Preparing for Observing

1. Mailing List
2. Manpower and Instrument Support
3. Arrival and Vehicle Logistics
4. Weather Multiplexing
5. alpha1, UIP, and Where to Put Your Macros
6. Entering Your Sources in the UIP Catalog
7. Bolocam Macro Structure
8. Changing Scan Direction or Coordinate System
9. Scan Angle and Dewar Rotation and Obtaining Full Sampling
   1. How Array Orientation Affects Sampling
   2. Important Coordinate Systems and Angles
   3. Converting Scan Angle to Rotator Angle
   4. The Optimal Scan Offset Angle and the Available Rotation Range
   5. Enabling Dewar Rotation
10. Designing Your Scan Pattern and Calculating Expected Coverage (esp. for Small Maps)
11. Cross-Linking
12. Calibration Observations
    1. Pointing and Flux Calibrator Catalogs
    2. Suggested Calibration Schedule and Macros
    3. Calibrating the Dewar Rotator
13. Revision History

Mailing List

Manpower and Instrument Support

Arrival and Vehicle Logistics

Weather Multiplexing

alpha1, UIP, and Where to Put Your Macros

Entering Your Sources in the UIP Catalog

Bolocam Macro Structure

 

 

Changing Scan Direction or Coordinate System

c scan along RA
XRASTER_SCAN 120 2280 15 /STEP_SIZE = 162 /EQUATORIAL
    /ALTERNATE_DIRECTION /POSITION_ANGLE = 90 /SETTLING_TIME = 3

  

c scan along dec
XRASTER_SCAN 120 2280 15 /STEP_SIZE = 162 /EQUATORIAL
    /ALTERNATE_DIRECTION /POSITION_ANGLE = 0 /SETTLING_TIME = 3

  

c scan at 30 deg clockwise from a RA scan
XRASTER_SCAN 120 2280 15 /STEP_SIZE = 162 /EQUATORIAL
    /ALTERNATE_DIRECTION /POSITION_ANGLE = 60 /SETTLING_TIME = 3

  


Scan Angle and Dewar Rotation and Obtaining Full Sampling

How Array Orientation Affects Sampling

Full sampling can be achieved by rotating the array relative to the scan direction.  Full sampling is achieved after one full pass of the array across a line perpendicular to the scan direction, assuming that the array has no missing elements.  This is illustrated in the following pictures, which show where various elements' centroids cross the vertical line for a scans as 0, 10, 20, and 30 degrees relative to the array rows for a toy 37-element array.  Note how much variation there is!

  0 deg:     

    
10 deg:     

20 deg:     

30 deg:     

Important Coordinate Systems and Angles

• focal plane system: this is the coordinate system one uses when looking at the focal plane from the photon's perspective.  Angles are measured in the standard mathematical sense, counter-clockwise.
  
  
• sky system: due to the way the optics transform the focal plane, the appearance on the sky of the focal plane has the same handedness as its appearance in the focal plane system.  There is a 180 degree rotation of the array between the two systems, but all angles continue to be measured counter-clockwise.  Some important notes regarding signs:
  
  
  • The sky system naturally coincides with the altazimuthal system: the sky system +x axis coincides with positive azimuth and the sky system +y system coincides with positive altitude.
    
    
  • The sky system does not coincide with the equatorial system, because RA is measured "backward".
    
    
  • The sky system's definition of angles has the opposite sign asparallactic angle.  Parallactic angle is measured from the +dec axis to the +el axis, with positive being counter-clockwise.  Sources in the southwest have a positive parallactic angle.  That is, in order to maintain a fixed orientation of the array on a source as PA increases (the source moves west), the array must be rotated by a negative angle in the sky system.
    
    
• rotator system: this is the system that is natural for the person standing on the alidade platform behind the dewar.  Since one stands behind the dewar, one sees the array from behind (opposite of "photon's eye view").  So this system is mirror-inverted relative to the focal plane or sky systems.  Rotation angles are positive when counter-clockwise in this system also, therefore rotation angles in this system are the negative of their value in the focal plane or sky system.  That is, a positive angle in the rotator system yields a negative angle in the sky or focal plane system.  Fortunately, angles in the rotator system effectively have the same sign as parallactic angle, which (misleadingly) makes calculating rotator angle for a given parallactic angle relatively easy.
  

