[mmaimcal] draft results of holography sensitivity calculations

[Bryan Butler]

OK, more comments, since i just skimmed more deeply into the email...



> 
> "Mark" == Mark Holdaway <mholdawa at tuc.nrao.edu> writes:
> 
>>> > Furthermore, I use my canonical ``quiescent'' 3C273
>>> > spectrum, which pegs the non-flaring 90~GHz flux of 3C273 at 15 Jy.
>>> > Planets cannot be used for interferometric holography, and 3C273 will
>>> > be among the brightest of compact sources that could be used at 90
>>> > GHz.

the SiO masers (mostly from the envelopes of stars) will be better. 
couple of hundred Jy, if memory serves.  very compact (for the purposes 
of doing two-element interferometry, where you want the dishes close 
together).  variable (factor of 2 or so), but so what, since you're just 
doing holography.

>>> > I've made a simple holography simulation package in AIPS++/glish
>>> > (this software package is really great for things like this, I must
>>> > say;  it is such a pity that AIPS++/glish is so underappreciated
>>> > and underutilized).  

i made a similar simulation package in IDL, which i am happy to send to 
anybody if they want it.  it is described in VLBA test memos 57 (the 
theory) and 62 (describing the simulations).  i also implemented it in 
good old FORTRAN, which is significantly faster and doesn't need an IDL 
license, but doesn't give you a nice graphical display...  it allows for 
investigations of sensitivity to raster size, oversampling factor, SNR, 
phase rms, amplitude rms (gain fluctuations), pointing errors (both 
fixed offset and rms for both the fixed and rastering antennas), and 
type of transform...

 >>> > The problem is now: what does the
 >>> > peak SNR mean?  Darrel Emerson made a hand-waving argument that
 >>> > translates the peak SNR in the image plane to the sensitivity to
 >>> > surface errors in the aperture plane, and it is probably correct 
to within
 >>> > a factor of 2-4, depending on how we slice it.

you don't have to hand wave (and i'm sure darrel can calculate this 
properly, he's an expert in these things...).  the errors look like:
    e_{max} ~ l N / (pi SNR)
    e_{rms} ~ l N / (5 pi SNR)
for wavelength l, and raster size N.  again, see the above two VLBA 
memos for the derivation, theoretically, and the simulations...

	-bryan