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NexDome dome rotation & shutter kit positioning

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7 replies to this topic

#1 deonb

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Posted 19 October 2020 - 08:43 PM

I see this statement in the installation instructions:

"It doesn’t make any difference where the magnetic switch is located on the dome, but it is recommended to place the dome magnet on the south side of the dome when it is in parked position for domes installed in northern hemisphere, and placed on north side on domes in southern hemisphere."

Does anybody know what this recommendation is based on?

 

Reason for asking:

 

I have a set of cable conduits that terminate into a bay on the North-East side of my observatory. I would like to place the Dome Rotation Kit rotator, as well as the charger for the Shutter Motor Kit near it to avoid long unsightly cables. Is this possible? And does the magnet placement affect this at all?
 


Edited by deonb, 19 October 2020 - 08:45 PM.


#2 AstroBrett

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Posted 19 October 2020 - 08:47 PM

If it really matters, it may have to do with the inclination of the magnetic field which is different in different hemispheres, and that positioning may maximize the sensitivity of the sensor. 

 

Brett



#3 rpineau

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Posted 19 October 2020 - 08:59 PM

I have mine due north (so zero degree equal the 0 of the stepper counter) and the shutter battery charger in the "park" position at ~ 320 degrees.... it really doesn't matter as long has you properly set the home position in the application (ASCOM, X2 plugin, .... ).

I'm fairly sure that once I put the mount in and do a proper polar alignment I'll have to adjust these 2 values.



#4 kathyastro

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Posted 20 October 2020 - 05:14 PM

I suspect that it is just poorly worded, or perhaps "explained" by someone who didn't quite get it.

 

All things being equal, you want the home position to be when the dome slot is on the south side.  When the dome tracks past the home position, it will re-set the position, correcting for any position errors accumulated during previous movement.  Since we are more likely to be tracking objects on the south side of the zenith, this improves the chances of the azimuth staying accurate.

 

The actual sensor can be located anywhere on the dome's circumference, as long as it indicates when the slot is in the home position.  There is no reason to prefer one position of the sensor over another, as long as it indicates the home position.

 

It is no big deal if other factors make a home position on the north side preferable.  It just means that the azimuth gets corrected less often while tracking.  If the system doesn't lose track of its position very much, no harm done.

 

This is general home sensor setup for any dome.  If there is a legitimate reason why NexDome wants the actual sensor on the south side, they should explain, because that is just weird.


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#5 rpineau

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Posted 20 October 2020 - 09:09 PM

I suspect that it is just poorly worded, or perhaps "explained" by someone who didn't quite get it.

 

Was that for me ..!!! cloudy.gif  (no offense taken whatsoever.. being wrong is how you learn :) ).

 

 

Jus kidding,

I see your point about getting pass the home position more often ... but during a night you probably only track one target.. and probably only pass once over the home sensor if your target is in the south (and you are in the northern hemisphere).

And, as you rightfully said. it depends on what you image... most of my south sky is obstructed by tree so at best I might reacht he meridian but never pass it.. thus never passing the home sensor.. so I set it due north :).

Also.. you're giving a lot of credit to the firmware ;) .. the "old" firmware didn't reset the position when crossing home. I don't thing firmware 3.3 does either (I might be wrong on this so I'll check the code).

I spent time re-writing a big part of the 2.x firmware for my own purpose and fixing a lot of issues... but din't think of adding this , so I guess I know what I'm coding in my dome firmware this weekend :)

 

Thanks.

Rodolphe



#6 rpineau

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Posted 20 October 2020 - 09:54 PM

Well.. when I say the "old" firmware didn't reset the position when crossing home, I mean, not in an interrupt. it does when pulling so the precision of the position reset depends on the speed of the Arduino being used (which is why in my new Dome controller based on the 2.x firmware to which I already contributed a bit at the time, I use an Arduino DUE .. ARM at 84MHz.. I can even run the AccelStepper run() code in an interrupt on this Arduino).

 

Edit : I just check firmware 3.3 code.. : 

Triggered as an interrupt whenever the home sensor pin changes state.

so it does use an interrupt to detect home and set the home position to the current motor position.. so I was wrong :)


Edited by rpineau, 20 October 2020 - 09:58 PM.


#7 deonb

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Posted 25 October 2020 - 08:32 PM

When the dome tracks past the home position, it will re-set the position, correcting for any position errors accumulated during previous movement.

 

How do you get it to do this? Mine will only reset position if I explicitly go to "Home". It doesn't change anything if it just moves past it for other reasons.



#8 kathyastro

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Posted 26 October 2020 - 09:19 AM

How do you get it to do this? Mine will only reset position if I explicitly go to "Home". It doesn't change anything if it just moves past it for other reasons.

Maybe not.  I was assuming.

 

The first dome automation system I had did this, and it was such an obvious feature that I coded my own home-brew system like that.  I assumed that, because the instructions had a preference for the south side, that this was the reason.

 

If your system doesn't do this, then there is no good reason to prefer south over any other direction either for the dome slot's home position or for the home sensor.  In fact, you might prefer a home position with the slot facing north (in the northern hemisphere) so that you can easily open it for cooling without getting direct sunlight onto the scope.

 

Note that, on a system that does reset the azimuth as it passes home, the change should be small enough to be unnoticeable.  On my current system, the controller could lose 100 steps and the correction would be something like 225.25 degrees being reset to 225.00 degrees.




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