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During On The Fly (OTF) mapping, the data are recorded continuously while the telescope is scanning across the field. A common Off position is used for each row. Compared with point by point mapping, OTF mapping can be twice or more times as fast.
Setting up an OTF map is fairly complex. Plan your observations in advance! A simple example is presented. For a full discussion of the considerations involved, please consult with the CSO staff.
The (interrelated) parameters of an OTF map are:
OTF_MAP command has a plethora of options. Only some will be discussed:
OTF_MAP LONGITUDE_SIZE LATITUDE_SIZE [SCAN_SPEED [RAMP_UP_TIME [LONGITUDE_RESOLUTION [LATITUDE_RESOLUTION]]]] [/DIMENSION NCOLUMNS NROWS] [/POSITION_ANGLE POSITION_ANGLE | /SIDEWAYS] [/SKEW OFFSET] [/ZIGZAG] [/SKIP N] [/ALTAZIMUTHAL | /EQUATORIAL | /GALACTIC] [/NOFOCUS_ADJUST] [/NOROTATOR_ADJUST] [/NOLO_ADJUST] [/NOATTENUATOR_ADJUST] [/CALIBRATE [N] | /NOCALIBRATE] [/PAUSE_AFTER_CAL [TIME] /NOPAUSE_AFTER_CAL] [/OFFSET X_OFFSET [Y_OFFSET] | /AZO OFFSET | /DESIGNATED_OFF [NAME]] [/MOVE_ONLY]
Here's an example of OTF mapping of CS near the Galactic center.
otf /position 0
! tune receiver ! cs4-3 195.954217 GHz - frequency from Lovas catalog ! tune line in lsb so usb image avoids atmospheric water line at 183 GHz lo cs4-3 /lsb /if 5.5 ! alternate syntax if line name is not in the catalog lo /freq 195.9542 /lsb /if 5.5 ! set up spectrometer ! 11 sec integration time = sqrt (cells/row) * integration per cell = sqrt (30) * 2 sec spec 11 /ffts1w 5.5 /res /display kilauea:1 ! track source obs sgra* ! do six submaps, 3 x 2 ! although illustrated as a loop, it is probably best to do this manually for ii 1 to 3 for jj 1 to 2 ! offsets for submaps ! offset = cell size submap dimension * submap offset from center = (20 arcsec) * (30 cells) * index rao /field 20*30*(ii-2) deco /field 20*30*(jj-0.5) ! otf submap, roughly 45 min total ! submap size: 10 x 10 arcmin ! scan speed: 10 arcsec/sec ! => 2 sec integration per cell ! => 60 sec scan (row) length ! ramp time: 5 sec - may need some adjustment ! cell size: 20 x 20 arcsec - roughly Nyquist sampling at 200 GHz ! zig: alternate scan direction w->e, e->w, etc. ! cal: every 10 rows, about every 15 min ! pause: 10 sec after cal ! designated off position in catalog: sgra*:off_position otf 0 0 10 5 20 20 /dimension 30 30 /zig /cal 10 /pause 10 /designated_off ! end loops next next