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Dithering: Relationship Between POS TARGs and CCD Rows/Columns

- John Biretta, WFPC2 Scientist


I want do dither my exposures by exactly integral pixel amounts. What is the exact relationship between the POS TARGs we specify in the proposal and the CCD rows and columns?


The answer is that the POS TARG axes run exactly along the CCD rows and columns on the specified aperture. For example, if you specify aperture WF3, the POS TARG axes will run *exactly* along the rows and columns on WF3. For the other CCDs the POST-TARGs will run and only approximately along rows and columns on other, since there are small rotations (<0.5 degree) of the CCDs from their nominal alignments. Note that if WFALL is specified, then the rotation for WF3 is used.

For small dithers (<0.3 arcsec.) the rotations between CCDs are unimportant, as they imply pixel registration errors less than 3 milliarcseconds, which is roughly the nominal pointing and guiding stability.

In somewhat greater detail, the rough CCD orientations (modulo 90 degrees) are used to convert from POS TARG (X,Y) to SICS (Science Instrument Corrected System) by the TRANS scheduling software. Next, the exact CCD rotations in the PDB SIAF file are used to convert from SICS to actual spacecraft (V2,V3) motions. The angle used for this last transformation depends on the aperture specified. This is spelled out in the "Transformation Guide: Pointing and Orientation SCI90201B" section 2.1 (STScI internal document).

The "beta-x" and "beta-y" values in the table below give our current best estimates on the CCD rotations. The "beta-x" and "beta-y" angles are rotations CCW from +V3 axis to the CCD "rows" and "columns" axes, respectively. These values should be in place when processing Cycle 5 proposals.

The table also gives the best current estimates of the pixel scale (x-scale and y-scale), the CCD pixel position of the aperture definitions (X and Y), and the corresponding HST OTA focal plane positions of these apertures (V2 and V3). Additional information concerning CCD scales and orientations can be found in "The WFPC2 Scales and Alignments" by Colin Cox (report SOB-94-10-21 available from

NOTE: The table below contains values applicable to Cycle 5. For the latest positional information, see the Observatory Support Group's Webpage.

Aper-    V2      V3     x-scale    y-scale    beta-x   beta-y      X      Y
ture      (arcsec)       (arcsec/pixel)          (degrees)         (pixels)
-----  -----   -----   --------   --------    ------   ------    -----   -----
PC1     2.09  -30.95   0.045561   0.045541    134.75   224.75    420.0   424.5
WFALL   1.85    7.48   0.099647   0.099553    314.54    44.54    133.0   149.0
WF2   -51.69   -6.49   0.099573   0.099669    224.23   314.23    423.5   414.0
WF3    -0.42   48.01   0.099647   0.099553    314.54    44.54    436.5   424.5
WF4    54.70   -6.60   0.099612   0.099708     45.10   135.10    423.0   421.0

(Gary Curtis, Colin Cox, Glenn Miller, and Mark Giuliano contributed to this memo, 30 Jan. 1995.)