J2000 or JNow? What should I be doing Generic equipment discussions · Andy Wray · 1/28/2022 · 4 · 555 · 0

I've noticed a few times that Stellarium and my scope seem to be out of sync.  My setup is:

HEQ5 Pro Mount
PHD2 autoguiding with an orion starshooter and guide scope
APT as my main platform including platesolving
EQASCOM as the telescope driver
Stellarium as my planetarium 
Sharpcap for polar alignment

I think the best way to set this all up is to set them all up to J2000, but not quite sure where I need to do that.  I've set J2000 in APT and Stellarium; is there anywhere else I need to do that? i.e. within the platesolvers/Sharpcap etc

Andy Wray:
I've noticed a few times that Stellarium and my scope seem to be out of sync.  My setup is:

HEQ5 Pro Mount
PHD2 autoguiding with an orion starshooter and guide scope
APT as my main platform including platesolving
EQASCOM as the telescope driver
Stellarium as my planetarium 
Sharpcap for polar alignment

I think the best way to set this all up is to set them all up to J2000, but not quite sure where I need to do that.  I've set J2000 in APT and Stellarium; is there anywhere else I need to do that? i.e. within the platesolvers/Sharpcap etc

HEQ5 Pro Mount - see, EQMOD
PHD2 autoguiding with an orion starshooter and guide scope -no
APT as my main platform including platesolving - yes
EQMOD as the telescope driver - yes
Stellarium as my planetarium  - yes
Sharpcap for polar alignment - no

Stellarium or other planetarium software or APT will send target coordinates to EQMOD, which acts as the mount driver via ASCOM platform, so you need to ensure that all software along this communication is set to the same coordinate system, best is to use J2000 as currently commonly used.
The mount driver will translate this to JNow coordinates and to mechanical movement commands.

J2000 is a coordinate system, which has star positions as of the year 2000. Meanwhile we have 2021 and stars have moved, so some stars will show up in different positions due to their movement. These current positions are reflected in JNow. So before actually any calculation of mechanical movement the software needs to translate J2000 in JNow positions. This why you have to ensure that all components "speak" the same coordinate system to avoid confusion.
However the star's movements are small, about 1 arcsec / year and thus the difference between JNow and J2000 is about 20". Depending on your FOV this may be of relevance or less of relevance. Regarding other moving objects this is a different story.

Cheers
Gernot

andrea tasselli:

Gernot Schreider:
However the star's movements are small, about 1 arcsec / year and thus the difference between JNow and J2000 is about 20". Depending on your FOV this may be of relevance or less of relevance. Regarding other moving objects this is a different story.

The mean difference between astrometric J2000 coordinates and mean of the date/apparent coordinates is much larger than that! RA is more than 1-1.5min (angular more than 15') and Dec around 40-45 arcsec.

@andrea tasselli
you are right. The difference between J2000 and JNow is not only the proper motion of the star but also and mainly due the precession of the earth. The precession has a full cycle in 25,920 years, thus 1° in 72 years or 5' per year, varying with dec.


Cheers
Gernot