As described in Section 3, reference star celestial positions may be expressed in either of two formats. The most usual format includes proper motions and has an optional epoch which defaults to that of the equinox. The other common format, used for reference stars whose proper motions are assumed to be zero in an inertial frame, has no proper motions and must have an epoch as well as an equinox.
The first format (i.e. with proper motions) has the supplementary option of allowing the annual parallax to be specified, following or instead of the epoch. Here are two fictitious reference star records each of which includes parallax:
| 14 39 36.087 -60 50 07.14 -0.49486 +0.6960 J2000.0 0.752 * Ref 1 |
| 09 16 19.03 -10 52 23.2 -0.0401 -0.006 B1950.0 1978.9 0.032 * Ref 2 |
In the case where the parallax is supplied without an epoch, which of the two is meant is deduced from the size of the number given.
In the case where an epoch is supplied as well as well as a parallax, it is assumed that the parallax has yet to be applied. In other words, the option to have the parallax removed from a reference star at the given catalogue epoch and then put back in for the epoch of the plate is not provided. The parallax is only taken into account (except for second-order effects on the proper motion) when a reduction in observed place has been requested (by supplying observatory, time and refraction information – see Section 7). Note that no provision is made to specify the radial velocity of a reference star. This would only matter in cases where the plate epoch was very distant from the reference star epoch and where both radial velocity and parallax were large.
No provision exists in ASTROM for specifying the parallax (or proper motion) of the unknown stars.