Dither Strategies

The "library" of dither patterns provided here is intended to give ACS users a quick way to identify and use, with confidence, a carefully designed pointing pattern which suits their observational goals. These include small-scale dithers (for artifact rejection and/or sub-sampling) and large-scale mosaics (for increasing the field-of-view). Users can always design their own freelance patterns (see below), but for most, the patterns presented below should eliminate the need to calculate dither parameters oneself.

Click on any Pattern_Type to view and compare suggested patterns:

As of HST Cycle 31, new 2-, 3-, and 4-point compact dither patterns have been derived for joint optimization of sub-pixel phase sampling when ACS/WFC and WFC3 (both UVIS and IR) are used in prime+parallel imaging.  

These new dither patterns are described in ACS ISR 2023-04, and must (at time of Cyc31 writing) be entered into the APT Phase2 specification as a series of POSTARGs.  

Discussions are underway with the APT development team to implement these dither patterns as "pre-fabs", in the same manner that the ACS-specific patterns  above are available. 

General information about designing ACS pointing patterns, and the "convenience" patterns available for Phase II proposers is provided in ACS dither and mosaic pointing patterns (Mutchler & Cox, ACS Instrument Science Report 2001-07).  The parameters listed below were updated to reflect the latest distortion solutions, and so the distortion coefficients below supersede those found in ISR 2001-07. Although we don't expect these updates to significantly change the parameters in these pattern forms, you can calculate your own pointing parameters as follows:

\(X_{pos} = x*a_{11} + y*a_{10}\\ Y_{pos} = x*b_{11} + y*b_{10}\\ PointSpacing = \sqrt{X^2_{pos} + Y^2_{pos}}\\ PatternOrient = arctan\big(\frac{Y_{pos}}{X_{pos}}\big)\)

Where the current distortion coefficients (just the linear terms of the full distortion polynomial) are:

For two-dimensional "box" patterns, the Line_Spacing is calculated like the Point_Spacing, except using the (x,y) pixel shift from the 2nd to the 3rd pointing. The Angle_Between_Sides is the 180 degrees minus the Pattern_Orient of the second segment, minus the Pattern_Orient of the first segment.

The ACS pointing pattern forms defined in the Phase II Proposal Instructions (and available in APT) have logical default values, but can be modified extensively for a wide variety of purposes. The pattern library above contains both these default or "convenience" patterns plus many commonly-used non-default patterns. Also, generic LINE, BOX, and SPIRAL pattern forms can be used to define pointing patterns which exceed the logical limits imposed on the "named" ACS patterns. Further, all these patterns can be combined (as primary and secondary patterns) in APT, to create complex patterns that simultaneously achieve multiple goals. You are strongly advised to display and review your pointing patterns in the VTT before submitting your Phase II proposal.

Due to the rather extreme distortion and scale variation present in the ACS optics, the larger patterns lose their integrity near the edges of the field-of-view. For example, this diagram shows the variation of the WFC gap dither (5x60 pixel shift) over the entire field-of-view: the y shift (across the gap) varies from 57 to 63 pixels.