12.12.2020 Another look at farmer's domes?

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Saturday 12th Cold and heavily overcast again. 

As I wait for the new screws, to continue my rebuild of the RA drive system, I am seriously considering new dome options. The plywood dome is leaking like a sieve in rain through the seams and it is the middle of winter. Which excludes the warm and dry conditions necessary for fibre glassing the dome. Which I don't fancy doing anyway at the heights involved. I'd have to hang off a cantilevered ladder for much of the work.

Commercial domes get into very silly money once they exceed the 2.7m Pulsar. The Astrodome 3m "onion" just doesn't appeal on my raised situation.  It would look just like a giant lollipop!

Talking of GRP: When I was still considering building a dome in the past I had flirted with the idea of using a "calf rearing igloo." The weight and sheer size had put me off at the time. 4.4m diameter x 220kg was beyond my [imagined] capacity back then. That's 14'6" x 480lbs in Ye Olde Money!

I was 70 and had long been suffering from a niggling bad back. Though I still went ahead with the solo build of my two storey observatory. 160lbs each for the three [igloo] dome segments seemed an awful lot to handle alone.  History records that I still managed some heavy lifts using opposing stepladders and winches when needed. 

In hindsight I could have had the larger dome modified and finished in far shorter time than designing a trapezoid panel dome from scratch. Moreover it would have been waterproof. And, it would have cleared the full length dewshield of my 7" f/12 refractor. I have never been able to use the full dewshield without hitting the dome. Or having the eyepiece scrape along the dome floorboards.

Changing these farming igloos into observatory domes is really rather small beer. Particularly after my practice with the plywood dome. At least it is in comparison with multiple plywood panels over ribs. It does, however, still require warm weather for fibre-glassing plywood reinforcements into place.

The igloo would need a plywood base ring and reinforcing, plywood ribs for an observation slit up to the zenith. The slit would be cut out of the broad, middle segment. A cross bar, joining these ribs would stiffen things up nicely at the zenith. 

Fibre-glassing the ribs into place would ensure water tightness and increased strength. I would probably add ribs to the igloo's horizontal joints up as far as the zenith. These too would be bonded in to spread the loads. I might want to add a telescope winch at the zenith and the observation slit might weaken the dome slightly. They have a central lifting hook for farming purposes. So are amply strong until I saw great big bits out of them.

I am imagining bi-parting shutters. Not an up-and-over shutter. Bi-parting shutters need only heavy duty drawer slides, top and bottom. Plus plywood ribs to close against. There is a semicircular doorway 1.4m high x 2.2m wide in these igloos. Hence the name, I suppose. The doorway would be largely lost behind the shutters as the lower observation slit. A couple of plywood panels could easily close off any remaining open areas either side of the closed shutters.  Or I could mould new section on the dome itself.

I would double the number of supporting, rotation rollers to bring down the loading to help reduce friction. It would be a simple exercise to transfer the dome's chain driven, friction drive roller over to the larger dome. 

The lower gearing provided by the larger base ring would make life even easier than it is at present. Probably needing only a single finger on the crank to advance the dome in keeping with the driven telescope. KISS! Keep it simple [silly/stupid.] 

The best bit about these GRP calf domes? They have a solar reflective coating to protect the stock from overheating in hot, sunny weather.

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