Homemade observatory dome + accessories

[choochoo_baloo]

I wanted to start a topic about designing, manufacturing, and using a homemade observatory dome. The aim is to be able to efficiently ask for help and information when the need (frequently!!) arises.

I'll start with a 3D sketch of the dome structure - see below. It is to be made by the popular method of laminate plywood. All components will be fungal/damp treated and then I'll probably coat both halves with fibreglass. If not fibreglass, then yacht varnish will go on the outside surface. The components are with wood router company - greater accuracy compared with a jigsaw and compass drawn arcs etc.

• My first questions is: with moderate tools and DIY skills, is achieving a smooth finish to a fibreglass surface possible? I've been told that I need to buy: a roll of the glass sheet, pot of the paste stuff, dye for whatever colour I choose, a roller brush. I image that careful sanding is the secret?

• Secondly, I would really appreciate some guidance with aperture design. I've been browsing the Technical Innovations HOME-DOME observatories guide in the stickied thread in this forum. Of the available hemispherical dome designs, the "two piece nesting" type is my preference. See image below. However I'm pretty stuck with regards to implementation! Any advice would be most welcome.

• RE dome rotation: I plan to use O groove mounted wheels on a circular rail. I'm not comfortable going the usual route of standard castors running in a track - no matter the level of protection/chemical treatment wood always warps to a degree coupled with the additional friction this design produces. As I see it, rails remove all sideways play and are very low friction.

That's all for now. As I said, any help given is much appreciated.

Edited August 11, 2015 by choochoo_baloo

[Davey-T]

I've used something like this for flat roofs, bit runny so not sure how it will work on sloping surface but very durable.

Dave

http://www.cfsnet.co.uk/acatalog/1project-roofs.html

[Tinker1947]

There have been Dome DIY threads in the past with lots of useful info....do a search....

[JamesF]

The problem I forsee with the "two-piece nesting" shutter is the difficulty of controlling it.

It's not too hard to come up with a design where the lower section of the shutter would drag a free-running upper section up and down with it so you'd only need to control the lower section.  But that would probably lead to the upper section slamming into the stops at either end of its run once gravity took over.

Driving them both is somewhat more tricky.

Whilst it may not be as "elegant", the two-piece non-nesting solution is probably far easier to implement.

James

[Peter Drew]

Of the many domes I've made between 10ft and 30ft diameter I've found the sideways opening doors, usually opposing pairs, to be the least trouble and easiest to operate.  

[choochoo_baloo]

Hmm sounds appealing. How did you achieve a watertight seal on closing, and what type of motor did you use to operate it?

[choochoo_baloo]

Thanks Dave - I'll look into that.

[Ericgraham]

I'm building a dome with a one piece up and over shutter. Often this means losing some sky, however there are several ways around this. If you plan to mount two scopes on an altaz mount, one over the other, then by turning 12 hours in Azimuth and swinging the scopes though the fork, each scope can see (almost) all of the sky- though not at the same time.

Another way is to use a hyper-hemispheric dome which allows for a bigger one piece shutter.

If you are prepared to rotate the walls with the dome, then you can make even more room to park the shutter.

Which method am I going to use? Why, all three, of course! Pictures will follow soon- if it works...

[choochoo_baloo]

The problem I forsee with the "two-piece nesting" shutter is the difficulty of controlling it.

It's not too hard to come up with a design where the lower section of the shutter would drag a free-running upper section up and down with it so you'd only need to control the lower section. But that would probably lead to the upper section slamming into the stops at either end of its run once gravity took over.

Driving them both is somewhat more tricky.

Whilst it may not be as "elegant", the two-piece non-nesting solution is probably far easier to implement.

James

Thanks all for the input.

After some research etc etc I'll go for the "two piece non-nesting" approach as you mentioned James.

However I have to admit that I'm struggling to:

(a) design the top shutter, ( how to motorise both shutters.

RE (a) -- I have devised a, what I believe to be, overly complicated wheel & rail system. However I'm convinced that I'm overlooking something fundamental since most of the ready made dome market (eg Pulsar Observatories), use this 'two piece non-nesting' method. So it cannot be that hard to implement!

RE ( -- Much the same 'practicality' problem really. I have visions of using a chain and sprocket system to motorise the top shutter via rack and pinion. I guess it's not feasible to use one motor to operate both shutters?

James, would you be able to offer any details on the above please?

