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ACS pointing patterns

ACS-WFC-DITHER-LINE
pointing patterns

This is a one-dimensional dither pattern, with the following allowable parameter ranges:

     Pattern_Number: 1 
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 2-9
      Point_Spacing: 0.01 - 10.0 (arcseconds)
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 0.0 - 360.0 (degrees)     
Angle_Between_Sides: N/A  
     Center_Pattern: ?           (YES or NO)
The following versions of this Pattern_Type are projected onto the detector pixel space in the graphic above.
The default pattern has two points shifted roughly 5x60 pixels apart (not shown above) to span the WFC interchip gap, so that combined data will not have missing data there. This is the most commonly used ACS pointing pattern. Note that with only two exposures, the default pattern will not reject cosmic rays in the overlapping gaps (only one exposure there). If a third point is added (Number_Of_Points: 3), or if this gap dither is combined with any other small-scale dither (see below), then cosmic rays can be rejected in the gaps.
     Pattern_Number: 11
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 2   (use 3 points to reject cosmic rays in gap)
      Point_Spacing: 2.994
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 85.27
Angle_Between_Sides: N/A
     Center_Pattern: NO
POS TARG equivalent:     0.000   0.000
                         0.247   2.984
                         0.494   5.968 (3rd pointing rejects cosmic rays in gap)
Note that the significant ACS scale variation causes the larger pointing patterns to lose their integrity across the field-of-view. This diagram illustrates how this pattern, a 5x60 pixel shift at the chip center, varies from corner to corner. This variation was accounted for in the design of all the ACS patterns.
The 2-point hot pixel dither pattern is a 2x2 pixel shift which is very small, but a big improvement over a CR-SPLIT (no shift). It is optimized for rejection of both cosmic rays and hot pixels. This pattern was designed in response to the poor annealing rate of WFC hot pixels.
     Pattern_Number: 10
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 2
      Point_Spacing: 0.145
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 47.23
Angle_Between_Sides: N/A
     Center_Pattern: NO
POS TARG equivalent:  0.000,  0.000
                      0.099,  0.107


The 2-point artifact dither pattern is a 5x5 pixel shift which handles both hot pixels and uncalibrated pixels at the base of bad columns (blobs of saturated pixels in dark calibration files, sometimes bleeding several columns wide).
 
     Pattern_Number: 14
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 2
      Point_Spacing: 0.364
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 47.23
Angle_Between_Sides: N/A
     Center_Pattern: NO
POS TARG equivalent:  0.000,  0.000
                      0.247,  0.267 


The 2-point sub-pixel dither pattern does not actually sub-sample all by itself, but in conjunction with another pattern (e.g. the gap dither), it may be a good option.
     Pattern_Number: 12
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 2
      Point_Spacing: 0.149
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 34.25
Angle_Between_Sides: N/A
     Center_Pattern: NO
POS TARG equivalent:  0.000,  0.000
                      0.124,  0.084


The 3-point sub-pixel dither pattern also does not optimally sub-sample all by itself (usually the 3-point gap dither is a better alternative, for only 3 exposures).
     Pattern_Number: 13
       Pattern_Type: ACS-WFC-DITHER-LINE
    Pattern_Purpose: DITHER
   Number_Of_Points: 3
      Point_Spacing: 0.170
       Line_Spacing: N/A
   Coordinate_Frame: POS-TARG
     Pattern_Orient: 47.23
Angle_Between_Sides: N/A
     Center_Pattern: NO
POS TARG equivalent:  0.000,  0.000
                      0.115,  0.125
                      0.231,  0.249


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