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46.10 Data Accuracies and Problem Solving

In this section we characterize the accuracy obtainable with WF/PC-1 data and suggest steps to take to achieve the best accuracy from the data. The table below summarizes the estimated calibration accuracies and the following sections offer some final suggestions on improving the calibration.*


Estimated WF/PC-1 Calibration Accuracies

Attribute

Estimated Accuracy

Comments

Bias

0.2-0.3 DN

Files d8c08261w, d8c0826nw

Dark

~0.0006 DN/sec

Files dac14274w, dae14273w;

worse on hot pixels

Preflash

~0.5 DN

Assuming 8 sec preftime for PC,

30 sec for WF (files d8c0*)

Flatfields, in general

~3%

~25-30%

~5-10%

~1-2%

Small scales (<<1")

scales > 1" if flat contains F122M or F8ND

(includes most broad band filters)

scales > 1" if flat contains no ND filter

(includes most narrow band filters)

scales > 1" if high-fidelity flat used

Flatfields,

additional effects

up to 5-7%

~1% rms (4-5% peak)

few percent

Short exposure reciprocity problem, if flat

and data have very different exposure times.

To minimize, use flat with same exptime as data.

Measle effects, partially correctible (see text)

changes caused by decon; reduced with deltaflats

Relative photometry

5-30%

Depending on flat used

Absolute photometry

5-30%

Strongly dependent on flat used

(see "Flatfield Anomalies" on page 46-7 and "Improving the Flatfield Correction" on page 46-15)

Plate Scale

WFC: 0.1016" +/- ~0.0001"

PC: 0.0439" +/- ~0.0001"

46.10.1 Improving Bias, Preflash, and Dark Calibration

46.10.2 Improving the Flatfield Correction

Apply a Delta Flat

A delta flat correction may be necessary if there was a decontamination procedure between the epoch of the flatfield and the epoch of the science data, particularly for PC8 data. One of the archived delta flats could be used or a new one created from individual internal flatfield exposures (see "Choosing and Generating Delta Flats" on page 45-18).

Experiment with Flatfields

After the end of the WF/PC-1 mission, Closure and high-fidelity flatfields, as well as deltaflats, were generated and archived; these flats cover ~85% of the nearly 8000 external WF/PC-1 science images in the archive. Any of these flats may be retrieved from the Archive and used for recalibrations; the reference file memo on WWW lists all of the WF/PC-1 reference files available from the archive. Small scale errors in the flatfields are normally around 3% (the high-fidelity flatfields are generally good to nearly 1%) while 10% errors can be expected on larger scales. In addition, those flatfields which were generated from Earth calibration observations taken with the neutral density filter F122M are known to have 25% gradients across the four WF CCDs. The following suggestions may improve the flatfielding.

http://www.stsci.edu/ftp/instrument_news/WFPC/
wfpc1_memos.html

46.10.3 Improving the Photometric Calibration

The resultant relative photometry at visible and far-red wavelengths should be good to 5-15% in the PC and WF2, and 10-30% for the full field of the WFC. Somewhat better accuracy is possible for narrow and medium band filters if the flats were not observed through neutral density (ND) filters; in these cases, the accuracy will be around 5-10% for all cameras. If core photometry is used in conjunction with a linear magnitude correction (WF/PC-1 ISR 92-02), and corrections are made for throughput variations and flatfield problems (ND filter gradients, edge droop, measles, etc.), an accuracy around 3-5% should be possible. The accuracy for absolute photometry will ultimately be limited by the accuracy of the absolute calibration, which is around 5% (see WF/PC-1 ISR 92-09).



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