Gina's Mini Dome Observatory for Widefield Imaging Rig

[Gina]

Measurements (for a 2ft = 610mm diameter) from base (1.) to apex (10.)  :- Vertical interval = 53.3mm.  Total height = 479 (9x53.3)

1. 234mm
2. 230mm
3. 220mm
4. 203mm
5. 179mm
6. 150mm
7. 117mm
8. 80mm
9. 41mm
10. 0

These are symmetrical so the measurements want halving and marking out either side of a mid line.  Then the allowance for the overlap wants adding in.  What I might do is to cut out paper patterns and see how these fit onto the sheets of aluminium I have.  I think I am likely to need to buy more than I currently have.

[Gina]

Curses...  Went to get a coffee and my post disappeared

[Gina]

Been looking into the size of dome I want.  This depends on the mount it will contain as well as the size of the imaging rig.  A quick look back through my posts for previous mini observatory ideas has shown that the NEQ6 would probably need at least 3ft diameter.  Alternatively, a home made fork mount with my widefield imaging rig would be considerably more compact.  I have a rough design for a fork mount which might work for widefield imaging as the requirements are much reduced compared with a telescope and the very short exposures used with the ASI1600MM-Cool camera further reduce the requirements.

Maybe I need to get the new mount sorted out before proceeding with the dome.  An interim possibility might be using the NEQ6 mount in conjunction with the cylindrical roof observatory used manually.  I already have a lot of this built and and both time and money invested in it so it seems a bit wasteful to abandon it.  I really do want to make a fork mount and dome arrangement though.

[mwayne]

On 4/28/2017 at 09:27, Gina said:

 

There is the problem of sealing the door against horizontal rain. 

Here is a thought, sync the dome opening to a weather station and always keep the slot pointed into the wind. At least that way you will know which way the water ingress will be from and maybe make weather sealing easier. Or at least more reliable.

[Gina]

That's an interesting idea

[AKB]

12 hours ago, mwayne said:

Here is a thought, sync the dome opening to a weather station and always keep the slot pointed into the wind.

Actually, you could make this entirely mechanically driven (whilst not in use)  by having a small wind-powered rotor like old-fashioned windmills.  If it's not windy, it doesn't matter which direction it's facing... surely another 3D printer project?  Got to be easier than a grandfather clock.

[Gina]

Ha ha   I could do but I think I'll just make sure the sealing is adequate.

[Gina]

A 600mm (24") diameter dome will definitely be big enough for my widefield imaging rig on fork mount so I've been looking at how much aluminium it will require.  The dome sections can be cut from 500mm wide sheet and arranged alternately head-to-tail, I should easily get 8 pieces out of two sheets 1000mm long, which I already have.

[Gina]

The apex needs a bit of sorting out.  In the present design I have 4 (or 5) points on top of each other - not good.  One way of overcoming this is to design and 3D print a pair of top pieces and have straight edges on the top of the sheets instead of a point.  As a side benefit of this is that the sections can be extended downwards by a couple of inches or the dome size could be increased a bit for a bit of "future proofing" whilst still getting all the sections out of the two sheets of aluminium.

[Gina]

For ease of sealing the joining edges when closed, each half of the dome could have 5 sheets with half sheets at the ends.  This actually saves on aluminium in the laying out but some extra will be needed for the overlap/gutter. 

Since the shape is an octagon rather than a circle the inner half dome is actually quite a bit smaller than the outer half.  With the current design with the outer dome measuring 612mm across the points, the inner half measures 565mm across the points before any allowance for clearance.  The inside maximum diameter is actually less again due to the octagonal shape of the inner half.  Again without allowing for clearances this becomes 517mm (about 20").  The upshot of all this is that we've lost 100mm in diameter in 600mm.  Hmmmm...  The dome will want to be bigger - just as well it can be still from the two ali sheets.

Diagrams showing the internal size plus closed and open.

