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Dithered Imaging
WFI_img_obs_Dither

• Auto-dithering or you can define your own dithering pattern
• Combined Offsets: do you want the autoguider to follow the guidestar when you offset?
  
  • Note: For large offsets (>?? arcsec) this should be set to no. The guidestar will fall outside of the tracker chip after the offset and a new one will have to be selected
• Return to Origin: do you want the telescope to return to the original position after each dither pattern?
  
  • This should be set to true if you want to perform the same dither patter with more than 1 filter inside the same OB. Otherwise, the dither pattern of the 2nd filter will start at the last dither position of the first filter.
• Filter: should be the same one as you used in your aquisition to minimise overheads
• Subexposure time: How long do you want each exposure to be?
• Number of Subexposures: How many exposures do you want in the dither sequence?
  
  • This number can be 1 if you only want 1 exposure
  • If you are defining your own dither sequence, this number must match the number of offsets you have given
• CCD Binning Mode 1x1 is default
• Observation Type choose from menu to reflect the purpose of the observation

P2PP panel with example parameters

The above example shows an OB comprising a sequence of:

• An aquisition template that will execute a simple preset, turn the autoguider on, and put the V/89 filter in the beam
• Take 5 dithered exposures of 120 seconds each through the V/89 filter in a dithering pattern optimised for 5 exposures
• Autoguider follows the offsets and doesn't lose the guidestar
• Returns to the original preset position after dither sequence is finished
• Take 3 dithered exposures of 20 seconds through the B/123 filter in a user defined dither pattern.
  • The 1st exposure is offset from the preset position by 560,560 arcseconds
  .
  • The 2nd exposure is offset from the 1st position by -1000,-1020 arcseconds
  • the 3rd exposure is offset from the position of the 2nd exposure by 500,480 arcseconds (i.e. the 3rd exposure should be taken -60,-60 arcseconds from the preset position)
  • Note: The offsets are cumulative!
  • The telescope does not return to the preset position since Return to Origin = false
• Take a single exposure of 500 seconds through the I/203 filter at the position of the 3rd exposure of the above dither sequence
• This happens since we did not set Return to Origin = true in the preceding part of the OB

Focusing
WFI_cal_FocusSeq

Within the template, one needs to define:

• Filter: Which filter you wish to focus in?
• Sub-exposure time: How long you want each exposure to be?
  
• Number of sub-exposures: How many exposures do you want to define the focus sequence?
• Telescope Focus Offset: How many encoder units do you want to move M2 by between each exposure?
• Image charge shift: How many pixels do you want each image of the star separated by?

Click here to download a standard Focus OB (for the V/89 filter). You can then import this directly into your P2PP, duplicate it and edit the duplicates to make Focus sequences for other filters.

Send comments to : ls-wfi Last modified: Thu Jul 31 2003