Residual Bulk Image

RBI effect is inherent to the CCD detector itself and it has nothing to do with any particular camera model. It appears on Front-Illuminated Full-Frame detectors (do not confuse it with full-frame 24 × 36 mm detector format used in classical photography, Full-Frame denotes architecture of the CCD regardless of its physical dimensions). So-called Back-Illuminated (or thinned) CCD chips do not suffer from this effect and RBI is suppressed during CCD clear operation in the case of Interline-Transfer CCDs. Unfortunately, Back-Illuminated CCD detectors are prohibitively expensive compared to Front-Illuminated variants (often by several orders of magnitude), so they are out of reach for many applications. Interline-Transfer detectors on the other hand do not offer high quantum efficiency and suffer from higher dark current, while their advantages (electronic shuttering, which allows very short exposures) bring no use in astronomical applications, where exposure times are quite long. So Front-Illuminated detectors are widely spread and its RBI effect must be taken into account.

The basic principles of how CCD detectors work are generally known—photons incoming to silicon chip release electrons, which are accumulated in the potential wells (areas of the chip, surrounded by negative walls, which do not allow negative electrons to escape) during exposure. Such potential wells are also called pixels. Individual packets of electrons are then moved around the CCD (shifted to neighboring potential wells) during the detector readout until they are finally moved to the output node, which converts charge to voltage. Output voltage is then digitized by camera electronics and a number corresponding to each pixels charge is sent to the control computer.

But situation is always more complicated in reality compared to idealized model. Not all photons are captured within pixels, some of them (especially the ones with longer wavelengths) penetrate deep into the chip substrate, where they also release electrons. Majority of these electrons are drained by the substrate, but some of them are captured in charge traps, which appeared in the CCD substrate during the CCD manufacturing. The charge accumulated in these traps slowly disappears, unfortunately some of the electrons move back to the image pixels. So if the CCD is read (even without the access of light), a signal is observable on the areas, where electrons remained in the charge traps.

Let us take for example RBI effect of the KAF-16803 CCD detector. The following image shows a portion of 180 s long dark frame of this CCD, taken at -25°C:

This dark frame was acquired before the observing session, while waiting for complete darkness. The camera was kept in the dark (shutter closed) for approx. an hour prior to dark frame exposures. Dark frame exposures were started so they finished in time for actual observation begins.

The following dark frame was acquired on the end of observing session, after many hours of imaging of the same field of view. Although it was taken at the same temperature and with the same exposure time, it shows higher level of dark current (frame mean value is higher) and also contains clear traces of stars. The corresponding portion of the star field is shown prior to the dark frame for comparison.

RBI effect is clearly visible on the dark frame. Slightly higher background level corresponds to weak RBI effect caused by sky background. But the number of electrons trapped in substrate is much higher in the areas where the light from bright stars hit the CCD, which results to star “ghost” images.

How prominent is RBI effect? Even if the image is stretched so the RBI is well visible, especially in our case of KAF-16803 the mean value in the brightest “ghost” star images is only 14 ADU above the background level on 180 s long dark frame. This is approx. 1.5 times the detector read noise RMS. We can say without hesitation that the RBI is more an aesthetic problem than a real issue for photometry or astrometry in this case.

Even more prominent is the RBI after the flat fields were acquired. The whole CCD is often saturated many times when acquiring flat fields (especially when we wait for proper sky brightness at twilight—number of images are saturated before the flat field brightness is around 1/2 of the CCD dynamic range). An example of such flat field and subsequently acquired dark frame are on the following images:

The dark frame taken immediately after the flat frames were exposed shows RBI effect uniformly all over the detector area. Clearly visible arc structures demonstrate intensity of charge traps within the CCD substrate.

Let us summarize the RBI effect features:

• RBI is inherent to Front-Illuminated Full-Frame CCD chips and cannot be affected by camera construction. Of course camera design may be so bad, that image noise hides the RBI effect. But this can hardly be a desired goal and cannot be considered a solution of this problem.

• RBI effect intensity depends on the particular CCD detector. Generally CCDs with bigger pixels suffer from more intensive RBI effect.

