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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>​
cso/instruments/heterodyne/otfmap.txt · Last modified: 2015-05-29 02:00 by sradford
 
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