Edited August 25, 2015 by choochoo_baloo

[JamesF]

I'll have to give that some thought.  However, Steve Richards (steppenwolf) has a thread running at the moment about completely automating his dome observatory, so there may well be information there that would set you on the right track. 

I think I'd definitely go for separate motors for the two shutters though.  Trying to combine the two might look very nice but is quite likely to be asking a lot in terms of engineering.

Gonzo has some very neat lid openers on his remote observatory here: http://stargazerslounge.com/topic/198091-a-remote-unmanned-pico-observatory/page-8

Even just one of those might work for opening the lower section?

James

[Psychobilly]

I wish I had had all my plywood dome parts routed rather than laser cut but the laser cutting was free but meant a lot of "messy2 charring on the edges that had to be cleaned off before gluing...

What are you going to use as "bearings"... for the dome ring.. I used Ball Transfer Units and originally they were fine but over the years they have worn into the plywood forming a track and "detents" in the home position... I really should have had a stainless ring cut and fitted  or some other "track" for them to run in...

Look forward to seeing the project come together

Peter...

[choochoo_baloo]

Question (and update) time:

Can anyone recommend suitable12v DC motors of suitable torque for dome rotation and shutter movement? I've read that the "high torque" type is needed, but to someonewith no engineering background, "high" is ambiguous to me. Instead; what minimum Nm (Newton meter) output do I need?

I've also heard that car wiper motors are ideal, but again some confirmation would be appreciated.

Also, can anyone recommend suitable linear actuators (electric motor driven) for the lower shutter?

RE update: routed plywood due to arrive middle of next week. Going to order all the ancillary bits this weekend.

[skybadger]

You'd have to work that out from the weight of your dome and the rolling friction. Or get a spring scale and measure the starting force required and rolling force required to make it move. The torque can then be measured from that. One motor has to be able to exert that force at the radius of the drive wheel you are using. Two motors means you can reduce the required torque by say sqrt(2).

You have to remember that the windscreen motors use a worm wheel as the final drive so they get a big torque through a large gear reduction. I am using some other motors with a reduction gear box to the same effect, having used windscreen motors successfully before. The bigger issue s making sure you can get all that torque to the dome without slippage. Hence two or even three motors may be required depending on how circular the dome is and whether you can get pressure on both sides of the dome.

regs

Mike

[choochoo_baloo]

Thanks Mike.

I also wanted to thank you for the invaluable information on your sky badger website - I've ben browsing it for the past few weeks and have picked up lots of valuable information from it. I didn't realise you were active on SGL.

I'm also going to implement your suggestion of timing belt (as described on your website).

So if I may, can I briefly outline my plan to you for dome rotation, and if you are able to suggest any improvements that would be great:

Two suitable [will use a spring scale to measure] motors, each fitted with a timing pulley, 180 deg separation. Timing belt laid face inwards along inner circumference of base ring. Will fabricate steel motor brackets that hold the timing pulleys against the belt under spring compression - effectively a suspension system.

[hughgilhespie]

Hi Choo Choo,

I have recently travelled the same route you are proposing. My observatory is a Pulsar 2.2 metre, the more modern design which has an exposed internal dome flange suitable for attaching a timing belt. I went with a 25 mm wide T10 timing belt glued in place with CT1 adhesive. You need to be a bit careful to make sure that the tooth pitch is close to 10 mm across the bit where the ends of the belt meet but this isn't too difficult.

I didn't bother with measuring the torque needed as several others have used this system successfully using motors rated at 2 Newton Metres or better. I chose a geared stepper with a holding torque before the gearbox of 1.89 Nm and a 4.25:1gear reduction. This gave a theoretical maximum torque of around 10 Nm, more than enough. 

The other Pulsar domes that I know of that use the timing belt system only use one drive motor and that's what I did. 

The motor itself drives through an 18 tooth pinion. To keep the pinion in contact with the timing belt, there is a spring loaded pinch wheel running on the reverse side of the flange. The whole motor assembly is fitted on a pivot arm that is securely attached to the dome wall. These mechanical complications are required to cope with the distinct lack of circularity and the variations in thickness of the dome flange. If you are making your own they may not be necessary.

My dome automation is a work in progress but I have got the basic motor rotation of the dome working so I can confirm that this design works well. For various reasons I chose to use a stepper motor rather than a basic DC motor. Obviously this adds some complexity but it offers a lot of benefits too in terms of more precise control and an easy way to detect potentially serious problems such as the dome jamming and the motor burning out - or worse!

Hope this is helpful.