• bolometer angle: measured in the focal plane system, this is the angle from F01 to the bolometer of interest.  For example, E01 is at bolometer angle = +60 deg.  Since the orientation of the sky system is the same as that of the focal plane system, the bolometer angle is valid in the sky system also.
  
  
  
  array in focal plane system when array angle = 0
  
  
  
• array angle: this is the angle of F01 measured in the sky system from the sky system +x axis when the dewar is rotated to some arbitrary position.  Recall that the sky system is rotated from the focal plane system by 180 deg, so we have
  
  
  array in sky system when array angle = 0
  
  
  
• fiducial angle: this is the array angle (i.e., the angle of F01 measured in the sky system) when the dewar rotator is set to its nominal 0 position ("homed").  Due to the way we mount the dewar, this is always approximately +90 deg in the focal plane system and -90 deg in the sky system.
  
  
  
  
  array in sky system when homed (rotator angle = 0)
  
  
  
• rotator angle: this is the angle of the dewar rotator relative to the home position, measured in the rotator system (i.e. opposite sign convention to bolometer and fiducial angle)
  
  
• pixel offset angle: this is the angle of a bolometer in the sky system from the sky system +x axis when the dewar is rotated to some arbitrary position
  
  
• scan angle: this is the angle defined by the /POSITION_ANGLE parameter of the XRASTER_SCAN command.  The position angle is always measured counter-clockwise from the +y axis of the coordinate system, counter-clockwise being defined from the perspective of an observer on the earth looking up at the sky.
  
  
• scan offset angle: this is the angle by which we rotate the array relative to the scan angle to get optimal sampling of the sky.  We discuss below how this optimal offset is determined; for now, take it as a given.  It has the same counter-clockwise sign convention as the sky system angle and the scan angle.  Since it is an increment to the scan angle, its origin is not defined.  In practice, what we do is calculate, based on scan angle and the scan angle coordinate system, the array angle needed to line the rows of the array up with the scan angle.  Then we increment array angle by scan offset angle, and then calculate rotator angle from the incremented array angle.
  

Converting Scan Angle to Rotator Angle

• equatorial:
  
  
  First, convert scan angle from equatorial coordinates to altazimuthal coordinates.  Note the sign convention of parallactic angle is opposite that of the /POSITION_ANGLE parameter to the XRASTER_SCAN command.
  
  

  scan angle altaz = 90 + scan angle equat - parallactic angle
  

  
  This conversion accounts for all subtleties of spherical geometry.  Then calculate the optimal array angle:
  
  

  array angle = scan angle altaz + scan offset angle
  

  
  Then calculate the desired rotator angle:
  
  

  rotator angle = fiducial angle - array angle
  
  
• altazimuthal:
  
  
  Here, we are already in altazimuthal coordinates, so we simply define
  
  

  scan angle altaz = scan angle
  

  
  and repeat the rest of the equatorial version
  

The Optimal Scan Offset Angle and the Available Rotation Range

1. The 60 degree altazimuthal rotation range that is available at any time maps onto equatorial coordinates in a time-dependent manner.
   
   
2. This 60 degree altazimuthal rotation range does not rotate with the desired scan angle.
   

• When the rotator is set to its nominal home position (rotator angle = 0), the array would appear on the sky approximately as follows.   The array angle in this case is (approximately) -90 deg in the sky system and 90 deg in the focal plane system.  If scan angle altaz were a multiple of 60 deg (0, 60, 120, etc.), then the scans would be along the rows of the array (approximately!).
  
  
  
  
  array in sky system when homed (rotator angle = 0)
  
  
• The maximum rotator angle = +30 deg, which corresponds to the minimum array rotation angle available, array angle = -120 deg in the sky system and array angle = 60 deg in the focal plane system.
  
  
  
  
  
  
• The minimum rotator angle = -30 deg, which corresponds to the maximum array angle = -60 deg in the sky system and array angle = 120 deg in the focal plane system.
  
  
  

Enabling Dewar Rotation

• /ROTATOR_ADJUST = ONCE
• /ROTATOR_ADJUST = ALWAYS
  

Designing Your Scan Pattern and Calculating Expected Coverage (esp. for Small Maps)

1. Residual imperfect sampling.  While every point on the sky is hit by a beam when the array is rotated correctly, each beam has a nonuniform response to the sky.  This effect is minimized, but not completely removed, by array rotation.
   
