J2000 or JNow

[Theil]

If I use BackyardNikon together with ASAPT and CdC, should I use J2000 or JNow?

[astroman133]

This is nothing that BYN knows or really cares about.

You should probably use whatever Equatorial Coordinate System your mount uses. For ASCOM telescopes there are several supported choices are. Most amateur telescopes use Topocentric coordinates (JNOW, in slang). Here is a full list:

Symbol	Val	Description
equOther	0	Custom or unknown equinox and/or reference frame.
equTopocentric	1	Topocentric coordinates.
equJ2000	2	J2000 equator/equinox
equJ2050	3	J2050 equator/equinox
equB1950	4	B1950 equinox, FK4 reference frame

Applications should ask the telescope which coordinate system it supports and provide coordinates in that system. If so, then it is really transparent to the user. ASCOM provides an API method for converting between J2000 and topocentric coordinate values.

[admin]

either should work.

[astroman133]

Sorry, but I disagree.

ASCOM supports both coordinate systems, and provides a way for applications to convert between them, but typically telescope mounts only support one system and never two at the same time. ASCOM applications can query which system a mount is using via the read-only EquatorialSystem property. When an application sends equatorial slewing coordinates to a mount they should be referenced to the coordinate system that the mount is using. Otherwise there will be pointing errors. They both "will work", but one will work incorrectly and the other will work correctly.

According to what I have read, the topocentric (JNOW) coordinate system is almost exclusively used by amateur telescopes. Only professional observatory telescopes use J2000.

I will say that as far as BYE and BYN are concerned, since neither app can slew to equatorial (RA & Dec) coordinates, the coordinate system in the mount is irrelevant.