8.22 Calibration Accuracy

Table 8.16: 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 8.16: 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

~10%

F160BW; however, significant noise reduction achieved with use of correction flats

Relative photometry

Residuals in CTE correction

< 3% for the majority (~90%) of cases

up to 10% for extreme cases (e.g., very low backgrounds)

Long vs. short anomaly (uncorrected)

< 5%

Magnitude errors <1% for well-exposed stars but may be larger for fainter stars. Some studies have failed to confirm the effect

Aperture correction

4% rms focus dependence (1 pixel aperture)

Can (should) be determined from data

<1% focus dependence (> 5 pixel)

1-2% field dependence (1 pixel aperture)

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

< 1%

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

Astrometry

Relative

0.005" rms (after geometric and 34th-row corrections)

Same chip

0.1" (estimated)

Across chips

Absolute

1" rms (estimated)

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	Visible, near UV
	~10%	F160BW; however, significant noise reduction achieved with use of correction flats
Relative photometry
Relative photometry
Residuals in CTE correction	< 3% for the majority (~90%) of cases
Residuals in CTE correction	up to 10% for extreme cases (e.g., very low backgrounds)
Long vs. short anomaly (uncorrected)	< 5%	Magnitude errors <1% for well-exposed stars but may be larger for fainter stars. Some studies have failed to confirm the effect


Aperture correction	4% rms focus dependence (1 pixel aperture)	Can (should) be determined from data
	<1% focus dependence (> 5 pixel)
	1-2% field dependence (1 pixel aperture)
Contamination correction	3% rms max (28 days after decon) (F160BW)
Contamination correction	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	< 1%
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
Astrometry


Relative	0.005" rms (after geometric and 34th-row corrections)	Same chip
Relative	0.1" (estimated)	Across chips
Absolute	1" rms (estimated)