Listing of the physical meaning of the pointing constants
R. Mauersberger, updated, May 24th 2010
∆Hi=∆az*cos(el)
∆Vi
Significance
Origin a
When check/change automatically?
IA
-IA*cos(el)
0
B, A,
O
Pad change, Work/Problems w. az.encoder, changing
between OPT Æ radio
IE
0
IE
T, E, O
Work/problem w. el. encoder. Pad change, change of rx or
OPT or subreflector; when inclinometer run indicates change
of X zero point, change ACU
CA
-CA
el - arcsin [cos(CA)
* sin(el)] c
Az encoder zero &
flexure term for
OPT b
El encoder zero
& collimation El &
zero point of X
tiltmeter
Collimation az
T, O
Change in rx/OPT/subreflector; lateral focus; tilt of
subreflector
AN
-AN*sin(az)*sin(el)
-AN*cos(az)
B
AW
-AW*cos(az)*sin(el)
AW*sin(az)
NPAE d
-NPAE*sin(el)
0
Not necessary with metrology for Vertex; change w. pad
change, or if AW0 changes significantly
Not necessary with metrology for Vertex; change w. pad
change, or if AN0 changes significantly
For radio pointing, should be fixed to values measured at
vendor camp; change OPT Æ radio; when inclinometer run
indicates change of Y zero point, change ACU
a
Inclination NS
of az axis
Inclination WE
of az axis
Non-Perpend. az
& el. Axis; flexure
term for OPTb &
zero point of Y
B
El.
axis, O
B: base of the telescope (below el. axis), T: top of telescope (above el. axis), O: OPT, A: az. encoder, E: el. encoder
These flexure terms affecting OPT (which is mounted off-axis in the BUS) pointing, results from the homology deformation of the BUS as a function of
elevation. They look like an az zero point error or an NPAE error.
c
This is not currently in the pointing model of ALMA telescopes, but TPOINT uses that term to derive the pointing constants from the data. I am not sure about
the sign; This is the strict formula for the correction which is a good approximation to the formula derived by Pere Planesas: ∆E=
-(CA/642.3)^2*cos(2el)*tan(el)
d
The mechanical part (i.e. excluding the flexure term for the OTF) of NPAE has been measured mechanically at the vendors’ camps. These values should not
change and can possibly be used for radio pointing: DVO1: 13.5”, DV02: 22.2”, DV03: 15.0” (priv. comm. M. Mündnich), PM03: 11.6” (priv. comm. R. Hills,
TBC).
b
HECE
0
HECE*cos(el)
HESE
0
HESE*sin(el)
HASA
-HASA*sin(az)*cos(el)
0
HACA
HACA*cos(az)*cos(el)
0
HASA2
HACA2
HESA2
-HASA2*sin(2az)*cos(el)
HACA2*cos(2az)*cos(el)
0
0
0
HESA2*sin(az)
HECA2
0
-HECA2*cos(2az)
HESA3
0
HESA3*sin(3az)
HECA3
0
-HECA3*cos(3az)
HASA3
HACA3
-HASA3*sin(3az)*cos(el)
HACA3*cos(3az)*cos(el)
0
0
tiltmeter
Bending cos term &
el. encoder
centering
Bending sin term &
el. encoder
centering
Centering az
encoder
Centering az
encoder
az. encoger tilt, sin
az. encoger tilt, cos
El nod 2 times/az
rev
El nod 2 times/az
rev
El nod 3 times/az
rev
El nod 3 times/az
rev
Az encoder error
Az encoder error
T, E, O
Change OPTÆradio; mechanical work
in dish/rx cabin; work on elv encoders
T, E, O
Change OPTÆradio; mechanical work in dish/rx cabin; work
on elv encoders
A
Work on az encoder
A
Work on az encoder
A
A
B
Work on az encoder
Work on az encoder
Not expected to change
B
Not expected to change
B
Not expected to change
B
Not expected to change
A
A
Work on az encoder
Work on az encoder
Remark: azimuth is defined with North 0 and East 90 degrees. Internally, the harmonic terms (beginning with H) have South at 0
degrees and East at 90 degrees. So any terms that change the azimuth have to flip sign (because that as system is ccw) and all terms
that contain a cos- dependence also change sign (because of the 180 degree phase shift); see footnote on page 67 of the TPOINT
manual. These changes have been applied in the Table.
∆H=ΣHi ; ∆V=ΣVi
Example of a set of pointing constants during optical pointing with DV02 in 11/2008
IA 223.1900 ; IE 600.0000; CA -1154.0000; AN -2.0900; AW 42.5900, NPAE -13.3300 ; HESE -27.8700 ; HECE -24.8400; HESA
0.0000; HASA -3.3900 ; HACA 4.6800; HASA2 0.9400 ; HACA2 1.2700; HESA2 1.0800 ; HECA2 0.6200 ; HACA3 0.0000 ;
HECA3 -0.7300 ; HESA3 -0.6700
Terms that change when going from OPT Æ Radio: CA, IE, IA, NPAE, HESE, HECE,
Terms that change when going from one radio receiver to another: CA, IE
Terms that change when going from one pad to another: IA, AW, AN
Remarks and Conclusions:
1. We must urgently introduce the Planesas term (i.e. elevation change as a result of large CA) in the internal pointing model of
the antennas: as an example, the receivers in the cabin are typically offset by 200”. Add to this a nutator offset of 180”. The
total horizontal collimation term is then 380”. Even at a modest elevation of 85°, the ∆E becomes -4” and for elevation close
to 88°, ∆E=-10”).
2. We should check whether the pointing is the same at +180 degrees and at -180 degrees. At other telescopes (APEX,
Effelsberg) this is not the case. If necessary, introduce an additional pointing term.
3. The inclination terms AN0 and AW0 of the antenna should be monitored often using the inclinometers to check the daily and
seasonal behavior of the antennas and foundations. The differences (AN0-AN) and AW0-AW) are constant for nearby pads
and should only change when moving the telescope a large amount (several kilometers). If these are known for each pad, AW
and AN can be determined from tiltmeter measurements, e.g. as a starting point for a radio pointing when changing pads. Note
that the deviation of the vertical at the OSF is about 40” and at the high site it is not known. The pads have probably been
oriented gravitationally, while the radio pointing uses geographical zenith. So many sources in a first radio pointing would be
out of beam if that is not done.