MOSTWEAK (Sep2005)      Nordic_Optical_Telescope      MOSTWEAK (Sep2005)



NAME
    mostweak -- find telescope tweak parameters from  two  images  taken
    with a mask and of the object field.
    
    
USAGE
    mostweak maskimage objectimage maskdefinitionfile holesize
    
    
PARAMETERS
    
    maskimage
        Name of the image taken WITH a mask of a calibration lamp.
    
    objectimage
        Name of the image taken WITHOUT a mask of the target field.
    
    maskdefinitionfile
        Name of the MOS mask definition file (.ap file).
    
    holesize
        The  diameter  of  the  fiducial holes in arcsec.  In this task,
        this parameter affects the centroid algorithm.
    
    
DESCRIPTION
    This task determines the centroid positions of  the  fiducial  stars
    and  the  fiducial holes on an image of the mask and an image of the
    target object field. To measure the pixel coordinates on  the  image
    the interactive task imexamine is used.
    
    The differences in the measured positions of the stars and the holes
    are then transformed into telescope tweak  parameters:  offsets  and
    rotation.  Ask somebody who knows about it (maybe yourself) to apply
    this tweak, which should bring the  fiducial  stars  closer  to  the
    centre of the fiducial holes of the mask.
    
    
EXAMPLES
    Take  an  image  of  the  target  astronomical field at the required
    telescope position and rotator angle, and an image of a  calibration
    lamp (i.e. halogen) with the mask, and do
    
      mostweak /data/alfosc/jc240012.fits /data/alfosc/jc240011.fits MOS0006.ap
    
    Ask  yourself to apply the recommended tweak (first the offset, then
    the rotation). This tweak should bring your  fiducial  stars  closer
    to the centre of the mask holes.
    
    Iterate  the  tweak  by using the task finetweak until the stars are
    properly positioned in the holes.
    
    
THE INTERACTIVE PART OF THE TASK
    This task will first display  the  mask  image  which  contains  the
    fiducial-star  slits/holes;  the  slits are marked by a circle.  The
    user is advised to adjust the colour  scale  of  the  image  display
    tool  such  that  the  fiducial  stars  AND  the  fiducial holes are
    clearly visible.  Then the object  field  image  will  be  displayed
    with the fiducial stars marked as described above.
    
    The  user  has  to  point  the  cursor at the fiducial stars (slits)
    marked by a YELLOW circle, and has to press key -a- to activate  the
    centroid  algorithm.   If  key  -a-  is  pressed more than once, the
    final press will be evaluated.  Each press will be  acknowledged  by
    statistics  appearing on the terminal that runs IRAF.  To advance to
    the next star (slit), press -q-.
    
    On the image display the centroid positions of the slits  and  stars
    will be marked by a RED circle.
    
    When  all  fiducial slits and fiducial stars have been marked by the
    user, the tweak will be computed. Check the distances between  slits
    and stars as output in the IRAF window.  Check that the suggested XY
    and rotational offsets make sense, before applying  the  tweak  with
    mosmove.
    
    If  one  of the centroid positions of the stars or holes seems wrong
    (judging from the RED circles and/or the  computed  distances),  the
    task  can  be run again with the user discarding the star in the way
    described above.
    
    A good tweak fit needs no less than 3 stars. Nevertheless,  a  tweak
    can be computed on only 2 stars.
    
    
TIME REQUIREMENTS
    A lot less than doing it by hand.
    
    
BUGS
    email Jacob W. Clasen (jclasen@not.iac.es),
    for bug reports and constructive remarks
    
    
SEE ALSO
    mosmove, mosstrong