Epoch in FITS header

A basic question for my understanding: if I do an auto-centering routine (using ASTAP in my case) then acquire subs, what epoch gets saved for the RA/DEC values in the FITS header (JNow or J2000)? I presume those values are coming from the mount (with recent sync from auto-centering).


The mount information in the header is in J2000 coordinates. So the equinox is J2000. The epoch is in principle the time and date you took the image. So any minor planet position imaged or tiny star proper motion is corresponding with the time you took the image.

Coordinate system for equinox J2000. At first of January 2000, the Earth axis is by definition precisely pointing to the celestial pole for the J2000 coordinate system. Now it drifted a little away.

Epoch is by definition the time the image was taken and displaying the situation of that moment.

A star catalog the positions are typically given in J2000 coordinates but using the star proper motion values you could calculate the star positions for epoch year 2020 or 2025. So a star or minor planet position is defined by both the equinox and epoch. Since stars hardly move, an epoch error of 10 or 20 years doesn’t make much difference.