These coefficients are a (no longer recommended) alternative way to do what the gain table does by transforming PHA to energy and PI, using polynomial fits to make the correction more accurate on smaller scales then 32x32 pixel regions.

A key to the symbols used:

pha0 and rows are the original PHA and CHIPY values for each event.

For a given energy, the pulseheight versus row is represented as a 2nd order polynomial:

pha = (1. + (cti*rows) + (eps*rows^2.)) * pha0

where cti and eps are the 1st and 2nd order terms and are energy dependent (see below). (This cti is *not* what we typically define as CTI). In this way you can transform your row dependent pha0 to row-independent pha.

The energy dependence of cti and eps are both represented as 2nd order polynomials.

cti = a1 + b1*pha0 + c1*pha0^2. eps = a2 + b2*pha0 + c2*pha0^2.

a1, a2, ..., as listed in the table on the previous page, are constants fit from calibration data.

To transform pha to energy,

E = (x0 + pha) / x

where x0 is the offset (in ADU) and x is the slope (in ADU/keV) of the gain function. Since the row-dependence of the detector has been removed, the same gain function can be used for each node.

back to:
ACIS cti, -120 Page
ACIS cti, -110 Page

Note: As mentioned above, this method is no longer recommended by the ACIS cal team. New recommendations will be posted on the main Gain/CTI page in the near future.

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