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cso:instruments:heterodyne:otfmap [2013-12-14 08:52] sradford |
cso:instruments:heterodyne:otfmap [2015-05-29 02:00] (current) sradford |
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| + | ====== OTF mapping ====== | ||
| + | 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. | ||
| + | |||
| + | |||
| + | ==== Planning ==== | ||
| + | |||
| + | The (interrelated) parameters of an OTF map are: | ||
| + | * Cell size, typically about half the beam size. | ||
| + | * Map extent. It's probably best to limit the size to complete a map in one hour of observing time. Larger areas can be broken into submaps. | ||
| + | * Scan speed. The recommended scan (row) length is 45-75 sec. Together with the map size and cell size, this determines the scan speed and integration time per cell. | ||
| + | * Integration time per cell. The minimum FFTS1 integration time for otf mapping is 0.5 sec. | ||
| + | * Integration time for the Off position. The optimum is //N//<sup>1/2</sup> times the integration time per cell, where //N// is the number of cells per row. | ||
| + | |||
| + | |||
| + | ==== Observing ==== | ||
| + | |||
| + | The UIP ''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] '' | ||
| + | |||
| + | === Caveats === | ||
| + | |||
| + | |||
| + | ==== Example ==== | ||
| + | |||
| + | |||
| + | Here's an example of OTF mapping of CS near the Galactic center. | ||
| + | |||
| + | * The overall map, 30 x 20 arcmin, is broken up into 10 x 10 arcmin submaps. | ||
| + | * Each scan (row) is about 60 sec. The off position is 11 sec. Add overhead for cal and telescope motion for a total of about 45 min per submap. | ||
| + | * Then six submaps will take 4 hours. Add pointing, etc. You might be able to get two complete maps done in one night. | ||
| + | |||
| + | * If the map is repeated, it's recommended to rotate the scan by 90°: ''otf /position 0'' | ||
| + | |||
| + | * Mapping in galactic coordinates is not supported. But it is possible to rotate a map by a fixed position angle. Then the submap offsets must be appropriately calculated so the entire map is rotated properly. | ||
| + | |||
| + | * Before starting to observe test the map setup and verify the motions with ''/MOVE_ONLY'' option. | ||
| + | |||
| + | |||
| + | <code> | ||
| + | |||
| + | ! 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 | ||
| + | |||
| + | </code> | ||
