Tuesday, December 9, 2008

The Ochra South Observatory

If the Keck Observatory can sport a double moniker (Keck North and Keck South) then so can The Ochra Observatory!

Ok, so both of my observatories are just a wee bit smaller than either of the Keck jobs, and in the case of my Southern data gathering facility... smaller than a finder scope! But hay, In your face Keck!!!

Thursday, December 4, 2008

Stud Wall Outer Cladding

December 2008: With all eight stud wall frames done it was time to nail on the outer cladding. To help keep out the damp, I first stapled on a sheet of heavy gauge builders plastic on the outside of the stud wall frames. This was then covered with the outer cladding - 3/4" tongue and groove floor planking that had first been liberally painted in brown Protim wood preservative - I defy any woodworm to try dining on my observatory!! The plastic sheet was deliberately cut too large to allow for wrap-around on the top and sides, and to allow a skirt to hang down outside the floor joists.

Above: Moisture membrane on stud wall

Above: Partially clad wall with window.

Of the eight walls, the three north, north-east and north-west walls will be solid, while the west, south-west, south-east and east walls will all have windows, and the door will be in the south facing wall.

Above: South stud wall with main entrance.

The next step is to knock up a door and four windows and shutters and add a little decorative detail to take the garden shed look off it!

Wednesday, November 19, 2008

Stud Wall Pre-Assembly

Back to work! After a couple of weeks doing other work around the place, I decided to buy a decent table saw to cut and shape the stud wall end studs... I wish I had bought this thing earlier, as it has made life soooo much easier in general!

Anyway, I reduced my small mountain of raw lumber into lots of smaller pieces and set about cutting out the rebates in the lintle and foot-plates of the stud walls. The end studs on each of the eight walls must be cut to a trapezoidal shape to make everything fit snugly:

So, after a bit of fun with the glue pot and a large hammer, I knocked out 8 stud wall sections, which I was delighted to find all fit together perfectly. With each pair of walls aligned and G-clamped together, I drilled through the studs in three places and bolted them securely together. And here's the result!

It really is quite roomy inside - and I haven't even put on the roof yet!!

Next job is to drown every squate inch in builder's grade Protim to preserve and protect the wood - a smelly, messy job!

Tuesday, October 28, 2008

Dome & Wall Ring Sub-assembly

Having cut out all 32 x 45degree sectors for the dome ring and wall ring out of 8 x 3/4" sheets of hardwood marine ply, the sectors were glued and screwed together in groups of three, offset to lend maximum strength and support to each other, with each sub-assembly representing 90degrees of the full ring.

After gluing and screwing the three sectors together into a double thickness of 3/4" ply, a small forest of G-clamps was used to squeeze out the excess glue and assist in a uniform laminate.

Laying this lot out on the ground, in a full circle, gave me a real sense of just how large a 16' dome was going to be! Man, this thing is big!! It was gratifying to see that my trigonometry skills haven't completely deserted me - all the sectors were exactly the same size - to the millimetre!

Thursday, October 16, 2008

Cutting the Dome Rings

To support the lower rim of the dome, lend it more rigidity, and provide a hard flat surface for the dome transport wheels, I cut out out 16 x 45 degree annular sectors of 3/4" marine ply that will be glued and screwed together to form a double thickness (1.5") ring. This Dome Ring has an outside diameter of 2500mm, exactly matching the outside radius of the dome itself, and is 150mm wide.

Another, double thickness ring was similarly cut and will be attached to the top edge of the octagonal observatory walls. This Wall Ring is 50mm smaller in diameter than the upper dome ring, allowing the dome ring to overhang the wall ring by one inch all round. The wall ring is also wider than the dome ring, at 330mm, so that the eight outside corners of the walls are aligned with the outer circumference of the ring and the inner corners with the inner circumference.

At a later stage a thin, 6" wide metal strip will be attached to the outer edge of the dome ring. This strip will hang down below the wall ring and prevent rain from being blown into the gap between the rings. A draft-excluder brush strip, as used on the bottom of doors, will also be attached to the outer edge of the wall ring with the brush pointing up and gently rubbing against the dome ring to keep out insects and dust.

