3.8 Is Proper Motion or Parallax Relevant?

If a small aperture or occulting spot is to be used, even a relatively small proper motion or parallax may cause difficulties in acquiring the target. In such cases, the Proper Motion/Parallax data must be provided. Note, however, that proper motion and parallax values may not be specified for a target which is specified by a positional offset. Such targets will be taken to have the same proper motion and parallax as the reference target.

The observer must determine whether or not proper motion or parallax is relevant. In general, this will depend on the size of the acquisition aperture of the SI that will be used and the epoch of the coordinates that have been provided. For example, the STIS uses a target acquisition area of 5 arcsec square. For a star whose coordinates are given for the epoch 1950, and that will be observed in 1997, a proper motion of approximately 0.5*2.5/(1997-1950) = 0.027 arcsec per year would be "relevant," with a resulting offset of half the minimum center-to-edge distance.

3.8.1 Proper Motion and Parallax Data

The following information is required for targets where proper motion and parallax are "relevant"; note that uncertainties for RA_PM, Dec_PM, and Annual_Parallax are not required. If a sign is not given for RA_PM or Dec_PM, a positive value will be assumed, but it is better to be explicit.

• RA_PM: For Proper Motion in RA, the value should be in units of seconds of time/year. To convert from arcsec/year to seconds of time/year, divide by 15 cos DEC.
• Dec_PM: For Proper Motion in DEC, the value should be in units of arcsec/year.
• Epoch: The "Epoch of position" is the date of the "Target Position" which was defined in the Target List (see Section 3.4 ).
• The "Epoch of position" may or may not be the same as the date of "Equinox for Coordinates" in the Target List (see Section 3.5 ). Remember that the "Epoch of position" is the date the target position is referred to, whereas the "Equinox of Coordinates" is the date of orientation of the coordinate system in which the position is measured. For example, a star with a large proper motion may have its coordinates given in the J2000 system, but the numbers themselves are for epoch 1984, meaning that the star was at the specified position on January 1, 1984. Epoch should be of the form 20yy.y or 19yy.y.
• Ordinarily the epoch of position is earlier than the present date. In the Guide Star Catalog (GSC), the equinox is J2000 while the epoch depends on the individual plate. Do not adjust your coordinates to be those that would be measured if the plate were taken in the year 2000. However, some catalogs contain coordinates already adjusted to an epoch of J2000: the Hipparcos Input Catalogue (often used in the GSC for stars brighter than m(V) ~= 9) and the PPM Star Catalog. When these catalogs are being used, it is appropriate to specify an epoch of J2000. (These remarks do not apply to the Hipparcos Output Catalog.)
• Annual_Parallax: The unit for parallax is arcsec.

The example below is for the object DM-9D697 (Epsilon Eridani), where the proper motion data are taken from the SAO Catalog.

Keyword	Explanation	Units	Example
RA_PM:	Proper Motion in RA	sec/yr	-0.0662
DEC_PM:	Proper Motion in DEC	arcsec/yr	0.019
Epoch:	Epoch of Position	20yy.y or 19yy.y	1984.5
Annual_Parallax:	Annual parallax	arcsec	0.30488

For some notes on proper motions and units, see Section 3.13 .