   
2. Bad bolometers.  Some bolometers in the array simply do not work.  Thus, even when scanning at the optimal angle, some regions will see fewer bolometers pass by than other regions, so the coverage will experience dips where bad bolometers pass. 
   

• Plots of scans taken to optimize the scan offset angle.  The corresponding command would be
  
  
  XRASTER_SCAN 120 900 35 /STEP_SIZE = 26 /ALTAZIMUTHAL /ALTERNATE_DIRECTION
       /POSITION_ANGLE = 90 /SETTLING_TIME = 3
  
  The source position was fixed at (AZ, EL) = (0, 0) since this was only a test to find the optimal rotation angle.
  
  
1. First, we show an extremely bad case, setting scan_offset_angle = 0 deg so that the rows of the array line up with the scan direction.  The striping in the raw map is evident.  This histograms that follow show the poor coverage uniformity more dramatically.  The histogram to focus on is the blue (solid) one, which corresponds to the blue square in the maps; this is the good-coverage region, the part of the map that the array has fully passed over.  The other histogram (black, dashed) corresponds to the white square in the map and is the any-coverage region, roughly the area than some part (but not necessarily all) of the array has hit.  The good-coverage region is the region where the coverage is pretty uniform because the entire array has passed by.  The red trace in the maps shows the path of the telescope boresight and the blue diamond shows the source position.
   
   
     
   
       
   
   
   
2. Next, we show the coverage one gets when the optimal array rotation angle is used, which is scan_offset_angle = 10.7 deg as mentioned above.  Note the significant improvement in uniformity in the unsmoothed plots.
   
   
     
   
       
   
   
   
3. Using scan_offset_angle = 4.5 deg also works reasonably well:
   
   
     
   
       
   
   
   
4. We also show a scan taken with /POSITION_ANGLE = 0 and scan_offset_angle = 10.7 deg.  Remember that, because the array can only rotate over a 60 degree range (see the discussion of the rotator in the previous section), the orientation of the array relative to the scan direction is different; it is 60 degrees off from what it would be if we had simply rotated the array by the 90 degrees that /POSITION_ANGLE was changed by.  The difference in coverage reflects the fact that the bad bolometers are distributed nonuniformly across the array (the array is not perfectly 6-fold symmetric).  But the coverage pattern variation is not much worse than in the /POSITION_ANGLE = 90 case, so we live with it.
   
   
     
   
       
   
   
• A more realistic example is provided in the following, where we show the coverage map found from performing the same scan geometry, but doing it in equatorial units and on a source specified in equatorial coordinates at (RA, dec) (J2000) = (6h, 60deg).  The command used was
  
  
  XRASTER_SCAN 120 900 35 /STEP_SIZE = 26 /EQUATORIAL /ALTERNATE_DIRECTION
       /POSITION_ANGLE = 90 /SETTLING_TIME = 3
  
  The scan angle in horizontal coordinates was 47.2 deg (parallactic angle = 132.8 deg).  The array was rotated to scan_offset_angle = 10.7 deg (i.e., 10.7 deg counter-clockwise from the scan angle) and then shifted by multiples of 60 deg to allow for the in situ -120 deg to -60 deg range for array_angle, which gave array_angle = -62.1 deg (in altazimuthal coordinates).
  
  
    
  
      

Cross-Linking

Calibration Observations

Pointing and Flux Calibrator Catalogs

• main flux calibration page: http://www.jach.hawaii.edu/JCMT/continuum/calibration/sens/calib_flux.html
  
  
• planets: http://www.jach.hawaii.edu/jac-bin/planetflux.pl
  
  
• secondary calibrators: http://www.jach.hawaii.edu/JCMT/continuum/calibration/sens/secondary_2004.html and Sandell (1994)
  
  
• main pointing calibration page: http://www.jach.hawaii.edu/JCMT/telescope/pointing/
  
  
• pointing sources (which include secondary calibrators):
  http://www.jach.hawaii.edu/JCMT/telescope/pointing/pointing_catalog
  

