Lew Cook’s HJD calculator

Enter the RA and Dec of the target star (J2000), and the date and time UT. It outputs the Julian Date, heliocentric correction, and Heliocentric Julian Date. Note for nearly all purposes you can take the heliocentric correction for a given star, on a given night, as constant. Just find it for the middle observation of the night’s series on your star and use that to correct all observations on it for that night.