# Use of the MF derotator¶

The K-band 7-feed receiver is provided with a rotating dewar, in order to prevent field rotation during long acquisitions (while performing OTF/raster mapping or simple sidereal tracking). When the receiver is commanded a setup, this derotator is automatically set up in the default FIXED configuration - i.e. it will be placed in its rest position and it will not update its position along time.

## Rest position¶

When the dewar angle is set to 0°,the MF receiver is positioned as displayed in the following figure. Feeds 3-0-6 are aligned along a line that is parallel to the horizon, with feed number 3 being on the right side of the central feed. Note that the cardinal directions refer to southern azimuth quadrants.

The static x-y offsets corresponding to this rest position, also provided in the output FITS files (see dedicated documentation), are:

Feed x-offset (arcsec) y-offset (arcsec)
1 68.800 -119.165
2 -68.800 -119.165
3 -137.600 0.000
4 -68.800 119.165
5 68.800 119.165
6 137.600 0.000

In order to be transformed in Az-El offsets, a cos(El) factor must be taken into account for the Azimuth (x) offsets.

### Convention on angles¶

The dewar rotation is considered positive when moving clockwise.

## Configurations¶

To select the derotator configuration, use the following command in the operatorInput panel:

> derotatorSetConfiguration=[conf]


where [conf] can have one of the following values: FIXED, BSC, CUSTOM. Here follows a brief description of these different options.

### FIXED¶

The dewar keeps a fixed position wrt the horizon, i.e. no derotation is applied. It is possible to command a specific static angle:

> derotatorSetPosition=[ang]d


e.g. > derotatorSetPosition=10.0d rotates the dewar 10° clockwise

To read back the dewar position, use:

> derotatorGetPosition


### BSC¶

The Best Space Coverage configuration is useful for scans. It automatically rotates the dewar in order to best cover the scanned area, according to system-defined parameters which are specific to the receiver in use and taking into account the reference frame of the observation (Equatorial, Galactic and Horizontal).

If scans (OTF or raster) are being held in a celestial frame (i.e. EQ or GAL), this means that the dewar will be positioned so that the feed tracks on the sky are monospaced along the axis which is orthogonal to the scanning axis, and this geometry will be kept along the whole acquisition - i.e. the derotation is active.

For the Horizontal frame (scans taking place along Azimuth or Elevation) the derotation is not enabled: the dewar is in a fixed position, where the specific angle is chosen by the system in order to obtain the equally-spaced scans.

As in BSC mode the needed angles are pre-defined, any use of the derotatorSetPosition command will raise an error.

### CUSTOM¶

In this configuration it is the user to choose which angle the dewar axis must form with the y-axis of the scanning frame. Once this angle is set by means of the derotatorSetPosition command, and the scans are commanded, this angle is kept for the whole duration of the acquisition - i.e. the derotation is active. As above, when acquisitions are performed in the Horizontal frame, the derotation is inhibited and the initial angle is kept fixed wrt the horizon.

## Rewinding¶

The dewar can rotate within a limited range due to physical constraints. For the K-band 7-feed receiver this range, wrt the rest position, goes from -85.77° to +125.23°. This means that it is necessary to periodically rewind it in order to achieve the desired feed positions. It can happen both when a new scan is commanded and during long acquisitions, when the parallactic angle variation overcomes the available rotation range.

The system is in charge of automatically commanding a dewar rewind whenever it is required. This means that pauses might take place between consecutive subscans, and that a certain sky position will be observed by two different feeds before and after the rewind: a rewind implies the rotation by N*60° (where N can be 1, 2 or 3). The system will always choose the maximum allowed rewind, in order to delay the following rewind as much as possible.

When a rewind is necessary and during its execution, the antenna status will be labeled as “off-source” (this involving the tracking flag associated to any data sample which might still be under acquisition). The rewinding operations will not stop an ongoing subscan/acquisition, but no further acquisitions will be commanded until the antenna is back on-source.