Edited May 2, 2017 by Gina

[Gina]

Been having second thoughts about this method of making a dome.  With just eight sections, bending the edges over to join the sections is going to be difficult as the edge is a curve.  More sections would be better but more work and more aluminium used in the joins.  One solution would be 3D printed ribs to which to attach the sheets.  Or I could go back to 3D printing the whole dome - or rather two half domes.  Not in one piece but in sections.  Since ABS can be solvent welded, it could be geodesic.  Or a geodesic dome could be made from aluminium plates.

I quite like the idea of 3D printing as the bottom can be circular and even have teeth included to drive the dome halves.  Overlaps and seals are easily designed and produced.  It would be best to leave actual production until I have my larger "GinaRep Titan" 3D printer working properly as it has about a foot cube printing volume and the dome could be printed in fewer pieces.

Edited May 2, 2017 by Gina

[Gina]

Made a model in SketchUp and a 45° section of the dome will fit easily into the print volume of my Titan printer.  I have taken the top out to print separately to avoid the need for support material for the main sections.

[Gina]

A half size scale model to print on my "Pilot" printer would take just over 4 hours!  3D printers are not fast

[Gina]

Running test shows that support material is indeed being generated which accounts for the long time.

Edited May 2, 2017 by Gina
Added photo

[Gina]

Changed the height from 80° to 70° and the printing time has gone down to 1hr 16m so running another test.  I would expect the same model at full size to take about the same time on my "Titan" printer as this has twice the size of nozzle of the "Pilot".

[Gina]

Still producing a little support material but much less than before.

Edited May 2, 2017 by Gina

[Gina]

With the design above there was insufficient ABS attached to the print bed to hold it well enough, something I rather suspected.  The main part of the print fell over!  In the final version, the outer half dome will have an outside flange forming a spur gear whilst the inner half will have a flange inwards with an interior set of gear teeth.  Alternatively, I might have both flanges pointing inwards and one above the other.  I think this might be easier.

[Demonperformer]

On 29/04/2017 at 12:21, Gina said:

Maybe I need to get the new mount sorted out before proceeding with the dome. 

Don't want to 'jump' on your enthusiasm for the dome too much, but, if the NEQ6 option would require a bigger dome, isn't the mount the place to start, as you suggested above? - unless I have missed something (quite possible)? Would hate to see you put all the work into creating the dome and then come unstuck with the mount.

[Gina]

Thank you for your concern   I'm working on both at the same time.  In fact I'm just about to post in the mount thread.

[Gina]

I think I have shown that my "Titan" 3D printer should be capable of printing a dome of 24" diameter or a bit bigger using 8 pieces curved  in both directions to form part of a sphere plus top pieces to complete the dome.  I might print separate pieces to extend the dome about an extra 3" if required though I think this cylindrical part could be combined with the spherical parts.  The half scale model printed fine on my smaller printer.

[wimvb]

On ‎2017‎-‎04‎-‎28 at 22:43, Gina said:

Now the real problem - the shape of the sections.  This is a wire-frame model.

Looks like a '70-s umbrella. Or: Cut one in pieces, then scale it up.

How large is your obsy going to be, and what material? (sorry, you answered that last one already)

[Gina]

600mm diameter or maybe a little larger.

[Dr_Ju_ju]

One thought to reduce some of the hard work, would be to use a pre-formed dome e.g. http://www.ebay.co.uk/itm/Perspex-Acrylic-Dome-700mm-dia-/332184848462?hash=item4d57c2884e:g:NyYAAOSwx-9W1bt6 which could then be cut\modified\painted etc...

[Gina]

Interesting, I didn't know they made them that big.  But the amount of work is really quite small for the dome itself using 3D printing.  It's mainly a question of time but I can let the print run overnight while I sleep   The printer cools down and automatically turns itself off after printing.  As I said, I shall leave the printing until I've got my bigger printer working better.  I'm hoping it will be as accurate as my smaller one which is working really well now.  I shall also get the fork mount made first.

[wimvb]

1 hour ago, Gina said:

600mm diameter or maybe a little larger.

Which is about the same size as an umbrella.

I think that I would make ribs from plywood (4 semicircles), and cut the cover from alumium sheets.

Just a thought.