Pulsar 2.2m dome DIY option

[robin-m]

The motor unit is bolted to the dome rather than the observatory walls, a toothed belt is glued inside the walls onto the circular plywood base I made for the dome rail to sit on. Power is transferred by two slip rings, one being the dome track and the second being another aluminium rail sitting inside the first. A small wheeled pickup runs on each rail which delivers 24V. I decided that although power transmission was more complicated the advantages of having power to the dome itself outweighed the disadvantages, especially as I intend to motorise the shutter as well. Communication to the arduino which sits in the dome is via xbee wireless from the dome computer. Slaving is handled by an ascom driver written for the dome which runs on the dome computer and communicates via serial over the xbee link with the arduino. When I have some time I'll post take and post some photos which should explain things more clearly.

[Gina]

How long have you had this system working? Do you have any dust/dirt problems with the track and pickups? It sounds a great engineering job

[Barfbagger]

I am really interested in how you have implemented this because I am planning something similar for my off-the-shelf Pulsar 2.7m dome. I forked out for the Rigel rotation system (vastly overpriced but I'm time poor and don't have kids). I was looking to use an Arduino over ethernet to trigger the wireless rotation from my warm room and use a magnetometer to get the azimuth of the shutter. After that I was considering a scope mounted webcam to visually check that the shutter was in the right place but also thinking about computing the desired azimuth of the dome by somehow reading the RA and dec from the ASCOM settings and computing the correct position with additional input from position sensors to ensure that the OTA was more or less centred in the shutter slit.

The next problem is how to motorise the shutter. The Pulsar system seems to use dual rope and pulley systems but I was wondering about a central belt drive, perhaps from a modified garage door opener.

[robin-m]

Gina, so far total run time has been a couple of hours, hardly a long term test but so far no problems. I am however considering modifying my design, rather than powering the arduino and motor directly from the slip rings I'm thinking of putting a couple of 12ah 12v batteries in the dome and building a trickle charge system for them powered by the slip rings. That way any momentary power interruptions will not be a problem and in addition I can implement a heartbeat between the arduino and dome computer so that in case of power failure to the obsy the arduino can close the shutter and home.

Barf, If you look at ascom POTH or Dome controller they automatically calculate the slit azimuth from the telescope taking into account offsets for the mount etc. It makes life an awful lot easier interfacing to them rather than the scope itself. As far as the shutter goes I considered opening via via a belt drive and powerful motor however the weight involved made the engineering too complex. I have decided to split the shutter in half vertically and have the two halves open sideways with linear actuators built from geared motors and screwthreads. The slit will be weather proofed by a flange on the inside of one half with a drip groove to channel any leakage out of the dome. That however is still firmly on the drawing board...

Robin

[Gina]

I was going to suggest batteries to provide the power with charging from the slip rings if you had any intermittent contact problems. But you clearly have everything well in hand Congratulations - not an easy thing to achieve

[Doshy]

The wood will draw any moisture up through the concrete.

Why would there be moisture in the concrete? My cheap garden shed has been sitting on a concrete base for years and is fine......

[Gina]

Maybe your shed has a damp course. Any concrete or brick will allow water to soak upwards. That's why buildings have damp courses. It provides a waterproof layer to stop rising damp.

[robin-m]

As concrete dries after being poured it leaves micro channels where the water escapes. These act as capillaries and draw moisture. I wasn't worried about moisture coming through the concrete but rather sideways from the unprotected concrete when it rains. Try dropping some water onto a concrete floor and see how far the damp patch spreads.

In my case I jacked up the dome slipped some DPC in and sealed job done...

Robin

[Doshy]

That's why buildings have damp courses. It provides a waterproof layer to stop rising damp.

No such thing as rising damp. Myth.

[Gina]

No such thing as rising damp. Myth.

[earth titan]

No such thing as rising damp. Myth.

So I'm led to believe.

Old cottages are built direct on the ground, with no foundations and no DPC. And no damp......

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[Steep]

No such thing as rising damp. Myth.

