. (...)
Peter, you are of course free to verbatim re-state your thoughts as much as you like.
The restating still does not transform them in universal binding Commandments.
Indeed, what you say it sounds true *to you* only for *you* have set *your* limit in *your own mind*.
Of course, being it your own limit, it is very true for you.
You see, I have a friend who absolutely hates standing at the telescope. Understandably enough: being a planetary and double-star observer his sessions at the eyepiece are several hours long.
Now, for him a Newton is a no-telescope, for he has to stand (or, anyhow, uncomfortably sit/stand on a ladder) to observe.
To him, say, a 20" Zambuto-powered Newton is worth nothing vis-à-vis, say, a 4-5" refractor or a C-8 or anything similar...
Would you agree? I guess not. For what it matters, I certainly would not.
BUT for him *this* is the truth. The Truth (capital T).
For him it is not a matter of price, or size, or weight. His key criterion is "I have to be comfortable when I observe". Full stop.
He is comfortable behind his refractors and the C8. Not so with the Newts (any brand, any size).
Is this a limit in the optics? Is the Newton a bad telescope? Not at all, of course. Rather the opposite.
But *for him* the Commandment is: "No Newton is a good telescope". Period.
Would you argue with this? Optically speaking of course yes.
But, as per his own comfortableness and choice, you simply cannot. Exactly for it is *his own* comfortableness and choice.
Given for granted his own legitimate preferences, what remains disconcerting is of course the capital T. Indeed.
Like every-time a capital letter is added to justify and reinforce what is nothing more than a (legittimate) personal preference.
Getting back to us (to you, actually), I wonder if you realise that you set a limit as well, i.e. your actual present mount (or the one you are ready to buy and/or you are comfortable with).
But this Limit (capital L) is not valid *per se*. It is just a limit you set for yourself.
Of course, this limit of yours could be shared by many others.
But for many others -- to me, for instance (and for many other like me) -- it simply does not apply
Mind you, it does not mean I do not have my own limit -- I do, of course (like anyone else).
Though it is not a Commandment, rather a mere practical usage limit.
In practice, when I am alone, my limit is trivially "What I can carry and mount all by myself".
And, according to my own limit, your statement is once more false.
Let me show you why.
As already mentioned above, this is my portable setup, i.e., this is the larger refractor I can mount all by myself: a TEC 200 f/9 on a Titan mount.
Actually this is an old picture I have at hand: now tripod and telescope are the same, but I've shifted first to an AP1200 (larger worm wheels both in RA and Dec) and then to an old HGM-200 (larger Dec worm wheel than AP1200).
Of course I could further improve the situation with an AP1600 or anything similar, but the HGM-200 has *to me* some advantages (that is of no utility to mention here); so, at present, I am fine with that.
Tomorrow, who knows.
My aim remains to improve the quality of my observing, which means the mount HAS to be transparent and let me observe at 600-800x with minimal (possibly zero) vibrations.
Thus I do not want to save weight on the mount, rather I wish to have it as big as possible -- of course within the limit of my own capabilities.
As per my own capabilities, for instance, an AP 3600 is out of the question -- out of *MY* question -- as per the portable setup I can load on the 4WD, bring with me at remote locations, mount and then unmount.
You might have noticed that, in all this mumbling, I am NOT talking at all about the telescope. That was clear already: the largest availableI can mount all by myself.
Now, in accordance with your statement, you would suggest that, to improve the stability even further I could shift to a shorter-focus "better-glass" instrument.
But, nop, that won't work. And I tell you why.
Yes, I could get myself even, say, a 200 f/7 (FPL53, CaF2, OK4, Unobtanium... whatever). Yes it's shorter an lighter. Maybe I'd save 5-6 kgs.
Who cares? I don't.
That is a better photographic instrument but visually it would be worst, with a lower level of correction of all possible aberrations, from CA to spherochromatism.
After I drive for hours and unload and mount the setup you have seen, sorry, I do not want to set for a lower level of correction, just to save a few kgs (that I know I can handle in any case).
Or, I could get a FPL55 or CaF2 f/8 version which could be just barely shorter, save me some 3-4 kgs, and would slightly improve the final correction of about 7% below 480 nanometers...
Have been there: when you also sum up the different behaviour of the whole optical system when getting down from f/9 to f/8, the gap is even reduced.
The difference in weight and torque is insignificant (both to me and to that mount).
Visually, the difference in the whites is like having # FFFFFF instead of # FFFFFD (using Hex Codes). Let's be serious...
And this is between CaF2 and a "lesser" (but extremely high quality) FPL51-flavoured objective.
Now, if instead of being f/9, that instrument would be f/11, there yes I would be ready to shift to a 20% longer instrument, which would be certainly a bit heavier (but still within my own capabilities), still properly manageable for my mounts, and have significant (to me) improvement on the final correction (i.e. not 5-7% but rather a more substantive 20-22%, which is 3-4 times more!). In that case, yes, *I* would be ready to accept the added hassle for the benefit. But unfortunately, something like that is today not available and has to be custom-ordered (which can be done, of course, if top manufacturers would not be so busy in producing photographic instruments).
You see? The criterion I apply here is personal and clearly stated as such.
And, incidentally, it si exactly opposite to yours. Not to mention, mathematically and physically explained in details.
And if you suggest, instead a 9" f/7 -- i.e., in accordance with your statement, "shorter but larger" -- that has two huge limitations in this case that would make it not viable:
A. it cannot be done at a decent level of correction (i.e., in that diameter, short f/ ratio instruments have so many residual aberrations that pose actual manufacturing limits);
B. it would be at least 15 kgs heavier -- THAT I know I can still move around, but certainly CANNOT mount and unmount late at night all by myself (not just "comfortably", but "in safety").
So, you see, all this to plainly show you that your supposedly universal statement (that you keep restating restating restating) is not so universal in the end, but it varies according to the preferences/choices of each of us.
To me, for instance, does not apply for the reasons above.
Therefore it cannot be generalized.
It is as simple as it sounds.
And it requires no repeated statements using capital T, C, L or other letters to be explained.
Just plain reasoning.