A peculiar case could be the U,V,W coordinates. With very long baselines
(20km with ALMA) and short wavelengths (0.3mm), one is tempted to say
that U,V,W should be real(8), as absolute baseline lengths are
measured (and defined by the ultimate antenna stability) to a fraction of
wavelength, say 40 m for example. 40
m/20km equals
which exceeds the precision of real(4) numbers by 2
digits. So in principle, real(4) are insufficient.
But this is only true if full astrometric calibration is required. GILDAS
UV tables are intended for imaging purposes, including self-calibration.
real(4) precision will only limit the field of view for a given
angular resolution. The phase error due to a (relative) numerical precision
is given by
![]() |
(2) |
![]() |
(3) |
In summary, the real(4) precision do not allow to perform normal calibration (phase calibrators being in general more than a (few) degree(s) away), but it is quite sufficient for even wide field imaging and self-calibration.