Determine Average Off-Axis Angle of a Source

 

For some tasks, e.g. the calculation of a model point spread function, the off-axis angle of a source is required. Due to the spacecraft wobble the off-axis angle of a source (angle between detector centrum and source) changes during the observation. The exact procedure for calculating a model PSF would be to create a histogram of the off-axis angle, calculate a PSF for each value of the off-axis angle, and add the resulting PSFs.  

A much faster (but low-precision) approach is to take an average of the off-axis angle within the accepted time intervals, and calculate an approximate PSF corresponding to this average off-axis angle.

The average off-axis angle can easily be obtained by the command

AVERAGE/OFF_AXIS_ANGLE [attitude] [xsky,ysky] [timref] [display]

 

• attitude : Attitude table, defaulted to attitude.tbl
• xsky,ysky : X- and Y-position of the source, in units of sky pixels. Defaulted to 0.0
• timref : Name of file containing the reference time selections (descriptor TIM_SEL). Can be either an image or a table. Defaulted to image1.bdf.
• display : Display flag. Default is d (display output on screen). Any other input suppresses the screen output.

The off-axis angle of a source at sky coordinates xsky,ysky is calculated for each time bin within the selected time intervals specified in descriptor TIM_SEL of the timref file. The output is a simple average of these values in units of arcmin. The number of used time bins is stored in keyword OUTPUTI(1), the average off-axis angle in units of arcmin is stored in keyword OUTPUTR(1).

Examples are:

AVERAGE/OFF 
AVERAGE/OFF myattitude 123,456 myimage n