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Motorised Dome pointing for side-by-side GEM - a solution

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Last October I started a topic (here) seeking help for a "challenge" I had with pointing side-by-side telescopes through the narrow opening in my motorised dome.

After a lot of research, going back to first principles on spherical geometry and teaching myself how to do Visual Basic programming - I've come up with a software solution I've called DPoint. It's running on my Windows 10 Observatory PC and acts as a "pointing controller" for the dome.

I set up the dome and telescope geometry measurements in DPoint and tell DPoint how to connect to the dome  and telescope. The software then gets the Alt-Az data from the scope, performs some maths using my scope configuration and hey presto, DPoint works out where the dome needs to point and tells the dome to go there. DPoint polls the scope every few seconds and tells the dome where to go - this lets the dome follow the scope when the scope is slewing.

I've been really pleased with how well this has worked - I can select the scope I want to use and DPoint positions the dome opening so the scope is right in the middle of the opening.

Here's some screen shots of what DPoint looks like. First is the configuration tab where you set up the dome and scope geometry, for some reason I built it to support five scopes (I only have two but clearly had aspirations for more?):

Dome Geometry.png

Next is what I called the Setup tab. This is where you connect to the scope and dome and select which scope you want DPoint to slave the dome to. DPoint remembers everything you configure but I also added some Import/Export buttons to make it easy to backup or moving the configuration data around:


DPoint is the "operational" tab. In the top half I show where the scope is pointing, where DPoint thinks the dome should be pointing (Target) and where the dome is actually pointing (Current). To slave the dome to the scope you just tick the "Slave Dome to Scope" checkbox.

I added the bottom half of this tab to give me full manual control of the dome - I can nudge it left or right, slew the dome to a specific azimuth, Park the dome, Sync the dome to the Target azimuth, and as always, have a big Stop button:


The two controls on this tab I haven't mentioned ("Use DPoints" and "Add") are for another pointing mechanism I built into DPoint but haven't fully tested - because I found I didn't need it. If you find that DPoint has quite centred the scope in the middle of the dome opening you can manually move the dome and when it's in the right position press the "Add" button. This adds a "DPoint" coordinate to a database DPoint builds up. If you have enough DPoints in the database, you can tick the "Use DPoints" checkbox and DPoint calculates the Dome Target as normal but then looks at the nearest DPoints in the database and works out a new Dome Target by blending the DPoints data with the original calculation. This seemed like a neat idea at the time to compensate for any non-linearity in the dome geometry or dome pointing (e.g. my dome is not actually level !). However, the maths works fine without having to get the DPoints stuff working.

So I can now happily leave the observatory to run my photographic command scripts overnight with the dome following the scope around - I even (eventually) got the setup to handle meridian flips in the middle of the night.

All I need is a dark sky and no clouds ....



Edited by Mart
Deleted wrong picture
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Hi Mart,

I have been trying to do vaguely similar things, so I can appreciate what an acheivement you have come up with. I don't know whether you have any feelings about sharing your code, but I would love to see it if you are happy with that. Of course, I will understand totally if you don't want to. Anyway, well done and I hope it works really well for you.

Regards, Hugh

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Hi Hugh,

happy for you to play with the app - but the code is not a pretty sight.

Come to that the app might not be that pretty - e.g. if you haven't got ASCOM installed you may get an "unhandled exception" rather than a friendly user error.

I've zipped up and attached the programme. As mentioned above, I'm running it on Windows 10. I know it doesn't run on Windows XP so can't vouch for any other operating system.

Happy to share my "experience" from putting it together if there's a problem you've had doing similar things.

Cheers, Mart


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Hi Mart,


Many thanks for sending the programme. A quick test shows that it installs just fine on my computer - Windows 10 with the latest ASCOM version installed.

Also many thanks for your offer to share your experience. I will send you a PM about that.

Regards, Hugh


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