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27.3 Dark Current Subtraction Errors

27.3.1 Electronic Dark Current

At the operating temperature of -88 C, maintained after April 23, 1994, the WFPC2 CCDs have a low dark background, ranging between 0.002 and 0.01 e^-/s/pixel. A relatively small number of pixels have dark currents many times the value. These warm pixels are discussed in great detail in Chapter 26. To remove the dark current, the standard pipeline procedure takes a dark reference file (which contains the average dark background in DN/s), multiplies it by the dark time (determined by the header keyword DARKTIME), and subtracts this from the bias subtracted image. Prior to April 23, 1994, the CCDs were operated at -77 C. The correction procedure is the same for these early data, but the average dark current was about an order of magnitude higher due to the higher temperature. Hence, the dark current correction is both more important and less accurate than for later data.

The dark time is usually close to the exposure time, but it can exceed the latter substantially if the exposure was interrupted and the shutter closed temporarily, for example as a consequence of loss of lock. Such instances are rare and should be identified in ther header keyword EXPFLAG and in the data quality comments for each observation, but will also be indicated by a difference between the exposure start and end time which is greater than the total exposure time. The true dark time differs slightly from pixel to pixel because of the different time elapsed between reset and readout. To the extent that dark current is constant with time, this small differential is present both in the bias image and in the observation itself, and therefore is automatically corrected by the bias subtraction.

New dark reference files are delivered on a weekly basis, but because of the necessary processing, they are usually available a week or two later than the observation itself. As a result, the dark used in the pipeline is not the same as the dark reference file recommended by StarView. The primary difference between successive darks is in the location and value of hot pixels. This difference will be most notable if a decontamination occurred between the images used to create the dark and the observation itself. However, because direct treatment of the warm pixels themselves, rather than dark file subtraction, now appears the best way to remove warm pixels, many users will find that they do not need to reprocess with the most up-to-date dark file. For more details, see "Warm Pixels" on page 26-16.

Until August 1996, the weekly standard darks were based on a relatively small number (10) of exposures, taken over a time span of two weeks, in order to track the variable warm pixels. However, these darks can be a significant component of the total noise in deep images. Observers whose (pre-August 1996) images are formed from exposures totalling more than five orbits may therefore wish to recalibrate their data using the so-called superdarks, which have been generated by combining over 100 individual exposures (see the reference file memo on the WFPC2 Web page).

Since August 1996, the weekly standard darks have been produced by combining the relevant superdark with the warm pixel information in the dark frames taken that week. The combined file is obtained by using the superdark value for all pixels that appear normal in the weekly dark, namely, for which the dark current value in the weekly dark does not differ from the superdark value by more than 3 . For pixels that do deviate more than 3 , the weekly dark value is used. This compromise allows a timely tracking of warm pixels, while maintaining the low noise properties of the superdark for pixels that do not appear to change. Recalibration may still be appropriate, because the weekly standard dark is not yet available when the image is processed and archived at STScI.

27.3.2 Dark Glow

While the electronic dark current is relatively stable between observations, a component of the "dark current" has been seen to vary between observations. The intensity of this dark glow is correlated with the observed cosmic ray rate, and the glow is believed to be due to luminescence in the MgF₂ CCD windows under cosmic ray bombardment. As a result of the geometry of the windows, the dark glow is not constant across the chip, but rather shows a characteristic edge drop of about 50%. The dark glow is significantly stronger in the PC, where it dominates the total dark background, and weakest in the WF2. The average total signal at the center of each camera is 0.006 e^-/s in the PC, 0.004 e^-/s in WF3 and WF4, and 0.0025 e^-/s in WF2; of this, the true dark current is approximately 0.0015 e^-/s, and the rest is dark glow. For more details, see the WFPC2 Instrument Handbook, Version 4.0, pages 71-75.

Because of the variability in the dark glow contribution, the standard dark correction may leave a slight curvature in the background. For the vast majority of observations, this is not a significant problem, because of the very low level of the error (worst-case center-to-edge difference of 2 e^-/pixel) and its slow variation across the chips. However, if an observation requires careful determination of the absolute background level, observers are encouraged to contact their WFPC2 contact scientist or the Help Desk (help@stsci.edu).

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