[mmaimcal] Polarization question

[Al Wootten]

At the bottom of page 7 in the Baars ACA memo
(http://www.alma.nrao.edu/memos/html-memos/abstracts/abs339.html),
Jaap states:

' The following should, however, be noted. Because the feeds are laterally 
displaced from the focus, the optics is slightly tilted with respect to 
the axis (maximally 2.8°) for a proper illumination of the secondary. 
This alignment will have to be adjusted to the different angle of the 
ACA antenna of about 5.5° maximally. '

Chapter 5.2 of the Project Book 
(http://www.tuc.nrao.edu/~demerson/almapbk/construc/cons_toc.htm)
details the optics plan for the B7 prime polarization receiver on pages 8 and 9:
'Draft: 3/9/2001 Figure 5 presents the configuration for Band 7. The 
dewar window is radially offset by 130 mm from the dewar axis. The 
beam passing through the window and infrared filters is reflected through 
an angle of 116 degrees by a plane mirror. It is then reflected by an 
ellipsoidal mirror where it is turned by 26 degrees and focused onto a 
grid with a waist radius of about 2.4 mm. The grid separates the two 
linear polarizations, which are then incident on two identical ellipsoidal 
mirrors. They are then reflected down by 26 degrees to converge into the 
corrugated horns by way of two flat mirrors. Figure 4: Optics layout and 
dimensions for Band 6.

 Losses due to the beam distortion and cross-polarization are low due to 
the small incidence angles of the ellipsoidal mirrors. Although it appears 
complex, this optical arrangement allows some flexibility in mixer design 
and LO injection.'

If the various receivers each have mirrors shaped for the configuration
of the main 12m antenna in order to achieve polarization purity, won't
this be compromised on the smaller antennas by the differing geometry?

Clear skies,
Al