J2000 or JNow in Gemini?


Hi, I am using a Gemini equipped GM-8 and I am not sure wether I should check the option “Gemini expects J2000 co-ordinates” in the Gemini advanced setup (since SGPro uses J2000 Epoch).

Also, when connecting to my mount from Sky Safari (on my Android phone) through SGPro, I’ve noticed that there is a slight difference in position. I will put an example: I have Vega perfectly centered in my frame in SGPro but in Sky Safari it appears slightly off-center -in the rectangle that represents my camera/scope field of view. Does this have to do with the Epoch used? (I tried using both J2000 and JNow in Sky Safari’s settings, but with practically no difference). I don’t know if this is an issue of Sky Safari, Gemini settings or SGPro settings.

Thanks for your help!



It should not matter. That’s assuming that the Gemini implements the ASCOM EquatorialSystem property correctly. This tells SGP what epoch the mount is using for coordinates and SGP converts to that epoch.

It’s not possible even to speculate about the cause of a difference in position without knowing more. A “slight difference” could mean different things to different people. Can you give a numeric example of the positions in the different systems.



Thanks Chris for your reply. Actually tonight I have been having other issues with regards to the mount’s position: I enclose a screen grab where you can see that the position reported by Gemini and that reported by SGPro are different (only a few minutes of asc). Any ideas of what causes this difference? Maybe J2000/JNOW calculations…?

As far a Sky Safari, when I mentioned “a slight difference” what I was trying to say is simply that it was different from the real position I got by taking a frame where Vega was in the center of the crosshairs, but wasn’t in the center of the crosshairs in Sky Safari.




I believe the Gemini Controller always replies with JNow but you can set it to accept J2000 which provides a weird communication paradigm. I would set it to JNow. And set everything else to JNow as well.



Thank you Jared. Is there a way to make SGPro read/write JNow? (instead of J2000).

Any thoughs on the discrepancies in reported position between Gemini and SGPro?

Thanks again,



SGP uses J2000 internally and read/writes either J2000 or JNow to the mount depending on what the mount says it wants. As to your original post, I wonder if you could have a little bit of cone error? That could explain the slight difference between the two programs. Sky Safari does not know about any cone error while the actual image would include the cone error.


Thanks DesertSky. Please excuse my ignorance but what is cone error?



Cone error is a misalignment of the telescope axis away from or towards the plane of the mount puck. This means that the front of the telescope is either pointing slightly inwards (towards the puck) or outwards (away from the puck).


Ok, I see what you mean. But when I do a plate solve and sync the mount, the co-ordinates shown in SGPro and in Gemini sholud be those of the actual area the telescope & camera are pointing to, and this should also be reflected in Sky Safari, so all three should show the same position. Is this so or I am missing something? Thanks!



If you have cone error, then where the optical tube is pointed is offset from the mount by the cone error. So when you plate solve, the value returned is offset from the actual mount value by the cone error. So the mount sync is off by that amount. How all this applies to your situation is to be determined. It just one factor to consider.