At the milliarcsecond scale, most of the compact extragalactic radio sources exhibit spatially-extended intrinsic structures that are variable in both time and frequency. Such radio structures set limits on the accuracy of source positions determined with VLBI astrometry unless their effects are accounted for. The astrometric suitability of each radio source can be evaluated based on its observed structure.

• Look at structure correction maps
  • These maps (gif-format images) show the magnitude of intrinsic source structure effects in the bandwidth synthesis delay as a fonction of interferometer resolution.

• Examine structure indices
  • The structure index defines the astrometric quality of a radio source according to the median structure effect in the bandwidth synthesis delay.

    structure index	astrometric quality	median effect (ps)	observed structure
    1	very good	0-3	point-like
    2	good	3-10	resolved
    3	use with caution	10-30	extended
    4	not acceptable	> 30	very extended

• Read the Astrophysical Journal Supplement Series Article, "VLBA Observations of Radio Reference Frame Sources. II. Astrometric Suitability Based on Observed Structure".
  • or see the July 1997 issue (Vol. 111, No 1, Pages 95-142)

• Read the Astrophysical Journal Supplement Series Article, "VLBA Observations of Radio Reference Frame Sources. III. Astrometric Suitability of an Additional 225 Sources".
  • or see the May 2000 issue (Vol. 128, No 1, Pages 17-83)