Surface error: ruze curves.
The error
in the reflectors will cause the gain to
decrease as the wavelength decreases. The ruze formula tells how much
this is:
- Gain=Gain0 * exp(-(4*pi*delta/lambda)^2)
- Gain0 is the gain for a perfect reflector (or
long wavelength).
- Delta is the rms surface error (which should be
random)
- Lambda is the wavelength in the same units as
delta.
The measured telescope gain from jul08 thru feb09
was used to compute the ruze curves.
The plot shows (.pdf):
- 3409 gain measurements were used (lband thru
xband).
- Gain0 was taken to be 10.75 Jy. This value was
picked because it
went thru the highest lband point.
- 2762 m^2 is 1 K/Jy
- A 225 spherical aperture has 39760 m^2 or 14.4
K/Jy.
- This assumes a uniform flux across the area.
The edge taper
will decrease this:
- 12K/Jy/14.4K/Jy = .83 aperture efficiency
- 10.75 K/Jy/14.4 = .75 aperture
efficiency.
- The dashed color lines are the gain vs frequency
from the ruze
equation using various surface errors:
- black: rms= 1.5 mm
- red: rms=2.0 mm
- green: rms=2.5 mm
- blue: rms=3.0 mm
- The 2.5 mm comes closest to the data. Some of the
errors in the
data are:
- cbh (6-8 Ghz) under illuminates the tertiary
(so the gain is
low).
- The cals could be a bit off.
- For higher frequencies, some of the sources
with 15" extent or
more are no longer point sources.
- The ruze formula assumes the surface errors are
uncorrelated. Large scale errors will probably cause larger sidelobes.
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