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A peculiar case are the U,V,W coordinates. With very long baselines
(20 km with ALMA) and short wavelengths (0.3 mm), 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/ 20 km equals
which exceeds the precision of Real(4) numbers by 2
decimals...
So in principle, Real(4) are insufficient. But this is only true if a
full astrometric calibration is required. GILDAS UV tables are
intended for imaging purposes only, although this should include
selfcalibration.
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) 
so to obtain for example less than 0.1 radian (6 degree), we must have

(3) 
Plugging extreme numbers (
, km, mm) we obtain
radian, or
, which is quite a wide field of view.
So, the Real(4) precision do not allow to perform normal calibration
(phase calibrators being in general more than a (few) degree(s) away),
but is quite sufficient for even wide field imaging and
selfcalibration.
Next: Complex Visibilities
Up: Dap positions and Size
Previous: Restriction on Real(8) daps
Contents
Gildas manager
20140701