Regards, Hugh

[choochoo_baloo]

Yes, very helpful info. thanks Hugh. Would you mind if I PM you in the future with the odd technical query as it arises?

[hughgilhespie]

Hi Choo Choo,

I am happy to help in any way I can so please feel free to contact me.

Regards, Hugh

[Horwig]

Here's my attempt at a dome, same kind of construction, feel free to ask away, I'll help where I can.

Huw

http://stargazerslounge.com/topic/112937-if-ikea-made-telescopes/page-5

[Davey-T]

I used a 12volt ram from these folks for my telescopic pier, may be of use to you.

Dave

http://www.gimsonrobotics.co.uk/12V-linear-actuators.html

[skybadger]

Thanks Mike.

I also wanted to thank you for the invaluable information on your sky badger website - I've ben browsing it for the past few weeks and have picked up lots of valuable information from it. I didn't realise you were active on SGL.

I'm also going to implement your suggestion of timing belt (as described on your website).

So if I may, can I briefly outline my plan to you for dome rotation, and if you are able to suggest any improvements that would be great:

Two suitable [will use a spring scale to measure] motors, each fitted with a timing pulley, 180 deg separation. Timing belt laid face inwards along inner circumference of base ring. Will fabricate steel motor brackets that hold the timing pulleys against the belt under spring compression - effectively a suspension system.

Thanks for the reference. Glad its useful..

Hugh has got one of those new fangled domes  - my replacement dome is a polaris with large internal edges preventing a wheel from simply driving the inside edge while pinching against the outer one for tension. My previous dome could also only drive the inside edge for a number of other reasons. So recently I have been doing the same as you and trialling driving the dome with some new drive bogies turning sticky banebot wheels against the edge. My bogies have to fit the narrow gap (80mm) between external wall and internal dome dropped edge. Testing is about to start and some tinkering will be necessary I think.

In the end instead of a sprung lever to hold the motor I used a pair of steel rods, some linear shaft bearings and springs to hold the drive drive the wheel in parallel against the outer lip of the inside edge (not complicated at all really...). I'll continue to use the magnetometer to read the bearing until I rig up an encoder. Either way I''ll still need a wireless comms link to the dome to run the dome door opener when I rig that up.

Robin from SGL came up with the linear bogies pattern so creds to him.

Mike

[choochoo_baloo]

Thanks all for the information, as ever much appreciated.

I believe I am fairly sorted mentally with construction of the various components now. However there's one component design I cannot settle on: the upper sliding shutter. Each design to date has been overly complicated. It is a popular design used by amateurs and manufacturers alike yet I cannot create a suitable design! I've considered angle iron with flanged wheels, U channel and ball transfer units...

For starters, the Pulsar upper shutter has little offset from the protruding dome main rails. See image below. Yet my designs have considerable offset of the upper shutter. Obviously this increases vulnerability to wind and rain. 

I suppose what I'm asking is whether someone could kindly post a photo or two of the upper shutter design used in their Pulsar (or the like) dome.

Thanks in advance.

[Davey-T]

Pulsar dome on this thread but using all Pulsar stuff to electrify dome and shutter which I believe adds an extra couple of thousand pounds on top of the dome price .

Dave

http://stargazerslounge.com/topic/210602-pulsar-observatories-22m-dome-and-shelyak-dome-tracker/

[Horwig]

Hmmm, don't know if this is what you're asking after, but this is how my shutter works on my wooden dome

An aluminium flat bar is screwed to the main vertical spar of the dome, the shutter rides on it, with nylon blocks under to secure the shutter, quite weatherproof.

The upper shutter is built up at the front to go over the lower shutter,seems to work well, the lower one just lifts off manually.

Again, don't know if this is what you were asking about, but hope it helps.

Huw

[choochoo_baloo]

Thanks Dave. I know what you mean RE Pulsar prices. I'm glad I'm doing this DIY method - laminate plywood dome structure, fibreglass cladding (by the way as we discussed near the start of this thread - I'm going to buy a fibreglass kit from these chaps: http://www.fibreglassdirect.co.uk/grp-fibreglass-kits.html)and high torque motors via rack and pinion.

This compared to what's described in Pulsar literature seems to be of a 'better' standard, and for a fraction (<£1000) of the price!

[choochoo_baloo]

Thanks Huw, yes that is exactly what I'm after. Could I just check that I've understood you correctly? Please see the attached sketch of your upper shutter in cross section.

If correct, I assume that you have no problems with smooth running/friction?

Edited September 1, 2015 by choochoo_baloo