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Why do I see the HRC occulting finger in my image?

HRC Occulting Finger
Why do I see the HRC occulting finger in my HRC image?

Reply
There is a 0.8" wide, 5" long occulting finger permanently
located at the window of the HRC CCD dewar which is used to
prevent saturation of bright targets when used for direct
imaging. Since it is not in the image plane, there is some
vignetting around the edges of the finger. The HRC flat field
reference file is masked to unity in a 1.6" wide, 6.5" long
region covering the finger and the vignetted region. These
pixels are given a value of 8 in the DQ array, indicating
the flag "masked by aperture feature." Using the default
PyDrizzle parameters, the corresponding pixels in the science
image will be replaced with a value of zero. Observers should
consider dithering HRC observations by at least 1.6" in the
x-direction if photometry in this region is required. Work is
in progress to reduce the size of the occulting finger mask by
determining the appropriate flat field correction in the vignetted
regions around the finger.

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Later addition:

The following pattern parameter form 
(and POS TARG equivalent) defines an HRC dither 
shift that spans the area flagged for the HRC 
occulting finger (~1.6 arcsec or ~56 pixels wide), 
with an extra ~0.3 arcsec or ~10 pixels of overlap.
The shift is along the detector x-axis
(which differs slightly from the POS TARG x-axis).
This is RPS2 syntax for use in Phase II proposals:

    Pattern_Number: 12
   Primary_Pattern:
      Pattern_Type: ACS-HRC-DITHER-LINE
   Pattern_Purpose: DITHER
  Number_Of_Points: 2
     Point_Spacing: 1.90
  Coordinate_Frame: POS-TARG
    Pattern_Orient: 5.65
    Center_Pattern: NO
  
  POS TARG equivalent:  1.890, 0.187