Big refractor? Fair warning 2.

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Unless you lie flat on a camping mat on the wet lawn you can't usually reach the eyepiece of a modern refractor. Not even with a star diagonal once the telescope tube is vertical. There is a minimum comfortable height of eyepiece for most people even if you lean over. A personal equation depending on your height, build and fitness.

This minimum eyepiece height has to be added to the lower half of the OTA hanging below the massive mounting. Lets assume a normal dining chair height for your personal comfort when viewing vertically overhead. Now measure your eye level above the ground or floor.

Don't cheat! Or it will be a rod for your own back. Now add the bottom half of your OTA's length and you have a minimum figure for pier height on which to plop your heavy mounting. Crane hire, anybody?

6" f/15 = 90/2 = 45" + eye height when seated. 7" x 15 = 105." 8 x15 = 120". That's a ten foot tall tube in old money. Half the tube is 5' + eye height [comfortably seated] is the minimum pier height for viewing overhead with a star diagonal. Your pier should be at least 8" diameter, thick wall tube if it is not to waver. A pier tube of the same diameter as the lens is usually a good idea.

And, no, you can't cheat and use a "faster" f/ratio as the lens gets bigger. In fact you really should go much longer to minimize false colour with an achromat.

It gets much worse with increasing objective diameter and not in a good way. Why do you think they made classical refractors in f/18, f/20 and much longer still? It was not the lack of some magical modern glass. It was basic optics. The science hasn't improved your chances of success over time.

Conrady and Sidgwick are your new friends to avoid certain disappointment. Green means go. Yellow means stop and think seriously before proceeding. Red means you are running a red light while delirious and blindfolded. Your planned kaleidoscope refractor will be dead on arrival and is not to be resuscitated. 👎

If larger APOs weren't so hideously expensive they might be a far more logical choice than an f/20 8".  Even though the large APO objective will be very heavy the much shorter OTA could actually save some money on premium mountings and towering piers for your kaleidoscope big refractor. Aperture counts more than length so just build or buy a handy sized reflector with a quality mirror. 

If you think a cheap, flat roofed, roll-off-roof observatory [ROR] will house your big, shiny new, classical refractor you might be in for a nasty shock. The roof height to cover your ridiculously tall pier, mounting and precious [horizontally parked] OTA might just need a steeplejack.

The roof will probably have to be a pitched but without any cross struts to reinforce your triangles. The roof structure has to safely clear the OTA when it is parked. Plywood triangles with U-shaped cutouts for the horizontal OTA might work. Though at a very high price for materials and weight compared with a conventional timber construction.

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