[mmaimcal] antennas

[Al Wootten]

I guess this didn't go to everyone...here's my 2 cents.

Bryan Butler writes:
 > 
 >  1 - regarding the nutating subreflector.  does the pointing spec
 >      need to be met while the subreflector is nutating?
 > 
 >  2 - regarding offset pointing.  are we (our group) comfortable with
 >      the idea of offset pointing correcting some substantial fraction
 >      of the pointing error?  especially in regards to wind buffeting.
 Thanks, Bryan, I put these on the agenda.  Question 1 asks, doesn't it,
 if the total power pointing spec is the same as the mosaicing pointing spec?
 We may end up taking much of the total power data in OTF mode, particularly
 for larger fields.  For smaller fields, e.g. the BIMA SONG galaxies for
 example, I don't know whether we win with nutating total power or OTF but
 the turn around times may work against the efficiency of OTF.
 This is something we learn from Mark's proto-memo.  In that memo, Mark
 certainly assumes that pointing meets specs when OTFing.  The time
 required to settle on source when nutating is subsumed into the parameter
 ta.  Since he does't attempt to model excitation of antenna modes by
 the nutation, this parameter is not really addressed in his modeling.
 I think that at the end of blanking in each nutation duty cycle the
 spec must be met if the final map is to register properly and achieve high
 image fidelity in the submillimeter frequency range.  That is exactly the
 range where atmospheric subtraction through nutation is most critical.
 Furthermore, since the primary beam is so small and the emission so extended,
 most of the flux will be in the total power maps.  If those maps have a 
 lot of jiggle in them, I cannot imagine that there will be  good image
 fidelity in the mosaic images.

 As currently practiced, offset pointing is done semi-hourly or so and so
 can't entirely work against wind buffeting.  I presume that you are considering
 an offset pointing done on the cal at each cycle of fast switching.  We
discussed this at the MAC meeting.  Someone--Jack Welch?--suggested that it
would only take a few seconds to accomplish.  However, this
 will limit the fast switching duty cycle it seems to me.  We need to model
 fast switching with pointing included in the cycles, which we have not done.
 This might take out some pointing jitter on sub-minute time scales.  However,
 I thought Mark's memo showed that at lower elevations, where the cross section
 of the antenna to the wind is largest, we were already pressed to get enough
 time on the source during the duty cycles he investigated; trying to put
 pointing into the duty cycle also can only make things worse.  I think
 that too heavy a dependence on offset pointing, especially to take out wind,
 might mean a heavy penalty in terms of science.

[Bryan replied that he didn't mean fast cycle reference pointing but referred
to the strategy in MMA Memo 159 which the Europeans may cite to try to relax
the pointing specs.  From Mark's reply, it sounds to me as if he thinks he may
have been optimistic in that memo.]

 My off the top of the head 2 cents...

 Clear skies,
 Al