1 PHASE_SCREEN Generate a 2-D phase screen to simulate atmospheric contribution
PHASE_SCREEN
Warning: This tasks is encapsulated into the "phaseeen.map" procedure
which enables an easy viewing of the result.
To compute the atmospheric phase errors, this tasks generate a 2-D phase
screen on a grid with sufficient spatial resolution to sample the anten-
na diameter (typically 4-m pixels for ALMA, and 4-m pixels for Bure).
The phase screen is generated in the Fourier plane with the constraint
that its 2nd order structure function is a combination of 3 power laws
in 3 different spatial ranges. The resulting phase screen is averaged
over the effective dish diameter. Because of the size limitation imposed
by the FFT, very long phase screen are built as a linear combination of
independently generated screens. This is correct since atmospheric path-
length variations are completely uncorrelated at large distances.
Dynamic (anomalous) refraction is directly proportional to the phase
gradient. This tasks thus computes the phase gradient associated to the
phase screen, in order to obtain a coherent derivation of the dynamic
refraction term. As the phase screen, the phase gradient is averaged
over the effective dish diameter.
2 LS_FILENAME
TASK\FILE "Name of the long output phase screen" LS_FILENAME$
This phase screen is made of the short output phase screens. It has pe-
riodic boundary conditions at least in its largest direction. The 1st
plane of this data cube stores the phase screen, the 2nd and 3rd plane
store the gradient components.
2 LA_FILENAME
TASK\FILE "Name of the long averaged phase screen" LA_FILENAME$
Same as the long phase screen but convolved with the phase antenna beam.
2 SS_FILENAME
TASK\FILE "Name of the short output phase screens" SS_FILENAME$
There are as many planes as needed to make the long phase screen by lin-
ear combination of the short phase screens.
2 SA_FILENAME
TASK\FILE "Name of the short averaged phase screens" SA_FILENAME$
Same as the short phase screen but convolved with the phase antenna
beam.
2 PIX_SIZE$
TASK\REAL "Pixel size [m]" PIX_SIZE$[2]
Size of the phase screen pixel. It should be a fraction (typically 1/3)
of the antenna diameter.
2 L_SCREEN_SIZE$[2]
TASK\REAL "Long screen size [m]" L_SCREEN_SIZE$[2]
Size of the phase screen which will really be used in the simulation.
2 S_SCREEN_SIZE$[2]
TASK\REAL "Short screen size [m]" S_SCREEN_SIZE$[2]
Size of an intermediate phase screen. This size should be small enough
so that the intermediate phase screen FFT transform is easily computable
on your computer. However the size should be large enough to be able to
consider that atmospheric pathlength variations are completely uncorre-
lated from one side to the other of this intermediate phase screen.
2 SF_VALUE_300M$
TASK\REAL "Structure function value at 300m (in square degree)"
SF_VALUE_300M$
The atmospheric phase rms on the 300-m baseline.
2 EXPONENTS$[3]
TASK\REAL "Structure function exponents" EXPONENTS$[3]
Exponents of the SQRT(2nd order structure function of the phase). There
should be 3 values that may be identical.
2 BASE_RANGES$[2]
TASK\REAL "Ranges of baselines for the previous exponents [m]"
BASE_RANGES$[2]
EXPONENTS$[1] will be used between below BASE_RANGES$[1] meters, EXPO-
NENTS$[2] will be used between BASE_RANGES$[1] and BASE_RANGES$[2] me-
ters and EXPONENTS$[3] will be used above BASE_RANGES$[3] meters
2 FOURIER_SPACE$
TASK\LOGI "Compute the power spectrum directly in Fourier space?"
FOURIER_SPACE$ NO
Switch to test an alternative way to compute the phase screen. This must
be set to NO as this is *not* working. This option and the associated
code should be removed.
2 DIAM$
TASK\REAL "Antenna Diameter [m]" DIAM$
Diameter of interferometer antenna used in the convolution of the phase
screen with the phase antenna beam.
2 SF_VERIF$
TASK\LOGI "Verify the structure function of the output screen?"
SF_VERIF$
Toggle computation of the structure function of the output screen, main-
ly for test purposes.
1 ENDOFHELP