I’ve installed a Nexdome rotation kit on my 8’ Exploradome. The installation went really well, the stepper has no problems rotating the dome and the software plays nice with SGP. My problem is the sync of the dome slit to the scope, it’s consistently off by roughly 8 degrees on all targets. I’ve remeasured the parameters used in the Ascom hub and even tried adjusting each one to no avail. The home position magnet is set properly and it syncs to home.
How should I go about troubleshooting this problem? Is there one parameter I should be looking at? Any help would be appreciated.
Alex
The Dome geometry is all done in the software, not in the controller. So it has to be set properly in ASCOM and/or SGP (or any other software involved in the sync between the telescope and the dome).
The dome only knows about where it's being told to go and doesn't know anything about where the telescope is pointing. This is all up to ASCOM/SGP.
So make sure the distance and dimensions are correct (including signs).
I don't use ASCOM (or Windows) so that about as much as I can do to help.
Thx for the reply Rodolphe. I did remeasure the distances numerous times and entered them in the Ascom Hub Setup. I assumed that since the scope can also be connected in the Ascom Hub and it can be synced to the dome that they both would be in sync. It’s interesting that the error is identical for all targets and I tried numerous ones N S E and W. Is there a way to enter an offset for the Azimuth since the error seems to be fixed?
this sounds like the home position is not set to the right Azimuth so the origin for all measurement it off. Would your local magnetic deviation be about 8 degrees by any chance ... and you used a magnetic compas to set the home position ?
Actually it’s very close at -10 degrees. I did use a digital compass corrected to True North but I’m not 100% certain. I’m going to check that tomorrow. Great catch Rodolphe!
I hope that’s the problem. Thx!
I rechecked the home position and it’s spot on. I really don’t know what else I can adjust. Would it make any difference that my mount is a centre balanced gem mount (CEM70G)?
If you made all the measurement fro the point where the Ra and Dec axis cross it shouldn't make any difference as this is the origin for all calculation :
I’ve searched for othets who have had similar issues and they talk about performing a calibration before syncing the dome and scope. I’ve looked but can’t find where the option is to do a calibration. Is it somewhere in the dome Ascom Hub?
Rodolphe, is there a diagram like this for fork-mounted telescopes? I have a Meade LX600. Thanks!
@Dean Neubauer
On a fork mount the axis is located where the OTA attaches to the fork. So that's you origin for all measurement.
I don't know if the ASCOM stuff can do a calibration.... because i don't use ASCOM .. or Windows ..
I use TheSkyX and my X2 plugin has a calibration routine... not sure the ASCOM driver has a calibration routine.
The default number of steps per rotation should be fine though (I ran some high resolution test using my own controller and I got to within 8 micro steps of the default .. that's an error of 0.006 degrees..).
Calibration is really needed if you don't use a NexDome with your rotation kit. The default for the NexDome dome is correct.
The issue here is really an offset somewhere. Home position accuracy, some measurements ... and this get even more complicated as it involves multiple piece of software and hopefully they are not all trying to do the offsetting and geometry calculation on top of each other.
Edit : I just re-read the top .. you have an Exploradome ... so yes the default value of the number of steps per rotation is probably not correct and would explain part of the issue.
Unfortunately the 3.x firmware doesn't include a calibration command.
This is going to be a pain to figure out the number of steps per rotation with the 3.x firmware.
There might be a way to figure out something close enough to get you going.
Go to the home position. then do a goto to the opposite side (so 180º appart).
Then measure by how much it is off.
Let's call this D.
We know that the default value is 55080 full steps per 360º.
So if you multiply D by 2 this gives you how much off you are in relation to a full rotation.
Then you can do (55080/360) * (360+(2*D)) , this is the actual number of steps per rotation you need to set on your controller.
The accuracy is tied to how accurately you can measure the offset.
That's the best I can come up with as there is no calibration command on the 3.x firmware (and 4.x).
Rodolphe
Thx Rodolphe for that idea. I just don’t know where i can enter the new number of steps per rotation. I use Ascom Hub and can’t see anywhere to modify the calibration steps.
Argn .. there is always something ;) .. and as I don't use Windows I had assumed (wrongly) that there was a way to change the number of steps per rotation in the ASCOM NexDome settings.
Ok, once you have figured out the value, you can update it using a serial terminal and sending the proper command.
I use CoolTerm usually but any serial terminal will do.
Connect at 115200 bauds, 8 bit, no parity, 1 stop bit ( aka 115200 8N1).
The send this command with a carriage return and line feed at the end :
@RWR,<new step value>
So if the new step value is let's say 54123 you would send @RWR,54123 (hitting enter should send the CRLF (\r\n), specially if you use CoolTerm as it's the default).
Rodolphe
Thx for that procedure Rodolphe. I did the measurements and it was off a fair bit. The new step size I calculated was 59976. I used Coolterm but when I connect I get a non ending loop of XB—>WaitAT, so I can’t send the string. Any ideas are greatly appreciated.
just ignore these, they are sent by the controller.
so you kind of have to blindly type the command
This "XB—>WaitAT" means the firmware is still talking to the XBee to configure it.
it should stop after a while but you can still type thing in there (not easy to see what you are typing indeed).
That did the trick, the slews to target are spot one. Thank you so much Rodolphe for your invaluable help!!
I'm glad it's sorted out :)
I wrote the NexDome X2 plugin for TheSkyX Pro and in my settings dialog I allow the user to set the steps per rotation.
I wish the same thing was available on the ASCOM side for people using it on different dome than the NexDome.
The old 2.x firmware had the capability to calibrate on its own (and so does my own firmware on my own controller) to allow for use in any dome with any stepper motor.
Hopefully this will become available one day for this v3/v4 firmware (not really Babak's fault as he trusted the firmware developer and so far the feature delivery hasn't been what he expected... ).