# MB Drift Scanning

Version 1.0 - CAJ, 2000-22-06
Version 1.1 - RFM, 2000-23-06

We sit the telescope still and let the sky go by!

A one-page summary of drift-scan observing is available here

General idea is that we take the telescope pointing South ie . 180. Azimuth and parked at 90. elevation. The receiver is then rotated to 79.1 degrees from the basic beam 2 up beam 4 down position (at 45 degrees), which gives 4 beams equally spaced in DEC. (A closer setting than the 60 degree rotation from 45 degrees used for RA scans).

Unfortunately we can't guarantee that the telescope will be parked at 180. Azimuth, and if it isn't the receiver should first be rotated to counteract the azimuth rotation and then taken around by 79.1 degrees.

The algorithm for the receiver rotation is therefore

rotation = 180.0 - azimuth + 79.1 + 45
or
rotation = 304.1 - azimuth

If the azimuth is in the range 270 - 340 degrees, then this equation gives a value in or near the no go area (330 degrees to 30 degrees). We therefore make use of the rotational symmetry of the multibeam receiver and turn it through 180 degrees, giving the equation:

rotation = 180.0 - azimuth + 259.1 + 45
or
rotation = 484.1 - azimuth

All results should be Modulo 360 - if you go all the way around to, say, 370 then subtract 360 to get 10. This should give results similar to those in table 1.

 Azimuth Rotator Angle Sched file to use 0 304.1 drift79.sch 30 274.1 drift79.sch 60 244.1 drift79.sch 90 214.1 drift79.sch 120 184.1 drift79.sch 150 154.1 drift79.sch 180 124.1 drift79.sch 210 94.1 drift79.sch 240 64.1 drift79.sch 270 214.1 drift259.sch 300 184.1 drift259.sch 330 154.1 drift259.sch

There are two sched files available for use, one for each receiver position. Each scan is one hour long, so it is necessary to tell it to repeat a number of times using the 'Repeats:' box on tkmulti. Check this after starting, as it seems to have a habit of reverting to '1' - which means that the drift-scan will only run for an hour.

#### DRIFT79.SCH (79 degree rotation)

``` \$ unit     1
disable fcc
freq = 1405.
bandwidth 64
config = mb4mod
fcc_p_trk = disabled
source = drft
fitsname = drft79.mbf
cycles =   720
az = 180.
el = 90.
fcc_rot = 79.1
track
closef
stop
```

#### DRIFT259.SCH (259 degree rotation)

``` \$ unit     1
disable fcc
freq = 1405.
bandwidth 64
config = mb4mod
fcc_p_trk = disabled
source = drft
fitsname = drft79.mbf
cycles =   720
az = 180.
el = 90.
fcc_rot = 259.1
track
closef
stop
```
To start drift scanning:
• In the observing room, change the LO frequency to 391.25 MHz. (The LO is produced by the Rohde & Schwarz Signal Generator on top of the main Observing room equipment racks. To change to a new value, enter the setting eg 391.25 on the numeric keypad and press the MHz button to the right of the keypad)
• Connect the cable labelled Correlator Cal to the cable labelled "To Cal Diode". (These cables should be found by the pen recorder in front of the main Observing room equipment racks)
• Get all the multi/tkmulti/mbcor software running. See Running the Multibeam Correlator
• Ask the controller to rotate the receiver to the position calculated to match the azimuth.
• In tkmulti's SCHED box, type either DRIFT79 or DRIFT259 (Return)and check in the logging display box (RHS) that the schedule file has been loaded.
• Enter a suitable number of repeats of the schedule (each repeat lasts an hour)
• Pull down Sched menu, and click START
• If at any time the controller has to change the telescope azimuth:
• STOP scanning. (tkmulti - STOP button)
• Recalculate receiver rotation and get it repositioned.
• If necessary change the sched file and re-enter the number of repeats
• Start scanning as above
• To stop drift scanning click on the STOP button in tkmulti and type CLOSE in the multi command box (bottom RHS)
After stopping the drift scanning, REMEMBER to reset the following equipment for pulsars!
• Pulsar CAL cable to Cal Diode cable
• LO to 386 MHz