A while back Ales and Mike discussed the low level of posting here.
So you can blame them for this:
Well, I'm a little late to the party but the OP raises a fascinating question:
Why 7" instead of 6" ? The optical advantages may not amount to much in some minds.
To my mind the 7" [180mm] is a remarkable sweet spot from many different points of view.
Perhaps the question should really be: "Why not a 7" instead of an 8"?"
The 7" is not remotely as large as an 8". A 7" f:12 is still only 7' long.
That's only a head height taller than a Hollywood hero in his socks.
It will also fit under most ceilings and go through doorways.
The dewshield should be removable for storage and the big carry out to the mounting.
Make the dewshield a decent length for your climate and you can put it back on outside.
Make it ultra-light from black, mottled, self curling plastic, like I do.
This will really help your OTA balance nicely and, more importantly, greatly reduce its moment arm!
All this makes the 7" easier to store, easier to handle and very much easier to mount than an 8".
The step up from the 6" classical refractor to the 7" is not really that huge.
It is manageable and the 7" remains human in scale up to about f:12.
The 7" is just manageable in both size and weight and can be carried by one person.
It can be lifted onto an equatorial mounting about 7' high into the open rings with the dec axis horizontal and locked.
A pair of well spaced and sturdy U-handles make a real difference to your lifting power and fine control.
Remember to space them where the rings won't intrude.
Unless you make your own equatorial mountings the cost of supporting an 8" equatorially is a really massive and costly step up.
The sheer bulk and weight of an f:12 or f:15 8" achromatic refractor is something you'd have to mock up to really understand.
Try visiting an outlet which sells big drainage pipes and take along your tape measure.
The really big pipes are usually stored outside in the yard. There's a definite clue there.
Try lifting one of those PVC monsters. Now do you believe me?
Consider the greater diameter of the tube to house an 8" lens and its much greater length compared with a 7".
A 9" minimum diameter or [much better] a 10" tube leaves room for internal airflow around your edge-relieved, ventilated baffles.
While an 8" diameter tube leaves no room for even the thinnest baffles at the business end of the 8".
There's hardly room to lay on some flocking just behind the lens. That will cost you contrast.
An 8" f:12 is a foot longer than the 7" f:12 and two feet longer than a 6".
An 8" f:15 is
much longer than the 7" and
very much longer than the 6".
You do the maths.
The weight of the bare, 10" alloy tube alone adds to the much greater moment of the much larger and heavier 8" lens in its cell.
The classical 8" is a two man job to carry out to the mounting.
Unless you can mount it permanently you'll need a strong and willing friend
and two stepladders every time you want to observe.
With the 7" you can just carry the OTA out and start observing alone when the whim takes you.
Anything which gets between you and observing is a potential hurdle to not observing at all.
The CA ratio of even the longest achromats, in the 8" size, puts them in a more "colourful" range than the 7".
CA is
aperture and focal length dependent: NOT just focal length dependent.
With the 8" you ought to be seriously thinking about that f:15 rather than the f:12.
The 7" falls in a much better range of colour correction and is quite happy at f:12.
The 8" refractor not only needs a much bigger mounting but a much taller pier.
A taller pier needs to be much more sturdy with a massive concrete foundation.
Think heavy wall, steel pipe, the diameter of your lens and going down a yard and a half for a rough guide.
Remember that the eyepiece has to clear the ground with enough room for you to reach it comfortably.
Thinking you can get away with a tripod with an 8" f:15? You'd better start ordering some lengths of 2x6.
Then consider how to attach them so firmly that they won't twist at the head where they join the mounting.
Those 2"x6" legs had better be painted white so you don't fall over them in the dark.
We're really talking "War of the Worlds" tripod scale here!
The 10' length of the 8" f:15 will enjoy considerable moment with that big chunk of glass up at the far end.
Need more ground clearance and a "prettier" balance point?
Then you'd better start adding counterweights at the focuser end.
But then, just as you were screwing them on, it clouds over and starts raining.
You now have to carry a wet or icy 10' tube 10" in diameter back indoors? Really?
All those hurdles between you and your observing time with the 8" are beginning fall to your unintentionally, clumsy steps.
With the 7" you just set the tube horizontal. Lock up the dec axis, open the rings and walk away with your OTA.
The clever TCR design offers an escape from some of your weight problems and certainly offers lots of useful handles.
It's just that you can't yet get a TCR in 8" f:12 or f:15.
Perhaps iStar should offer the bare rings as kits for the different lens diameters and you fit your own rails?
The Rx lenses offer a way out too. You can go shorter F/L without your views getting too "Kaleidoscopic."
You could mount almost any refractor in a sturdy, offset fork, with Dobsonian bearings if you can just lift the tube.... eek... vertical.
I built one back in the 70s for my classical f:15 5" refractor and it was superb away from the zenith.
The mounting still needs to be on a really sturdy post or pier though, or it will literally shake like a jelly.
Since the Dobsonian reflector was invented a lot of really clever minds have applied themselves to its design.
The refractor has yet to enjoy such fierce attention for private pride and practicality, star party competition or commercial advantage.
The classical refractor has been stuck in a strange, 19th century time warp, usually under a very large and very expensive dome.
The TCR, Rx and iStar have certainly begun on the long path to the level of perfection we now see in many large Dobs.
I just hope we don't have to wait for the application of human ingenuity, over a similar time period, to enjoy really practical refractors.
Meanwhile, for those of us denied an APO by sheer expense, we can always choose a 7" achromatic refractor.
Mike
Surely you live the dream, while I only dream 
. 
