Warning: Sheer size and weight of a 7" f/12 refractor and its mounting.

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An interest is being shown in ED lenses from iStar in the 160mm and 180mm apertures at f/9 and f/11.

As a favour to those who are becoming round eyed at the prospect of an affordable,  highly desirable, long focus, semi-Apo-ED OTA I offer the following images and advice:

Here is the reality: I am only an inch, or so, shy of 6' in my scruffy ATM clothing. Note the industrial safety boots with steel toecaps for handling large and heavy objects more safely.

The cords on the OTA are to stop it slipping down through the rings. No affordable 8" tube rings were available. So I tried to pack larger rings with closed cell foam strips. It didn't work no matter how tight I made the [sticky] sponge fit. The cords were made as Prussic loops and worked after a fashion. I replaced the foam with birch plywood packing rings and this works well. Though now the tube rings each weigh several pounds extra! At least they will look the part when finally painted.

The massive [several hundred pound] welded steel pier is about as high as I am before the hefty, 100lb Fullerscopes MkIV mounting is perched on top.  This old mounting has seriously large castings, 1.25" solid steel shafts and 7" wormwheels. It dwarfs modern Chinese mountings. The heavy, webbed saddle casting is over 2' long.

The OTA needs over 50lbs of counterweights just to balance it. Note the large wheels and jacks just to move the whole thing about to see past local trees and hedges. A task requiring great patience and care to avoid tipping due to the high center of gravity!

The OTA is built around a very thin, steel, 8" diameter [seamed ventilation duct] tube. The basic OTA weighs over 50lbs without additional guide telescopes or finders. Nor even a heavier or larger focuser like Feather Touch at ~5lbs. Think steel is heavy? An 8" aluminium tube would be even heavier at a minimum 3mm [1/8"] thick wall necessary in this size of OTA. Long tubes go oval where they bend unless reinforced by sturdy baffle rings.

I need that 3' industrial stepladder [top image] just to reach the open mounting rings once I have lifted the huge and heavy OTA into place. I use a spare tube ring to "hook" the OTA temporarily in place while I perform the double climb. This all takes place with the telescope pointing at the Pole Star. Others have suggested starting with the tube horizontal or diagonal but I have never managed the huge lift.

Leaving the rings on the mounting saves the weight of carrying them about. Not to mention fitting a vital and very substantial dovetail required for an OTA of this sheer size and weight.  Is there a really heavy duty dovetail a couple of feet long? Now imagine the loads on the dovetail fitting on the mounting with the huge leverage applied by a ten pound objective on the far end of that 8' long tube. I prefer to place my trust in a really solid saddle.


With the full [and usually very necessary] dewshield in place the OTA reaches well over 11' into the air. My large and heavy mounting is totally inadequate for such a large and long OTA! Do you think an EQ6 would do any better?

The OTA/mounting and pier allow comfortably seated observing of objects overhead from a standard, dining chair, seat height when using a star diagonal. Now think about how far the eyepiece moves as object altitude or azimuth change. Both ends of the OTA sweep out huge circles as it covers the sky. This thing is never going to fit in an affordable dome under 3m or 10' in diameter.

Remember how difficult it was for major manufacturers to mount those "dinky" little 6" f/8 refractors? Wobbling all over the place in a breeze and impossible to use above 45 degrees without sitting on the wet grass or in the even wetter and colder snow? Now imagine what it is like to mount an 8" diameter tube 4' longer and still be able to get under it to look upwards.

Think you'll optically 'fold' the OTA with a couple of [very expensive, guaranteed 1/20th wave 5" and 4" ] precision optical flats? I thought that too. Sadly, the folded skeleton OTA weighs as much as the straight tubed OTA. Just lifting it on and off the mounting means re-collimation. [Every time.] At least the straight tube can be stood on its 'nose' for storage. Though most domestic ceilings will be too low! I have no ceiling in my shed so the focuser fits right up above the bare joists.

I am a strong advocate for bayonet fitting objectives. Removing the objective in its cell makes life so much easier. You can save 10lbs of extra weight when carrying and loading the huge and heavy OTA in and out of storage. Lifting the OTA onto the mounting is much easier but still reminds me of "tossing the caber." A Scottish traditional sport where a strong man throws a telegraph pole in the air after balancing it upright in his bare hands. I haven't been tempted to try throwing it yet. Though I have come so close to dropping it several times that I should be having nightmares!

Here are most of my refractors posing in my trailer. I have arrowed the popular [black] 6" f/8, including its standard dewshield, as it lies next to the completely bare 7" f/12 tube.

Do you still think of the 6" f/8 as a large and hefty OTA?  The 6" f/8 is absolutely tiny in comparison with the 7" f/12! Note the bare 5" f/15 tube nearer the camera. Imagine how difficult it would be to mount a 6" f/15 which would not even remotely fit in my trailer!
 

Click on any image for an enlargement.