Residual bulk image (RBI) is a phenomenon observed in certain types of front side-illuminated charge-coupled devices (CCDs). A CCD is an electronic light sensor used in digital cameras. In simplest terms, the sensor exhibits a memory of prior exposures resulting in ghost images appearing in subsequent images. This deferred charge can cause a number of problems in cooled long-exposure scientific applications. At a minimum, the ghost images can create the illusion of a non-existent object (Figure 1, left). Equally serious, they can lead to significant errors in quantitative measurements required for photometric applications.

Figure 1. Astronomical exposures showing a normal star field (right) and the same star field with RBI (left).
In Figure 1, the right-hand image shows a normal star field captured in a 15-minute astronomical exposure. The image on the left shows the same field, but it also includes a non-existent nebular-appearing feature that is caused by RBI. In this case, the three nebular dots are residual images from a star used for focusing the system prior to taking the exposure. During the exposure, the trapped charge leaked into the image, creating the illusion of three non-existent nebular objects. Left unchecked, the RBI renders the camera unusable for this sort of scientific application. Fortunately, there are solutions to the RBI problem.

Figure 2. Plot of trapped charge leakage
For some applications RBI can be avoided by removing the longer wavelength light with a filter. However, if near infrared (NIR) and other longer wavelength light are to be studied, another method must be used. One such technique floods the sensor using NIR light and then flushes it before starting each exposure1. This method pre-fills the traps before the exposure destroys any residual image. Because the filled traps will leak charge during the exposure, the overall dark current shot noise is increased by this additional charge. Cooling can reduce the leakage since it is strongly influenced by temperature.

It is customary in scientific CCD applications to define the maximum practical exposure limit for any camera operating temperature as that time for which the noise contribution from dark current begins to exceed the camera’s read noise. For a longer exposure, the camera noise increases beyond the read noise floor.

For the RBI solution to not add noise beyond this limit for the planned exposure lengths, the dark signal shot noise from the combined thermal and trap leakage components should not exceed the camera’s read noise. The operating point of the cooling system is, therefore, chosen to meet this constraint.

The trap leakage versus temperature behavior of the sensor must first be learned in order to determine the target maximum operating temperature for the planned maximum exposure length. Measuring the traps’ leakage characteristics is a straightforward but time-consuming process. First, a set of reference dark exposures is taken at different temperatures using a common exposure time. In this work, the standard exposure time was five minutes. Once this reference data is collected, the sensor is flooded using NIR light and a series of dark exposures are taken matching the length of the reference dark. After each dark exposure is captured, it is compared to the reference dark, and the difference is recorded. It is important to accurately measure and remove the off-set from each dark frame. A good way to do that is by over scanning the horizontal shift register during readout and carefully measuring the zero signal level in the over scan region. Once the data is collected at several different operating temperatures, it can be plotted as shown in Figure 2.

Thermal rate-based phenomena are often analyzed graphically by making an Arrhenius plot. This plots the logarithm of a thermally sensitive rate quantity versus inverse temperature (1/Kelvin). For a single rate-limited thermally regulated process, the result plots as a straight line. The leakage data from Figure 2 was plotted in an Arrhenius plot and appears as Figure 3.