[mmaimcal] Sensitivities continued

[Al Wootten]

Thanks Steve.  I forwarded your comment along to Stephane, and this
discussion along to Marian.

  Until we have a real receiver design, of course, the expected number will 
not be well-determined.  For the moment, I leave it at 10 hnu/k.  Since
this is the only receiver band for which we expect a real ALMA prototype
receiver is to be mounted on the test antennas, we should know this better
before long.

Al

Steven T. Myers writes:
 > 
 > We are routinely getting noise temperatures of 12-14K over 10 GHz of
 > bandwidth at 32 GHz on both CBI and DASI now.  I think Al's estimates are
 > fine, though with the optics design one might expect as much as 8 K from
 > ground spillover (pessimistically).  Actually, I think this is the one
 > band where we DO have a good estimate of the performance, as there are now
 > more than 26 recievers out there using the NRAO HEMT (MAP) design.  
 > 
 > >  > 3) You use Trec = 14 K at 35 GHz. My impression is that no HEMT amplifiers
 > >  > have such a low noise
 > >  > at these frequencies. Furthermore, optical losses will most likely be larger
 > >  > at 35 GHz than at higher frequencies.
 > >  > I think we should use the 10 h.nu/k for that band.
 > > Roger.  Yes, without a real receiver design we don't have a good estimate
 > > for this band, for the bandwidth we want.  I called Marian Pospieszalski
 > > about this.  He thinks that over the 30-45 GHz band with cold horn, window
 > > and room temp. mixer, we should achieve 14 K or so.  So OK, let us add a
 > > few K for mirrors and perhaps something else.  10 hnu/k gives Trx=21K or so
 > > and the continuum sensitivity climbs to .02 mJy/min, with the line (1 km/s)
 > > sensitivity increasing to 5.2 mJy/min.  This is Tsys=38 K or so, and
 > > Carlstrom reported numbers as low as 34K for his BIMA/OVRO experiment over 
 > > narrower bands in the recent ApJ paper, so I think we are quite conservative
 > > with this.
 > 
 >