What is the best way to dither my ACS exposures so that I can get rid of hot pixels?The following pattern parameter forms (one for each ACS detector) define dither patterns which are optimized for rejecting hot pixels when combining multiple exposures. They are each 2x2 (integer) pixel dithers along the detector diagonal, which best matches pixel area (i.e. minimizes the effect of the ACS scale variation). They can be defined and then used in any HST proposal, as described in the HST Phase II Instructions.
The POS TARG equivalents included below are not actually a part of the pattern forms -- they are an alternate way to dither. However, we encourage the use of pattern forms, because our archive (queries) and calibration pipelines (drizzling) all key on them (and not on POS TARGs).
Pattern_Number: 10 Primary_Pattern: Pattern_Type: ACS-WFC-DITHER-LINE Pattern_Purpose: DITHER Number_Of_Points: 2 Point_Spacing: 0.146 Coordinate_Frame: POS-TARG Pattern_Orient: 47.17 Center_Pattern: NO POS TARG equivalent: 0.099, 0.107 Pattern_Number: 20 Primary_Pattern: Pattern_Type: ACS-HRC-DITHER-LINE Pattern_Purpose: DITHER Number_Of_Points: 2 Point_Spacing: 0.079 Coordinate_Frame: POS-TARG Pattern_Orient: 44.28 Center_Pattern: NO POS TARG equivalent: 0.057, 0.055 Pattern_Number: 30 Primary_Pattern: Pattern_Type: ACS-SBC-DITHER-LINE Pattern_Purpose: DITHER Number_Of_Points: 2 Point_Spacing: 0.094 Coordinate_Frame: POS-TARG Pattern_Orient: 44.40 Center_Pattern: NO POS TARG equivalent: 0.067, 0.066
These patterns are additional non-default variations of the DITHER-LINE pointing patterns defined in the following report (ACS ISR 01-07, which you may want to review for additional background):
The Pattern_Number can be any unique integer within a given proposal. The numbers here are consistent with the numbering used in the report mentioned above, which only provides a convenient way to uniquely identify each variation of each pattern. With APT (for Cycle 12), this number is now automatically assigned.