ACS-WFC-DITHER-BOX

This is a two-dimensional 4-point dither pattern, with the following allowable parameter ranges:

     Pattern_Number: 1 
       Pattern_Type: ACS-WFC-DITHER-BOX
    Pattern_Purpose: DITHER
   Number_Of_Points: 4
      Point_Spacing: 0.01 - 10.0  (arcseconds)
       Line_Spacing: 0.01 - 10.0  (arcseconds)
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 0.0 - 360.0  (degrees)     
Angle_Between_Sides: 0.0 - 360.0  (degrees) 
     Center_Pattern: ?            (YES or NO)

The following versions of this Pattern_Type are projected onto the detector pixel space in the graphic above.

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The default pattern has relative pixel coordinates
(0, 0), (5.0, 1.5), (2.5, 4.5), (-2.5, 3.0).
It is a parallelogram pattern designed for optimal half-pixel sampling in both x and y, with overall dimensions large enough to help reject the larger detector artifacts.

     Pattern_Number: 16
       Pattern_Type: ACS-WFC-DITHER-BOX
    Pattern_Purpose: DITHER
   Number_Of_Points: 4
      Point_Spacing: 0.2637
       Line_Spacing: 0.1856
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 20.7
Angle_Between_Sides: 69.02
     Center_Pattern: NO

POS TARG equivalent:  0.000,  0.000
                      0.247,  0.093
                      0.123,  0.232
                     -0.123,  0.139

This diagram shows how much this pattern's shape varies across the entire WFC field-of-view, as a result of scale variation.

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The following pattern was designed for the UDF observations. It is slightly more compact than the default pattern above, and therefore reduces the effect of the scale variation on the integrity of the pattern across the entire WFC field-of-view:

     Pattern_Number: 
       Pattern_Type: ACS-WFC-DITHER-BOX
    Pattern_Purpose: DITHER
   Number_Of_Points: 4
      Point_Spacing: 0.171
       Line_Spacing: 0.171
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 30.16
Angle_Between_Sides: 145.82
     Center_Pattern: NO

POS TARG equivalent: 0.000, 0.000
                     0.148, 0.086
                     0.222, 0.240
                     0.074, 0.154

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The compact pattern is about as compact as possible, therefore it minimizes the effect of the scale variation on the integrity of the pattern across the entire WFC field-of-view, at the expense of not rejecting some of the larger detector artifacts.

     Pattern_Number: 
       Pattern_Type: ACS-WFC-DITHER-BOX
    Pattern_Purpose: DITHER
   Number_Of_Points: 4
      Point_Spacing: 0.104
       Line_Spacing: 0.053
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 18.31
Angle_Between_Sides: 80.79
     Center_Pattern: NO

POS TARG equivalent: 0.000, 0.000
                     0.099, 0.033
                     0.074, 0.080
                    -0.025, 0.047

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This expanded pattern is designed to move an object completely off of its own PSF between exposures. It is similar to the other BOX patterns here, but has overall dimensions about 4 times larger. This is optimized for smallish objects near the defined aperture, i.e. when the shape of the pattern near the edges of the field-of-view is less important.

     Pattern_Number: 
       Pattern_Type: ACS-WFC-DITHER-BOX
    Pattern_Purpose: DITHER
   Number_Of_Points: 4
      Point_Spacing: 0.841
       Line_Spacing: 0.795
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 19.91
Angle_Between_Sides: 97.36
     Center_Pattern: NO

POS TARG equivalent:  0.000, 0.000
                      0.790, 0.286
                      0.618, 1.063
                     -0.173, 0.776

This pattern is not plotted above, but its relative dimensions can be viewed in this comparison plot.