[mmaimcal] ACA 7m Specs and Reqs

[Al Wootten]

Folks,

The intent is to go for contract on the ACA 7m antenna ~19 Jan.
Some specs and reqs differ from other ALMA antennas, in contradiction to
our usual policy of having them identical.  Therefore we will need to have
science comment and all sorts of approvals.

Let us start the comments now.  Here is an extract highlighting some
differences for comment.

ACA Spec and Reqs differences from ALMA Antenna Spec & Reqs

Section 4.3.2  The antenna shall be a symmetrical paraboloidal reflector
of 7m diameter with a Cassegrain geometry F/D = 0.37

OK.

Section 5.2.1  Velocity and Acceleration:
ALMA                       Melco 12m                    Melco 7m
Az > 18 deg/s2            Az > 10 deg/s2            Az >  9 deg/s2  
El >  9 deg/s2            El >  9 deg/s2            El >  4.5 deg/s2  

Needs discussion.

Section 5.4.1  Fast switching phase calibration
ALMA                       Melco 12m                    Melco 7m
Steps 1.5 deg on sky     Steps 1.5 deg on sky       Steps 1.5 deg on sky     
settle to within 3"      settle to within 3"        settle to within 3"
peak pnting error        peak pnting error          peak pnting error
all in 1.5s of time.     all in 1.8s of time.       all in 1.9s of time.

Needs discussion.

5.4.2  On the Fly TP Mapping
ALMA                       Melco 12m                    Melco 7m
settle within 0.8"        settle within 0.8"       settle within 1.0"

Needs discussion.

5.4.4  Step response 
 ALMA                       Melco 12m                    Melco 7m
3" within 1.5s and         3" within 1.8s and       3" within 1.9s and
0.6" within 2.0s (total)   0.6" within 2.3s (total)  0.6" within 2.4s (total)

Needs discussion.

5.5   Antenna surface accuracy
 ALMA                       Melco 12m                    Melco 7m
<25 microns               <25 microns                <20 microns

OK

5.5   Antenna surface accuracy
 ALMA                       Melco 12m                    Melco 7m
Error budget allocates     Error budget allocates   Error budget allocates    
10 microns rms for holo    10 microns rms for holo  8 microns rms for holo
accuracy                   accuracy                 accuracy 

OK

5.8 Low Noise
 ALMA                       Melco 12m                    Melco 7m
Total unweighted           Total unweighted             Total unweighted        
geometrical blockage       geometrical blockage         geometrical blockage
less than three per cent   less than three per cent     less than three per cent
(<3%)                      (<3%)                        (<5%)

Three is not five.

6.3.4 Thermal requirements 
 ALMA                       Melco 12m                    Melco 7m
It shall be possible        Same, except secondary       Same as Melco 12m
locally at the antenna      restrictions limited to
and remotely via the ACU    T<20 C
to set the temp inside
the cabin inside the range
of 16-22 C during any of the
primary and secondary 
operating conditions.

Needs FE group discussion I think.

One more on OPT.

Clear skies,
Al