WFPC2 Instrument Handbook for Cycle 11

Calibration Accuracy

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Table 8.11 summarizes the accuracy to be expected from WFPC2 observations in several areas. The numbers in the table should be used with care, and only after reading the relevant sections of this handbook and the documents referenced therein; they are presented in tabular form here for easy reference.

Table 8.11: Accuracy Expected in WFPC2 Observations

Procedure	Estimated Accuracy	Notes
Calibration (flatfielding, bias subtraction, dark correction)
Bias subtraction	0.1 DN rms	Unless bias jump is present
Dark subtraction	0.1 DN/hr rms	Error larger for warm pixels; absolute error uncertain because of dark glow
Flatfielding	<1% rms large scale	Visible, near UV
	0.3% rms small scale
	3-5% rms	F218W, F255W, F300W
	5-10% rms	F122M, F160BW, F170W, F185W; uncertain
Relative photometry
Residuals in CTE correction	< 1% rms (above 1000 DN)
	3% rms (at 100 DN, <1997)
	3% rms (100 to 500 DN, background <1 DN/pixel, >1997)
	5% rms (<100 DN, background <1 DN/pixel, >1997)
Long vs. short anomaly (uncorrected)	< 1% rms (above 10000 DN)
	5% (at 1000 DN)
	15% (at 100 DN)
Aperture correction	4% rms focus dependence (1 pixel aperture)	Can (should) be determined from data
	1-2% field dependence (1 pixel aperture)
	<1% focus dependence (> 5 pixel)
Contamination correction	3% rms max (28 days after decon) (F160BW)
	1% rms max (28 days after decon) (filters bluer than F555W)
Background determination	0.1 DN/pixel (background > 10 DN/pixel)	May be difficult to exceed, regardless of image S/N
Pixel centering	~ 2%
Absolute photometry
Sensitivity	< 2% rms for standard photometric filters	Red leaks are uncertain by ~10%
	2% rms for broad and intermediate filters in visible
	< 5% rms for narrow-band filters in visible
	2-8% rms for UV filters
	5-10% rms for linear ramp filters
Astrometry
Relative astrometry	0.005" rms (after geometric correction)	Same chip
	0.15" (measured)	Across chips
Absolute astrometry	1" rms (estimated)
Polarimetry
Fractional polarization	1.5% rms	Larger errors occasionally seen