Gina's Mini Dome Observatory for Widefield Imaging Rig

[Gina]

This is better.

[Gina]

Actually, I think I'll swap the 10mm and 4mm plates over.  I reckon the 4mm x 300mm x 200mm plate will quite adequately take the pillow block bearings whilst the thicker plate would be better for the base.

[Gina]

Been looking into the roof drive system.  The cross-section attached shows my original idea for connecting the big sprocket to the inner roof opening section.  Problem is that this means that the sprockets and chain would be on the outside to the roof box.  It might be possible to print a cover but this would add more length to the axle and increase the overall size.  I think having the drive inside the box makes much more sense.  To make this practical I think I would need a metal axle (aluminium) rather than 3D printed ABS.  There will be a considerable torque involved.

 

[Gina]

300mm x 200mm x 4mm and 280mm x 170mm x 10mm plates swapped over so that the thick one is the base.

Edited May 7, 2017 by Gina

[Gina]

This will be bolted onto the puck that goes in the top of the pillar mount.  Some of the PA adjustment drives can go underneath the base plate.  I'm waiting for the hinges to continue the construction but the aluminium channel has arrived so I shall be going ahead with the forks shortly.

Edited May 7, 2017 by Gina

[Gina]

Viewed from living room window.

[Gina]

Back to the roof drive.  I've ordered a large ball bearing so that I can have the bicycle chain wheel on the inside of the box with plenty of meat in the axle.  It's 50mm ID, 80mm OD and 16mm thick.  That will permit several long bolts to go right through and form a substantial coupling between the sprocket and the roof quadrant.  I might as well continue with the over-engineering   Like my main observatory this little one will have substantial timber work with good size coach bolts holding it all together.

I already have a 3D printed sprocket attached to a windscreen wiper motor for the drive.  This has 12 teeth.  The large sprocket has 44 teeth so a reduction ratio of 11:3 - nearly 4:1.  The control will consist of a relay (or maybe two) driven from the Raspberry Pi and limit switches using micro-switches.  I have several spare ones from my 3D printer projects   It can be powered from my main battery backed observatory supply with a heavy duty cable running from the warm room along the fence probably enclosed in MDPE water pipe.  This supply will also power the electronics.

Edited May 7, 2017 by Gina

[Gina]

Here is a rough cross-section diagram of the axle and roof drive system.

 

[Gina]

The new ball bearing has arrived - really looks the biz

[Gina]

Here's a new cross-sectional diagram.

[Gina]

SketchUp model of main drive axle.

Edited May 9, 2017 by Gina

[Gina]

Not having an 80mm hole saw I'm using the nearest above which is 90mm and a spacer ring as I had to do with the previous bearings.

Edited May 9, 2017 by Gina

[skybadger]

Machine mart do a set for a good price. Essential. As is a variable diameter tank cutter. I seem to use nothing but holes in my constructions.

I'm sure you could have bored it on the lathe too...

 

[Gina]

Yes, I have a good quality variable diameter tank cutter too.  A hole saw is best for cutting plywood though as it makes less mess than the other tools.  A router also makes a reasonable job but hole saw is easier.  A 2ft piece of plywood would be difficult to hold for cutting with the lathe   Anyway, 'tis done now

[Gina]

Drilled the four holes in the large sprocket for the bolts, which I've ordered as I didn't have the right size.

Edited May 11, 2017 by Gina

[Gina]

The M5 bolts have arrived so I might be able to progress this project a bit today - take a rest from the war on rats!!

[Gina]

The inside of the base box wants to be 400mm as a minimum - it can be wider if convenient.  Currently I have a 1m long roof section and I'm planning to roughly halve this and use the off-cut to make the inner moving roof section.  The difference between inner and outer roof section lengths is 28mm so I can make the outer one 514mm and the inner one 486mm.  The distance from outside of inner section to inside of fixed part is 33mm or 66mm in total.  Thus the inside of the box will be 486 - 66 = 420mm. 

This seems just about right though a bit more room for errors and tolerances would have been welcome.  As it is this will save me some work as I get two roof sections from one.

Drive side cross-section.

[Gina]

Further examination shows this is not quite as good an idea as I thought.  The inner moving section wants outward lips rather than bent inwards but I could use the off-cut for the fixed part instead.

[Gina]

In addition to the already bent 1000mm x 500mm x 0.7mm sheet, I have the following aluminium sheets :-

1. 1000mm x 500mm x 0.7mm
2. 750mm x 500mm x 0.7mm
3. 620mm x 500mm x 0.7mm
4. 750mm x 500mm x 1.0mm

[Gina]

If I were to use the off-cut from the outer section for the fixed section I wouldn't need to alter the lips and the overall size should be slightly bigger giving a better clearance from the imaging rig.  Difference in width of fixed section and outer moving section = 70mm (35mm x2).  This gives 500mm + 35mm = 535mm and 500mm - 35mm = 465mm.  Outside of fixed part = 465mm so inside = 465 - 12x2 = 421mm ????   That can't be right!

[Gina]

Trying again...

Referring to the cross-section diagram below :-

The difference between outer moving section and fixed section is 35mm each side or 70mm overall making the outer 535mm long and the fixed 465mm long, as before.  The inside of the box is two lots of plywood less than the outside ie. 465 - 24 = 441mm (not 421!).  That's better

[Gina]

The inner moving section wants to be 28mm (2x14mm) shorter than the outer.  ie. 535 - 28 = 507mm.  I'll probably take that out of the 620mm long piece of aluminium but I think I'll work out how I'm going to construct the rest of the box before deciding.

[Gina]

Looking at the overall view with roof, imaging rig and mount, here is a cross-section view.  The vertical dashed line shows the edge of the current box ends.  This shows that the south side will need extending both to the south and needs east and west sides extending to box in the mount.  One possibility would be to cover in the plywood ends and extend southwards with aluminium sheet  (if I have sufficient).  This needs some thought...

[Gina]

I think I might cut a pair of triangular pieces of the same marine plywood to extend the ends and then use aluminium sheet across the south side.

Here a cross-section diagram with the roof closed.

[Gina]

I think the box needs to be either the marine plywood or solid wood for strength and rigidity.  The pieces I cut for the original cylindrical roof mini observatory can be cut down and the vertical south facing piece can be set at an angle, held out by the triangular side pieces.  May need to add something like 3"x2" solid timber bracing pieces.  There's also the legs to sort out.

Edited May 14, 2017 by Gina