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Hubble Space Telescope
WFPC2 CTE Tool #3 Help Page

Gain Ratio: Select the gain ratio for the dataset in question. This can be found in the observation's header file with the keyword ATODGAIN.

MJD of Observation: Enter the Modified Julian Date of the observation's start time. This is the header keyword EXPSTART. The expected format is 5#####.####...

Measured Source Counts: Enter your measured counts, in DN, for the source you wish to estimate CTE effects.

X-Position on CCD: Enter the X-position of the source on the CCD. The expected range is from 1 - 800.

Y-Position on CCD: Enter the Y-position of the source on the CCD. The expected range is from 1 - 800.

Background Counts: Enter your measured background for the observation, in DN.

The CTE Correction Formulae:

For CTSobs < 4000 DN and BKG > 0.1 DN:

Y-CTE = 2.3 * 10 - 0.256 * logBKG * (1 + 0.245 * (0.0313 - 0.0087logCTSobs)
        * (MJD - 49471.0))

X-CTE = 2.5 * (1 + 0.341 * (0.00720 - 0.0020logCTSobs) * (MJD - 49471.0))

For CTSobs > 4000 DN and BKG > 0.1 DN:

Y-CTE = 2.3 * 10-0.256 * logBKG

X-CTE = 2.5

Total CTE:

CTScor = [1 + (Y-CTE/100 * Y/800) + (X-CTE/100 * X/800)]

Where:

CTScor = Number of counts in DN for the point
                    source, after CTE correction.

CTSobs = Raw number of counts in DN for the source.

Y-CTE = CTE loss in percent over 800 pixels in the Y-direction.

X-CTE = CTE loss in percent over 800 pixels in the X-direction.

X, Y = X, Y source position in pixels.

BKG = Mean number of counts in DN for a blank region of the
      background.

MJD = Modified Julian Date for the observation.

Note 1: Small uncertainties in the bias level can lead to the measurement of negative values for BKG when the background is very low (<0.05 DN). Our recommendation for cases with BKG < 0.1 DN is to linearly scale the value based on observations using the same filters but with longer exposures of the same field, if available. In cases where longer exposures are not available, estimates can be made using Table 1 in the Whitmore, et al. paper.