Wrong! I watched the series!

[Doshy]

Wrong! I watched the series!

[Steep]

Sorry, couldn't resist

[Gina]

Love it!

[nytecam]

"No such thing as rising damp. Myth". So I'm led to believe. Old cottages are built direct on the ground, with no foundations and no DPC. And no damp......

Don't think I'd rely on building expertise hereabouts. Yes - occasionally one can get lucky with no foundations and no DPC and have no rising damp but only a fool lives in such hope and are timewasters. Once visited a pretty old cottage in Devon with massive walls and the blazing log fire in August said it all

[Tinker1947]

Don't think I'd rely on building expertise hereabouts. Yes - occasionally one can get lucky with no foundations and no DPC and have no rising damp but only a fool lives in such hope and are timewasters. Once visited a pretty old cottage in Devon with massive walls and the blazing log fire in August said it all

My house in London had no foundations, it sat on a spread of 4 bricks, when i dug out the foundations for a kitchen extension the roots for the road side trees at the front of the property had grown right under and come out the back,,,,that said if a building is erected now it would be silly not to put in a damp course as the cost is next to nothing and prevention is better than a cure....

[jimmythemoonlight]

in my opinion a very professional job looks very smart i will be copying it down to the final detail thank you for all the good ideas and also the step by step instruction which is invaluble please keep us all updated on your fine project cheers jimmy

[Doshy]

Don't think I'd rely on building expertise hereabouts. Yes - occasionally one can get lucky with no foundations and no DPC and have no rising damp but only a fool lives in such hope and are timewasters. Once visited a pretty old cottage in Devon with massive walls and the blazing log fire in August said it all

Why do you assume one cannot rely on building expertise on this forum? Do you know everyones chosen career? I once had a 400yr old cottage and the whole point is that it's nice and cool in the summer. No DPC, no rising damp. Fool? Timewaster? go easy on the insults chap.

[Steep]

Why do you assume one cannot rely on building expertise on this forum? Do you know everyones chosen career? I once had a 400yr old cottage and the whole point is that it's nice and cool in the summer. No DPC, no rising damp. Fool? Timewaster? go easy on the insults chap.

Seems to me he's talking about the area that he lives in, not the forum. No insult intended I'm sure.

[Gina]

Yes, that's the menaing I placed on it.

[robin-m]

With my camera away for repair ( 4 weeks and counting ) I've had a chance to do a few jobs around the obsy that have needed doing for a long time, cheif amongst them being the mess of cables and power supplies. It was getting downright dangerous to walk around in there during the day let alone at night! In addition with the long term goal being a fully remote obsy I wanted to be able to remotely switch power for the mount, camera, etc. A Maplins aluminium case was bought which now holds an arduino, a single channel stepper motor control for the home brew focuser, a home made shield which has 4 12v 10a relays, microswitch input from the focusser and 5v 2a (2 * 7805) to power the usb hubs. There is one internal 4 port hub which handles the arduino and the USB-> serial connector for the HEQ5 leaving the external facing 4 port hub for camera, filter wheel etc. The whole thing is currently powered by a 12v 12a power supply next to the computer. I now have only 3 wires coming from the pier 2 * usb and 1 * 12v which will get hidden under the foam interlock flooring.

[robin-m]

I have been asked by another SGL member who is considering a pulsar dome to go into more detail about the rotation system that I built for mine.

Technically it's not that difficult, the only critical item is a ring inside the dome that is as near as possible to a perfect circle. The ring can either be fixed to the dome or the obsy walls. If using the walls as I did then the motor sits in the dome and moves with it. Otherwise the motor remains static and the ring moves. In my case the ring was also the base for the dome track. Glued to the ring is a length of 9mm belt from MotionCo.

