[mmaimcal] Band 6 Optics Question

[Mark Holdaway]

On Wed, 11 Jul 2001, Al Wootten wrote:

> I think -10 to -13 db is pretty good while -8 to -11 is marginal.  
> I'm not sure we have
> set an actual range; just that -12db center of band is the goal.
> 
> Comments on this???
> 

I don't think we've done any rigorous analysis on this count, have we?
Why do you suppose "-10 to -13 db is pretty good while -8 to -11 is
marginal"?  

I think there are two main sets of issues:

	- spillover, ground pickup, noise from spillover,
		aperture efficiency, position-
		dependent errors in total power

	- imaging issues


I am not prepared to address the first set.

For the imaging issues, I just calculated the beam for one instantiation
of surface errors.  I assumed 25 um rms errors ranging from 1m to 8m in
size, and fit out a parabaloid from the surface errors to simulate the
effects of performing pointing, focus, and best fit holography
measurements.  The taper is Gaussian, with the specified taper in
POWER at the dish edge.  No taper-dish misalignment has been included
in these calculations.  It is actually not correct to calculate the
primary beam from a single surface error instantiation (ie, no two
dishes will have identical sets of surface errors), but looking at
this primary beam will give us some qualitative feel for how things
will go.

I calculated beams for 243 GHz:

		peak sidelobe		FWHM
No Taper:	0.030			21".72
-9.6 dB		0.014			24".04
-11.6 dB	0.012			24."61	


and for 270 GHz:

		peak sidelobe		FWHM
No Taper:	0.035			19".55
-8.1 dB		0.020			21".64
-10.1 dB	0.017			22".15



The lower sidelobe levels will improve imaging and total power
reliability.  The wider beams will result in faster mosaicing or
wide field surveys, but with slightly less fidelity (ie, more image
plane pixels to solve for with the same number of visibilities).
So, as we increase the taper, the sidelobes go down and beam size
goes up -- these have an opposite effect on image fidelity.  I am
guessing that the sidelobe level will dominate, and that increasing
the taper will be slightly beneficial to imaging.

BUT:  it seems that the difference between -9.6 dB and -11.6 dB is
slight from the imaging perspective.



	-Mark