Dome Alignment

Paul, it was you alignment with Polaris that was troubling me – that’s why I asked if the counter-weight was still pointing down – something didn’t feel right about this step in your workflow but I was too dopey to see it. Kinch has hit the nail on the head. When the mount is set on CNP the counter-weight will be straight down and the dome slot should be facing north, the dome encoder (if connected) should read 0 (360) degrees and the mount Az should also read 0 (360) degrees. Depending upon the time of day, if you then centre on Polaris the counter-weight will be anywhere from horizontal East to horizontal west and (I think) there will be two points in time when it will be pointing straight down. I’ve just run up my gear. The first screen-shot of POTH is when the Mount is on Home Position (CNP). Mount Az shows nearly 0 degrees, the counterweight is straight down, and the Dome Az shows 000.0 degrees which is because the Arduino, which counts ‘steps’ (and calculates headings in degrees from the number of steps moved) and not absolute degrees (no encoders), re-sets to 000.0 on boot-up. I check the dome slot is facing north and then engage Slave (which was when the screen-shot was taken.

I then commanded a slew to Polaris (at around 0830 this morning, site is 30 miles NW of London) and you will see from the second screen-shot the Mount Az has hardly moved (less than 1 degree east) but the Dome Az (and hence the dome slot) has shifted 17 degrees West (the counter-weight bar is now about 30 degrees from vertical out to the east). These results look very similar to results you posted, so it looks like your setup is basically functioning.
The things to sort now are:

  1. establish the point in your workflow that you slave the dome,
  2. check your dome movement calibration in terms of motor-steps per degree of dome rotation and
  3. check your dome geometry settings
    For 1. I suggest you establish a home or park position on CNP, manually move you dome slot to north, check you dome and mount Az readings are both very close to 0 degrees, and engage Slave.
    For 2, the way I did this was to mark the inside of the observatory wall with N, S, E and W (also I have mine marked every 5 degrees) and put an arrow marker on the dome, centred in the middle of the slot and close above your N,S,E W markers. In POTH connect only your Dome, check your Dome Az is reading 0 Degrees and the slot arrow is above the N marker, type 090 in the box (to the right of the GoTo button) and hit GoTo. The dome should slew to E (090 degrees). Repeat for 180, 270 and finally 0 (360) and confirm the dome rests as near to S, W and N markers. Your Arduino will have a formula for translating degrees into motor steps and if the foregoing test misses the marks then you will need to adjust the formula. While you are doing this it is worth also checking that your dome always takes the shortest route when slewing. For example, on a meridian-flip, while your mount does a 360 degree move your dome should only traverse the relatively short distance across the meridian.
    For 3 – experimentation with different settings might be required
    Good luck.

thanks so much Chris, Kinch and Buggs for fantastic support. I’ll try out your suggestions and come back to you. We’ve done a lot of work between us and once it’s sorted, perhaps we could get it into an ASCOM help file if that’t the best way to make the info available.

all the best,

Hi all,

I have made progress, but had to go to the sitech controller forums (I have a mesu 200 with SiTech controller), as the mount can’t sync successfully CWD with scope looking at polaris. I have the scope sync’d now and can point it at polaris and sync the dome at POTH’s requested 330 degrees - scope and dome aperture aligned…

I send the scope to various parts of the sky and the dome follows it. The problem I have now is that the dome consistently slews to a position about 20 degrees to the left of the scope. So its all nicely aligned at the pole, but when I point the scope somewhere between 0 degrees and 180, the dome always travels that 20 degrees too far east (to the left looking from behind the scope).

I’ll try altering the setup parameters further, but nothing has worked so far.

Because it is a consistent angle of error, it would be easy to ‘offset’ in the arduino, but I’m not going there yet.


I have it sorted now - many thanks for guidance. My workflow is based on an initialised mount, set up with a spirit level CWD and scope horizontal pointing at the western horizon. Azimuth 270 with RA and Dec set as part of the initialisation.

I then set park and park in this initial position.

So from a powered down scope, the workflow is this

Power up the mount controller and the Sitech software
start POTH
Unpark the scope
connect to scope and dome in poth.
Slave dome
Run Cartes Du Ciel and connect scope
slew to an object just west of due south
manually move the dome so it aligns with the scope
set the dome encoder to the dome azimuth angle POTH has requested.
I can now slew to different objects and the dome slaves to within an acceptable margin for my C11 to see out of the dome.

It seems that the POTH setup values for the scope position had some effect and getting those more accurate was helpful in my case. They were only 37 and 107 mm, which is quite small in a 1.2 metre radius.

I also kept away from Polaris as an initialisation point. It worked great, but only for Polaris. I am not sure why.

Thanks again,

Good news Paul - a bit of science and a lot of trial and error. Sounds like you are there. Your next excitement will be the first time you try imaging something near the zenith - don’t get too alarmed but each time the dome position is updated it can move a long way and even change direction which doesn’t seem right. It’s fun to watch though. Good luck.

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