Why are there negative fluxes and flux densities in NED?
A.: NED attempts to represent the published literature as accurately as possible. Sometimes this will lead us to include negative fluxes and flux densities, and/or measurement errors larger than the detections themselves. In these cases, we encourage you to read the original papers to check the validity of the flux and flux density values. • Q.: What kind of redshifts are used in NED? A.: We use heliocentric redshifts. When we display them as velocities, we use the optical convention Vsun = czsun. Note that we do not make any relativistic correction to these velocities. John Huchra at the Harvard-Smithsonian Center for Astrophysics has kindly supplied more information on the definition of extragalactic redshifts. • Q.: Some of the reported redshifts in NED seem to show discrepancies with my estimates. Why? A.: As with the celestial coordinates which we adopt for the objects in NED, we attempt to use the best available redshifts. However, occasional errors creep into NED, in spite of ou