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Further online research into piers suggests that solid wooden piers are heavy but damp vibrations well. Clinging to my massive steel pier may offer solidity but makes its movement a vital part of every observing session. A square, removable pier would need a deep, square hole lined with a close fitting square pipe to be worth setting up each time. Probably a 6"x6" minimum size of timber would be required. Available from stock in sizes up to 8" x 8" square in oak locally. No doubt larger sections are possible to order. Unseasoned oak is 65lbs per cu/ft. So a 6" square oak post x 12' long weighs very nearly 200lbs! Eeek! There will be no Dorpat replicas on my lists of things to do, then. Though there is always pressure treated larch if I do decide on a removable post and moving it between various sites. That raised platform is beginning to look well worth building!
Curiosity
got the better of me so I set up a builder's step ladder. An iron bar
[the fulcrum] was placed across the rungs to bring the ladder close to the MkIV pier
and mounting. A cord with a Prussic loop was fixed around the pier and a long pole
passed through the tail loop and over the iron bar. My
vintage Salter scale read 14kg of down force on the far end of the bar with
a 5.25:1 lever ratio [210:40cm] between moment arms. Suggesting that
the MkIV on its pier weighs roughly 70kg, 154lbs or 11 stones in old
money.
From the struggle I had lifting it bodily I have
always imagined the pier and mounting together weighed considerably
more. So, the upshot of basic mechanics is that I need at least a
6:1 lever ratio to be able to lift the pier on my intended "fork lift
truck". The pier ideally needs a fairly high crossbar to allow a secure
fixing on the short arms of the lever above the center of gravity of the
pier/mounting.By sheer coincidence I had a perfect axle amongst the scrap metal for my experimental barrow. Now I just need suitably long arms to make my strange fork lift device into hideous reality. The rest can probably be thrown together from slotted angle iron. Even if the pier is never moved in anger at least the lawn will be better mown thanks to moving the pier occasionally. The only major difficulty I foresee is ensuring a decent lever ratio without the wheels colliding with the pier legs.
Further experimentation suggested that the pier could be towed around if the wheel axle was placed under one of the legs. Though it is not an ideal arrangement at present.The axle has to be restrained from twisting and rolling towards the end of the leg. The small castors provided some stability when the structure wobbled on uneven ground. This is certainly the first time I have been able to move the pier without an exhausting struggle. The castors are far too small and useless on anything less than smooth concrete. While the big fat tires roll effortlessly on grass or gravel. Far better, in fact, than the planned, puncture free, wheelbarrow tires.
The towing bar could be passed through large galvanized eye bolts [or exhaust clamps?] fixed through the leg for security. With a security pin dropped through a drilled hole in the pipe to stop the pipe pulling straight out. The cord lashing was just used for the experiment to see how well it went. An inverted channel section fixed beneath the leg could restrain the axle. Though the channel would need to be considerably extended and braced to stop the axle twisting around on the pier. The large hub flanges, welded onto the axle, do not lend themselves to being passed through a hole bored through the pier leg. That would have been the easy way out.
Despite these difficulties I would like to try a larger refractor on the MkIV mounting. Just to see how well it copes. The MkIV's limitations are its 40mm 1.25" shafts, plain bearings and the 6" wormwheels. The pier, thanks to its massive construction, is certainly adequate to the task. BUt is its own Achilles heel. The sheer weight makes it anything but portable. On open site it wold be perfect. Where high hedges and lots of trees obscure much of the sky it is a disaster! It should not be forgotten that any mounting is a combination of its own qualities and its means of support. OTA length, as well as its weight, is always a critical factor. I started a project on a 16" Dobsonian decades ago. I had a nice figure on the glass at f/5 but the PVC tube was so large it would not fit into my limited storage space. Regrinding the mirror blank to make it shorter focus proved that faster mirrors are far more difficult to figure. Particularly when using plate glass.
No doubt the amateur astronomical world has long been populated by such overambitious white elephants. With weight and moment arm rising rapidly it is no wonder that larger refractors have never become commonplace. The rapidly rising cost of larger apertures does not quite match that of Apochromats but has always been a major factor. The optical glass is increasingly costly the moment any attempt is made to use anything non-standard. The sheer size of the OTA makes storage, handling and mounting a nightmare. Even if the long tube were made of cobwebs the lens and focuser have their own weight toll with rising aperture. You can't just throw a tarpaulin over a long OTA on a very tall pier. It is no wonder such instruments are few and far between and most are permanently housed.
As always, the relatively low demand for something is reflected in the small number of providers. The economies of scale in manufacturing never quite provide cheap enough products to compete with the reflector. Buyers will pay a fortune for a tiny APO but will baulk at the cost of an achromat. Those with a long enough memory, or telescope history books, will know that only 50 years ago the reflector suffered from exactly the same problems. It was a heavy beast of limited aperture and very expensive to buy from only a few producers. Chinese manufacturing and the Dobsonian design changed all that. Rapid progress towards lighter and more portable, ever large apertures continues in reflectors. Sadly the same cannot be generally said for achromatic doublet refractors.
Click on any image for an enlargement.
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