Polarization Control (POS)

    The name POS refers to the polarization angle for a receiver situated in a focus of the telescope. By means of SPOS, APOS and OPOS one can set a polarization angle and track the parallactic angle $\pi$   relative to the descriptive coordinate system. For switching between the box-rotation in the primary focus and the secondary focus (''Drehstand'') there exists the command DREH.

The angle $\pi$ is measured in the normal astronomical sense, i.e. looking towards the sphere, $\pi$ is measured from the meridian passing through the pole in a clockwise direction from - 180^o to + 180^o . The position angle P (corresponding to $\pi$ ) in the azimut-elevation system is calculated every telescope cycle. P is also known as the ``feed angle" (FA) of a receiver.

The inputs for the POS commands are in degrees.

NOTE: If not reduced for technical reasons, the limits for movement are from 0^o to 360^o.

DREH < p > #

  For < p > = Y the polarization in the secondary focus (''Drehstand'') is activated, in the case < p > = N the box-rotation in the primary focus.

SPOS < $\pi_{s}^{}$ > #

  In the descriptive system, the position angle $\pi_{s}^{}$ is produced.If the OPOS option is not entered with a star, this angle is kept constant in the descriptive system by rotating the FA.

APOS < $\pi$ > #

  The value $\pi$ is added to $\pi_{s}^{}$

$\pi_{s}^{}$ (new) = $\pi_{s}^{}$ (old )+ $\pi$

NOTE: The value of $\pi_{s}^{}$ is increased by $\pi$ each time the command APOS is executed.

OPOS < O_$\pi$ > #

  The offset O_$\pi$ is added to $\pi_{s}^{}$ ; in the astronomical system

$\pi_{o}^{}$ = $\pi_{s}^{}$ + O_$\pi$

these values are used to drive the FA.

OPOS < O_$\pi$ > * #    The star-option :

If the star option is used, the astronomical meaning of $\pi$ is removed and the Feed Angle takes the value

$\pi_{o}^{}$ = FA = O_$\pi$

i.e. SPOS and APOS have no effect and O_$\pi$ does not have a direct astronomical meaning.

PPOS #

 

MPOS #

 

ZPOS #

  These values cause the offset in POS to be multiplied by + 1, - 1 and 0. (Star option is retained.)

VPOS < V_$\pi$ > #

  This command allows the rotation of the polarization angle at a rate of V_$\pi$ (degrees per min.) . It can be used with DPOS, etc. to set up a continuous or raster scan of polarization angle. The velocity of the FA is not controlled continously, but has fixed values.

DPOS < D_$\pi$ > #

  This command sets D_$\pi$ , the total change in rotation angle for a continuous or raster scan in polarization angle. DPOS is used in conjunction with the angular velocity, VPOS. For a raster scan set VPOS to $\pm$ 0 and specify STEP.

RPOS < R_$\pi$ > #

 

RPOS < R_$\pi$ > * #

This command defines a reference polarization angle. Measurements at reference positions are commanded by SRP3.

$\underline{\rm {RPOS\ without\ star\ :}}$ defines an additive offset relative to $\pi_{s}^{}$ .
$\underline{\rm {RPOS\ with\ star\ :}}$ defines an additive offset relative to $\pi_{o}^{}$

P is calculated according to the standard astronomical definition. The position angle is discontinuous at the zenith. North of the zenith (sources with Dec > 52^o 32') + 180^o is always added when calculating the actual feed angle from P. The addition of 180^o is not done if OPOS < O_$\pi$ > * # is used. In this case the subreflector unit is set to the angle O_$\pi$.

EXAMPLE:
OPOS 180 D * #
This gives the feed angle the subreflector unit would have at transit for a position angle of zero.