ao - ForOT Optical Turbulence Forecasts

WAVEFRONT DISTURBANCE BY REFRACTIVE INDEX (n) CHANGES
∆y
d
n 2
n 1
Sea Level
RH = 1
∆x = (n 2 -n 1 )*d
∆φ = ∆x/λ
∆x
NOTE: ∆x , dx/dy (tilt) and
d 2 x/dy 2 (curvature)
are achromatic if
n is λ-independent
CIRCLES: RH = 1 AIR
Cerro Paranal
RH = 0
RADIO
WAVES
R.J. Mather, 2004
ANOMALOUS DISPERSION EFFECTS
J H K L M
NOTE:
(i) in general refractive index is function of
temperature & water vapor (RH)
(ii) in optical/IR water vapor has little effect;
T (⇒ P & density) variations dominate seeing.
(iii) in radio & far-IR astronomy water vapor
variations dominate “seeing”
(iv) anomalous dispersion effects occur mostly
outside atmospheric transmission windows;
they have minor effects on n variations
(and do not “compensate seeing” as I had hoped many years ago)