9.7.17

Building the Octagon Pt.46 It works!

*

Yesterday evening I went out at 11.30 to have a look around. I was already too late for Jupiter which was hiding behind the larches to the West. Saturn was low down to the south, at only 12° altitude, soft and colourful. With no finder I was reduced to sighting along the main tube. With the 32mm eyepiece in the 2"  star diagonal for a slightly wider view. Naturally there was little to no detail to be seen and increasing power was pointless. A single star or moon was visible nearby. The Moon was still climbing from behind a neighbour's tree. It was just under full, sharper than Saturn but still rather colourful. [Warm yellow/orange.]

It ain't pretty! Certainly not good enough weather protection in the long term. The large, shiny dewshield rests in the short stepladder in the foreground. If nothing else does, then that dewshield gives one a better sense of scale with larger refractors. Everything is much bigger and far heavier than you could ever imagine.

Focusing did not produce the violent image wobbles I had feared. Though the heavy mounting is not remotely user friendly at present. The radial plastic clutches in the wormwheel bosses were tightened down hard which made pointing a jerky process with backlash. 

I should have been using the drives for slewing but have nowhere safe to put the electronics yet. Though I have a very good idea where they will go eventually. On the southern side of the pier which is strictly No Man's Land. I have quickly learned to put an empty crate just there to remind myself that repeatedly banging my head on the OTA is very amateur! I have already learned to head north when I reach the top of the steps. It was that, or wear my white safety helmet all the time.

The pier seems to work even without its intended, plywood, mini skirt. I made a "deliberate" mistake in leaning the hefty Jumbo steps against the pier. Now the image wobbled slightly when I literally jumped up and down on the loose obs. floorboards. Or hit the railings hard with the side of my fist. The leaning stepladder was short circuiting the pier's careful isolation. Parking the steps elsewhere instantly cured the unwanted feedback. 

It makes me wonder whether I would have got away with a sturdy pier resting directly on the wooden floor. Without its external cladding the building itself is not as stiff as I would like. Yet it still allowed the telescope to be used easily. No doubt high magnification imaging would demand more steadiness than visual observation. I am still awaiting the arrival of the wood treatment powder. So cannot fix the floorboards down just yet. 

Adding the ply covering at the top of the timber, pyramidal pier had worked wonders. As had screwing the pyramid's 4x4 posts to the concrete footing brackets. Timber cross bracing lower down helped almost not at all or actually made things worse. I am tempted to try a sheet of thinner ply to clad the pier's midriff. Thinner ply will add less mass while providing most of the desired stiffness.

The thick plywood top plate helped once it was firmly screwed down to the 4x4 internal timber reinforcement. As did tightening the azimuth pivot stud. Before that the mounting was able to rock slightly with really energetic shoves. The sheer weight of the mounting seems to have had a calming effect on the pier legs.  

After half an hour of looking around in semi-darkness I packed up at midnight. It was darker than I remembered from the last time I was outside at midnight. Too dark to read a newspaper. The last time it would have been easy. I put the leaf collection bag over the mounting and parked the OTA horizontal with the weights up. Then I wrapped a tarpaulin around the OTA and added cord in case the wind picked up unexpectedly. Another cord to the "railings" stopped it becoming a wind vane.

Later I removed the heavy tarpaulin as it was unbalancing the OTA and leaving the focuser exposed. I pointed the telescope straight up at the zenith with a token bin bag over its head. The leaf bag went over the mounting to offer rather more protection than before. A cord anchored the OTA at the bottom. It looks horribly amateurish, and is.  

Click on any image for an enlargement.
*

2 comments:

sang said...

Hello again, Chris.

Having my morning papers on your blog along with a delicious coffee, its a great start for a hot summer day.
I am very curious about your tall wood pier as I want something similar for my elevated platform. I will also use a slightly longer refractor like yours, an 216/2700 achro with a matched Chromacorr. Only for visual work but as you know, long moment arm its a pest to deal with.

2 questions, if I may:
What's the foot mark of your pier at the platform level? How much space it eats from your floor?
And what's the diameter of your octagonal dome?

I haven't settled about the shape/type of my platform as I see many inconvenience in both ROR or Dome builds. Ideally, I want a Dome type walls with at least half of a sliding roof.

Well, have a good day Chris. It's nice to know you're there.
Sorin

Chris.B said...

Hi Sorin

A very good morning to you!
We are having 63F, overcast skies and rain this morning.
The morning coffee and toast were still very good. ;-)

The nominal diameter of my dome/building is about 3m.
The pyramid, pier feet are placed just inside the lowest, octagon cross braces.
So [say] about 2.2-2.4m apart measured on the diagonal.

The pier at obs. floor height is about 80 cm square.
It only needs about about 1 cm clearance on all sides for isolation.
So a hole of about 85cm square is required.
Its size hardly matters because you won't usually stand in that space anyway.

These figures only relate to my own build.
You could make the pier's footprint smaller.
But you should really clad the entire pyramid _all over_ in plywood for extra stiffness.
My design is badly flawed by the position of the ladder.
It is convenient but I can't close the 4th side of the pier with plywood.

If you can get larger timber posts than 10cmx10cm it would help.
150x150 would be much stiffer and much less resonant over a longer length.
Very much heavier to lift too! Which is why I chose 100x100.

Your 216/2700 is a much larger instrument than my 'tiny' 180/2160.
You must have a very substantial mounting for such a long instrument.
I'd love to see some images of your telescope if possible.
Do you have a blog, website or images posted online?

Thanks and regards
Chris