Suggested Calibration Schedule and Macros

• Pointing observations: It is strongly suggested that observers use multiple (2 to 3) pointing sources near the field to be observed (within 10 to 20 deg), checking pointing once every 2 hours or so, more frequently if near zenith and the field's local coordinates are changing quickly.  The pointing sources should be spread out so that the pointing on the field of interest can be interpolated, not extrapolated, from the grid.  Pointing sources with fluxes of 2 Jy and higher are easiest to use, though lower-flux sources can be used if necessary.  As will be indicated below, only the center of the array need be scanned across the source to check pointing, reducing the amount of time such observations take.  A typical macro for pointing calibration is
  
  
  FLSIGNAL 128 /RESET
STARE 1
FLSIGNAL 128 /SET
XRASTER_SCAN 240 600 21 /STEP_SIZE = 11 /EQUATORIAL /ALTERNATE_DIRECTION
     /SETTLING_TIME = 3
  
  
  The step size has been reduced a great deal because we need dense sampling of the pointing source by a subset of bolometers to get good pointing information.  We do only need a subset of the bolometers (the central 18 is enough), so the scan only covers 20 x 11 arcsec = 220 arcsec in the cross-scan direction.  Depending on your scan mode (/EQUATORIAL or /ALTAZIMUTHAL) and /POSITION_ANGLE setting for your science fields, you may want to modify the above command; the pointing scans should be done in the same coordinates and at approximately the same angle as your science field data to ensure they are applicable.
  
  
• Flux calibrations: These are identical to pointing observations, except that the source should be a primary or high-quality (esp. non-variable) secondary calibrator.  Again, only the center of the array need be scanned over the source. You should hit at least one high-quality flux calibrator per night.
  
  
• Full array beam maps: These are essentially expanded pointing observations, scanning the entire array over the source.  It is preferred that a primary or secondary calibrator be used so that the absolute flux calibration of all array elements can be measured directly.  These observations are also used to measure the beam profiles.  The typical macro for this will be
  
  
  FLSIGNAL 128 /RESET
STARE 1
FLSIGNAL 128 /SET
XRASTER_SCAN 240 800 73 /STEP_SIZE = 11 /ALTAZIMUTHAL /ALTERNATE_DIRECTION
     /SETTLING_TIME = 3
  
  
  The macro is longer in the scan direction and much longer in the cross-scan direction (73 x 11 arcsec = 803 arcsec) to ensure the entire array sees the source.  Full array beam maps should always be done in altazimuthal coordinates because this is the system that we want to determine the beam shapes in.  Depending on the length of your run, you may or may not want to do a full-array beam map.  The decision should be based on when the last such map was done.  Contact the Bolocam support person for help on deciding whether to do one.
  

Calibrating the Dewar Rotator

• 2004/04/06 SG
  First version
  
  
• 2004/04/12 SG
  Much expanded
  
  
• 2004/04/18 SG
  Add section on calculating coverage, add some example tracks and coverage maps, add detailed macros for calibration.
  
  
• 2004/04/20 SG
  Original tar.gz file of coverage code was missing a couple routines, corrected version uploaded.
  
  
• 2004/04/23 SG
  Add information on connecting to alpha1, adding sources to catalogs.
  
  
• 2004/04/29 SG
  Add information on manpower and support staff.
  
  
• 2004/04/30 SG
  Add backup program information.
  
  
• 2004/05/02 SG
  Update backup program information.
  
  
• 2004/05/04 SG
  Add /ROTATOR_ADJUST = ONCE command
  
  
• 2004/05/09 SG
  Make note that /GALACTIC is not supported by XRASTER_SCAN, update naming of coverage code archive
  
  
• 2004/05/21 SG
  More explicit directions about editing macros via kilauea, warnings about sources not being in the catalog or being there with incorrect coordinates.
  
  
• 2004/06/20 SG
  Remove separate archive for coverage code; link only to script run_coverage_code.pro which is not in the archive.
  
  
• 2004/10/02 SG
  Add warning about macros that take a very long time.
  
  
• 2005/01/25 SG
  Post backup cut note and binomical_cl.pro routine.
  
  
• 2005/03/26 SG
  Updated weather multiplexing section based on policy arrived at during Feb/Mar 2005.
  
  
• 2005/06/05 SG
  Update SCUBA calibration links.
  
  
• 2005/06/18 SG
  Update SCUBA calibration links.
  
  
• 2005/12/09 SG
  Correct example 6-offset scan macro -- y offsets were calculated incorrectly.
  
  
• 2006/05/05 SG
  Replaced weather multiplexing discussion with link to official weather multiplexing policy page.