raawe Any reliable converter for J2000 to JNOW?
This might help, in case you want to brew it for yourself:
http://www.stargazing.net/kepler/b1950.html
I have included my Computational "library" into my mount simulator. I'd originally created this library back in the 2005 time frame, to build my Takahashi Temma mount controller.
http://www.w7ay.net/site/Applications/MountSim/
There is a link in that page that directs you to the Xcode project (sources, etc) viz:
http://www.w7ay.net/site/Downloads/MountSim/MountSim.proj.dmg
Download that, expand the dmg, and you should find folders like this:
The "Computational" library are straight C programs; you do not need to know Objective-C to read them.
positional.c , for example includes a routines called precessionMatrix() and nutationMatrix() -- these convert precession and nutation between epochs (see julian.c for more on Julian dates; for example routines like currentJulianDate (JNOW), julianDateForYear (J2000, for example)). Note: I found that nutation (the small oscillations cause by the primary precession circle) has a large effect, so make sure you don;t ignore it.
Now, these are purely precession based. If you are looking for specific stars, you also need to take their proper motion into account. I.e., Epoch conversion is for a coordinate in the sky for different epochs, and does not account for movement of stars (Barnard's star, for example).
It should take no more than an afternoon to cobble up a program to convert.
I also highly recommend that everybody interested in this kind of thing buys a copy of Jean Meeus' book:
I have been using it for years and years now. Worth every penny.
https://www.amazon.com/Astronomical-Algorithms-Jean-Meeus/dp/0943396611
https://shopatsky.com/products/astronomical-algorithms-2nd-edition
Chen