Big refractor? Fair warning 3.

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A commercial dome, to house your big refractor, is going to cost an absolute fortune! Unless, of course, you have a year or two, the skill and the bottomless purse to put a wooden [or metal] dome together yourself. My 3m [10'] dome is only just big enough for my 7" F/12 with careful positioning. A 7" f/15 would be FAR TOO LONG to fit in my 10' dome. So would a 6" f/15! So be warned!

Remember that a 7" is not even a large aperture telescope. A decent, optimized, 8" Newtonian might be just as good visually most of the time. A 10" almost certainly will be. 

Don't forget to factor in the price of a team of oxen to pull your wooden dome round. Because your leaky, white elephant [sic] will be far, far heavier than you ever planned. Or desperately hoped in your wildest, most optimistic dreams. Just think of the vet's fees and winter fodder for your beasts of burden. Or electricity for the heavy duty motor to drive it.

And no, you can't cheat with a carbon fiber refractor tube. Nor any other of the "magical" modern materials. All the weight of a refractor OTA is in the objective lens. All practical telescope tubes weigh very much the same. Only the stiffness varies, depending on what you can afford. Forget PVC tubes because they WILL bend. Or will soon will if  not already bent before you get it home.

Adding a large, FeatherTouch focuser to balance your heavy objective lens might be a good idea if luck is perpetually on your side. Which means you can afford the hideous expense. I wouldn't want to carry any large and heavy refractor with that much invested in just looking though the thing. A lead weight would be considerably cheaper and far less vulnerable. Particularly when you have to lift the whole thing almost vertically, above your head and onto the mounting unaided. And then back down later in the dark. When everything is quite literally covered in thick and slippery ice! Or were you actually planning to rest your $1000+ FeatherTouch on the muddy lawn? So you can have a desperately needed rest every few steps on your way back to your cramped storage facilities? 

Optically folding a refractor with flat mirrors is very expensive, awkward, fiddly to baffle properly and as heavy, or heavier, than a long, straight tube. You probably won't be able to cover your costs when you get fed up with it.

If it isn't permanently mounted it will be a dog to keep in collimation and no easier to use than the "factory chimney" it replaced. The expensive, flat mirrors will get filthy and will dew up if you are daft enough to leave them exposed in an open framework [like mine]. Capping them securely will help when not actively in use. Just make sure nobody touches the mirrors while the telescope is in storage. Hopefully you don't do any serious woodwork in your telescope storage facility?

The best telescope [you own] is the one which gets used most frequently. From personal experience that means one which is small enough. Or one set up permanently and ready to use at a whim moment's notice.

Comparisons are odious? This is the standard dewshield of a Vixen 90mm f/11 sitting beside the 10" diameter dewshield of the 7" f/12. The 2" Vixen focuser backplate fits the 8" diameter tube of the 7" refactor.

This is a doubling of aperture from a rather modest 3.5" refractor to an equally modest 7". The huge change in scale should help to open your eyes to the reality of owning bigger refractors.

As a footnote: iStar is one of the very few businesses still making objective lenses for private customers. They now offer only "shorter" focal lengths because they get so few requests for standard [classical focal length] objective lenses. They came up with a workaround for this by using heavy flint glass to reduce colour aberrations. My own R35[%] 7" f/12 is supposed to have a correction equivalent to a lens with f/16 focal ratio.

Not having a 7" f/16 to compare the two side by side I must take their word for it. There is certainly very little false colour on the limb of the Moon. It consistently holds double the power of my 6" f/8 Celestron CR150HD. Which I could only very rarely push above 120x to perhaps 150x once a year.

It was quite a revelation to discover that my constantly poor planetary views were not poor local seeing but rather the quality of the objective lens in the Celestron. How else do you explain being able to regularly use over 300x in the iStar?

The view from the ground is very restricted in my back yard/garden because of all the mature trees and ridiculously tall hedges. None of my neighbours has ever shown the slightest interest in clipping our shared hedges. So I had to do all the thankless work. These boundary hedges are supposed to be kept under 2.5 meters but are now over 7-8m high!

Hence the raised observatory. It will only be a matter of time before the hedges win again. The unfinished dome is 20' high in this image taken from the top of the low hill to the east looking west.

 I soon became involved in the building of my observatory so I have done very little observing with either instrument. More often than not they were only half finished or under constant modification.

That said, I was able to instantly see tiny craters in Plato with my 10" F/8 mirror which were always a struggle in the 7" lens. There were not simultaneous views as seeing and [Lunar] lighting might have been the important factors.

I cannot be certain that either instrument was ever perfectly collimated. It was a real struggle lifting down these big and heavy OTAs. Down from the high [above head height mounting and putting them back up again. Not to mention the equal struggle to put them safely into and back out of storage. Which had lower [bare] ceilings joists than their own length. This is not the ideal scenario for maintaining optical consistency! BTW> I'm no wimp despite my age.

D&G used to say that f/12 was very similar to f/15 if length mattered to the buyer. They no longer produce achromatic lenses for amateur customers. Though their lenses have a consistently fine reputation. They were standard achromats so displayed exactly the same colour correction as any other lens of the same f/ratio. Purchaser bias and aging eyes are excellent filters for violet halos.

The message I am trying to get across is that bigger refractors are wonderfully impressive. They do however have limitations. There are those who claim their best views ever [usually of planets] were through a fine refractor.

Hopefully the instruments involved were well and permanently mounted. Such that they could be used often. Rather than demanding enormous willpower to lift a huge and awkward load onto a high mounting. A mounting which can manage a 15" reflector will often struggle with the long tube of a refractor of only half the aperture. Housing the large refractor is a major hurdle to satisfactory and satisfying ownership. Imagine the huge wind drag of a horizontally mounted refractor covered in a tarpaulin!

There are easily available 'props' to judge the scale and weight of any planned refractor purchase or build. Just visit a builder's merchant or big shed DIY outlet which stocks large PVC drainage pipes. Remember to add at least an inch to the objective aperture for the main tube. Otherwise it will be impossible to baffle effectively against grazing incidence just behind the lens. These drainage pipes often come in useful lengths and weights to practice your refractor lifting skills and your own fitness for purpose. Just remember: If you drop it and break it. You pay for it! 

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