[mmaimcal] Re: [alma-config] Imaging simulations for ACA

[Jeff Mangum]

Hi All,

Mark makes a good point.  We *might* have a way to improve the
pointing by using optical tracking.  Even though our current design
for an optical pointing system is not best suited for optical
tracking, Jack Welch and I are working on a design which should be
well-suited for optical tracking.  It turns out that only a slight
modification to our current optical pointing system is needed
(amounting mainly to a more sensitive CCD detector).  Recall that we
tested optical tracking on the 12m last winter.  Note, though, that
anomolous refraction will still be a problem.

Cheers,

Jeff


"Mark" == Mark Holdaway <mholdawa at nrao.edu> writes:

>> >> 2. " smoothed
>> >> total power data instead of raw total power data from 12 m
>> >> antennas. Improvements of imaging quality have been obtained
>> >> by this method."
>> >> -- so do you efffectively use only up to ~ 6m spacings from the
>> >> 12m antennas ?
>> >
>> >Yes.  But, I should test different beam width for the smoothing.
>> 
>> In fact, this is the only way to minimize the effect of pointing errors
>> from the single-dish measurement. These result in phase errors proportional
>> to the UV distance, so improvement is obtained by only retaining the
>> shortest
>> spacings from the 12-m single-dish.  With 12-m SD, 6-m ACA, 12-m ALMA,
>> forgetting about close packing constraints, you would cover 0-4 m from the
>> 12-m
>> antennas, 4-8 m from 6-m ACA, and 8-12 m from the 12-m ALMA. Adding
>> close-packing
>> at 1.25 D, this becomes 0-5 m, 5-10 m, 10-14 m.
>> So I guess smoothing to the equivalent of a 5-m dish will give you nice
>> results.
>> 

Mark> There is another way: to make the 12-m dishes point better.
Mark> I would suggest a cost-benefit analysis here: from studies
Mark> like Morita-san's, we can specify a 12-m pointing error which
Mark> results in equivalent image quality to the 64 x 12m + 18 x 6m
Mark> + smoothed 12m data.  Which costs more, to build the ACA, or
Mark> to make the 12m antennas point better?


Mark> I would further argue that we don't need to make all the 12m
Mark> antennas point better, as it is mainly the total power measurementes
Mark> which are messing up the imaging (there is more flux to be thrown around
Mark> here), so it is only the number of antennas that are alotted to total
Mark> power measurements (yet another mess to be sorted out) that we need to
Mark> make point exceedingly well.

Mark> 	-Mark