The dome must rotate freely, the dome is heavy and any further friction will overcome the stepper stalling the dome. I had a problem with mine as some of the cladding on the outside of the dome had cupped and was interfering with the rotation. This was despite the cladding being firmly stapled but actually harks back to me building the framework kit for the dome before it arrived, I had to guess at the available clearance. I guessed a little wrong and was left with very little. As a solution I've removed a section of the cladding from just below the lip of the dome upwards. To prevent water getting into the cut ends a flashing strip of gaffer tape was used (whatever did we do before gaffer was invented???). Not ideal but it doesn't look intrusive and appears to work quite well.

The 'turnage' is provided by a 3.1nm stepper motor, they can be got cheaply from flea bay. I bought a job lot of 4 for less than £100 (cnc converting my lathe is another project I have in mind). For the stepper driver I got a few of these . They are based upon the TB6560 chip rather than the more common L298 based boards such as the arduino motor shield. I wanted the option to run at higher voltages / current ratings than allowed by the L298. I actually catastrophically 'smoke tested' a motor shield while building my focuser. I accidently connected it to 24v rather than 12. It didn't like it one bit and blew up quite spectacularly!. The brains of the outfit is an arduino with an xbee wireless shield to communicate with the dome pc.

Mounting the motor can be done in any way that allows for some movement against the ring and a bit of pressure holding the pulley against the belt. I chose to construct a rail / spring system but a simple lever arm / spring would work just as well.

As the motor, driver and arduino are housed in the dome rather than attached to the walls they need an independent power supply. I had initially put a second aluminium ring inside the dome ring and with small wheeled pickups picked up 24v from a power supply in the. After a bit of relection I decided that it would be better to have batteries and a charger at a single point ao the second ring was removed and a shelf holding batteries constructed inside the dome. I didn't want a battery charger powering the load while the dome was moving. Another slight change is that I have dropped the voltage to 12v and wired the batteries in parallel rather than series. I had initially specced 24v volts to overcome the inductance in the motor coils but the downside of running a higher voltage is that the stepping becomes more 'jerky' i.e. the motor snaps to it's next position rather than moving smoothly. When running at high revs this isn't noticeable but at the speeds I use it is. It won't give me any more run time as the motor just takes more current to do the work but it does run smoother.

For position I used a magnetometer board from love electronics. In use however it wasn't as reliable as I had hoped, maybe the amount of soft iron in th e pier is affecting it somehow but I spent an awful lot of coding time trying to get around the spurious readings. In the end I decided to just count steps for position. In 1/8 step mode there are 120.2 steps per degree. It was a simple matter to write a small program that allowed me to slew by a predefined number of steps then place a mark, perform 1 full rotation and count the number of steps used. Divide that by 360 and 120.2 popped out. So far in use the system has worked flawlessly with the dome always following the scope perfectly and ending up within 1 or 2mm of it's home position at the end of the night.

On the dome PC I have written an ASCOM driver for the system so that using POTH or any other 'dome aware' software the dome can be slaved to the scope. The ASCOM toolkit makes writing a driver simple but if anyone needs help I will happily do so.

Still to do:- At the moment I have rigged up an autmatic charging station at the domes home position, at the end of a session I just connect the batteries to a charger to charge until the next time. The charging station will just be two contacts and a microswitch. The arduino will just rotate the doem to the home position until the switch trips. The contacts will be wired through a relay so that the arduino can switch to charge. This is purely to prevent the contacts arcing as the dome contacts them.

I have typed enough! Any questions please feel free to ask and I'll do my best to answer them.

Robin

[robin-m]

*edit: I should have written "At the moment I have not rigged up an automatic charging station" rather than "At the moment I have rigged up an autmatic charging station" ... Brain fade.. too much typing...

[dmahon]

Thanks for that.

I am been humming and ahhing about what to get for months now. I really would like a dome but the lack of automated rotation has really put me off it. The motorised, but non-automated, setup from the manufacturer is a pretty poor offering.

I suspect that I will go for a dome now, and that I will be contacting you for advice (and help with the ASCOM driver) when the time comes. Basically, if you have something working I'd be very happy to copy it part for part! Steve1962 might send you a message too, as I visited his (exceptionally nice) dome the other day and he is trying to get his head around synchronising to the mount too.