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28.6 Accuracy of WFPC2 Results

Table 28.6 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.

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; uncertain

Relative photometry

Residuals in CTE correction

< 1% rms (above 1000 DN)

3% rms (at 100 DN)

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% 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 correction)

Same chip

0.1" (estimated)

Across chips

Absolute

1" rms (estimated)

**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; uncertain
Relative photometry
Residuals in CTE correction	< 1% rms (above 1000 DN)
3% rms (at 100 DN)
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% 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 correction)	Same chip
0.1" (estimated)	Across chips
Absolute	1" rms (estimated)

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