Using graph paper and compass I experimented with how many sectors I could cut out of each sheet of ply. The best fit was achieved with two dome ring sectors and two wall ring sectors nested tightly together. My 10:1 scale drawing suggested that I should have 30mm to spare... however, my pencil lines and slightly springy compass meant that my error factor was a little over 30mm! Cutting the first sheet was going to be an act of faith!

10:1 scale drawing of how two dome rings and two wall rings
fit onto a single sheet of 4' x 8'

Eight sheets of 3/4" x 4' x 8' Hardwood Marine Ply are required to cut out the 32 sectors required for the two rings. For those who haven't had the joy of trying to handle this stuff, a single sheet is humongously heavy even for two men. Dragging this stuff around my yard and into the garage single-handedly for cutting nearly cost me my life on numerous occasions! My arms hurt. My back hurts. Everything hurts!!

Before cutting each sector, the two ends of the sectors were carefully defined so that they would be accurate 45 degree sectors. This was done using simple geometry... on a large scale!

I knocked up a long radius arm for the router out of a length of skirting board, using a screw as the pivot point. The plywood sheets are raised off the ground by some lengths of scrap chipboard to prevent damaging the cutting bit. Three passes with the router were necessary to cut all the way through the very hard 3/4" ply.

Slow progress... as you can see, there is precious little space between the cuts
every millimetre is precious!

And the proof of the pudding is in the cutting...
Full house - they all fit!!!

Wednesday, October 8, 2008

Dome Control Hardware

There are a number of commercial systems on the market to control the dome (opening and closing the two-door shutters, and slaving the dome to the telescope so that they are both pointing in the same direction.) However they are rather expensive for the piddling amount of electronics and driver software involved. I was just about to reach out and 'touch' a friend to see if he could weave his magic (and soldering iron) to come up with his own control system, when I stumbled across the LesveDome group, run by Pierre de Ponthiere, who had developed dome control hardware based on a generic off-the-shelf USB controller board. The USB board is used to control a number of power relays for activating the dome shutter and azimuth motors, and to take inputs from a number of sensors and limit-switches. The ASCOM compliant driver software is still under active development by Pierre but is fully stable in its present incarnation. It's also dirt cheap!

There is a registration fee of €30 for the driver software (after 60 days free trial) and the USB controller board can be picked up either ready assembled or in kit form for around €56 including shipping. Being a consummate fiddler, I plumbed for the kit, which looks like this:

The kit is a doddle to assemble for anyone who can wield a soldering iron safely without putting out one of their own eyes or accidently setting fire to the cat, and requires no knowledge of electronics. After sorting out all the components and checking that nothing was missing, thirty minutes saw the unpopulated circuit board transformed from this:

...into this:

The kit comes with a CD of interface software for general use and for testing the completed board. I bought the USB Interface Board Kit from Ramsey Electronics in the USA using PayPal and had it in my hands less than a week later. Excellent service!

The next step is to knock up a small circuit board with the power relays and other ancillary gubbins - more on that later.

Tuesday, October 7, 2008

Full Size FSP Layout

To double check the final dimensions of the Folded Stevick-Paul optical tube assembly I laid out three 4'x8' sheets of plywood on the ground and drew out the optical path at full scale. The white rectangles in the photos are full scale drawings of the mirror cells that will be bolted onto the tube assembly. Doing this allowed me to double check all the dimensions and angles that I would need for cutting up the 1" box-section steel for the OTA and fine tune the whole support structure.

The photo, below, shows the light path of the convirging cone of light to its focus point, and how it passes close to the tertiary 10" flat folding mirror (in fact, through it's cell!) This mirror cell will have to be built in such a way so as not to impede the light path. Cooling fans on the tertiary mirror will heve to be ducted away from the light path to avoid turbulence.

It is interesting to note that in faster (f/12 and faster) versions of the Folded Stevick-Paul design the light cone actually grazes the edge of the tertiary mirror, which needs to be very carefully bevelled to minimise the obstruction. This is another reason I decided to design my FSP at f/15. I had hoped to make an f/20 system, but that would have meant building an even bigger observatory! Also, I can *just* test a 14" f/15 mirror inside my house. The mirror must be tested at its radius of curvature (twice its focal length) which is 10.5m!!!