• The charge captured in traps slowly disappears and the shadow image fades. The speed of fading again depends on the particular CCD type as well as on the detector temperature.

  Unfortunately with the lower temperature the speed of RBI fading also slows.

• Because the charge is trapped under the pixel structure, the speed of RBI fading is not affected by CCD read/clear operations. However, the CCD must be of course kept in dark if the RBI has to disappear.

How much the RBI effect affects observations? This depends on specific observation program and on the possibilities of its adaptation. It also depends on particular camera model (the speed of RBI fading) and on the time available for waiting for RBI to fade.

The experience shows that when a single field of view is imaged and the observing program is adopted (particularly dark frames are captured after some time during which the camera is kept in dark), RBI does not affect observations at all. The precision of photometric measurements is not compromised if the image does not move around the CCD detector too fast (exposures are guided). As already stated, intensity of RBI on many minutes long exposures are only a few electrons (or tens of electrons) per pixel, which is typically well below the influence of other disturbing effects (e.g. high clouds) and measurement uncertainties.

Similar situation is in the case of astrophotography—RBI does not affect observations if the single object is imaged and exposures are properly guided. Even if the filters are changed, traces of RBI from previous filter are not detectable. When we take into account that very long exposures are the trend in astrophotography (everybody wants the best images with very high S/N ratio), moving from one object to another during single night is not so common.

Near-IR Preflash

In situations when RBI is really unwanted and disturbing (e.g. objects like M31 galaxy and almost “empty” variable star fields are on the observing list), it is necessary to deal with the RBI caused shadow images.

The first possibility is naturally t wait for RBI to fade. When a camera with small pixels (~7 μm) is used, it is enough to wait for 10 to 20 minutes and RBI disappears.

Another alternative to deal with RBI is so-called “NIR preflash”. In fact this method does not remove RBI, but the opposite is true—it creates the RBI uniformly all over detector before each exposure (something like “if you cannot defeat the enemy, make it ally”). Uniformly does not mean that the RBI intensity is the same over the entire detector area. Different parts of the chip substrate are capable to accommodate different amount of charge, which makes the RBI intensity to vary. Arc-like structures are typical, they are related to production process of the silicon monocrystal, from which the CCD is made. These structures are well illustrated on the dark frame captured after the flat field (see image above), which means after the complete detector is saturated.

The whole CCD is flooded with light during Preflash, so its substrate is completely saturated. It is better to use near infra-red light (this is why “NIR” Preflash abbreviation is used), which penetrates to the substrate more easily. The detector is cleared after flooding with light to remove electrons from active detector areas (image pixels, horizontal register, output node, ...) so the regular exposure can be started. Because all portions of the CCD were completely saturated, CCD clear operation must be repeated several times, one clear is not enough.

If the above described procedure (saturation + repeated clear) is performed before every single exposure (dark, light, flat), initial conditions are always the same and calibration (dark frame subtraction) eliminates RBI. Ghost images are not visible, any residual charge left after previous exposure is overrun during light flooding to maximal value and removed during subsequent dark frame subtraction.

The fact the NIR Preflash electronics is present in the camera does not mean it must be used. This function is programmatically controlled and it is enough to define 0 s Preflash time and camera hardware will not perform light flooding (and subsequent clear operations).

NIR Preflash control is available in the Exposure tab of the SIPS CCD Camera tool window.

Optimal values of both parameters depend on specific model, CCD temperature etc. NIR LED within the Gx cameras used for light flooding are powerful enough to saturate the entire detector after a fraction of second. So Preflash time 2 to 3 s is enough with wide margin. Number of clears should be at last 2×, but specific number is again necessary to adjust, 3 to 4 clears still changes the intensity of dark frames acquired after Preflash.

It is necessary to take into account time delays when NIR Preflash is used. Especially in the case of small cameras (G2-0402, G2-1600, ...) with digitization time from a fraction of second to several seconds, the delay of several seconds for Preflash and subsequent clear operations significantly increases interval between exposures.

NIR Preflash is also supported in Gx CCD camera drivers for other software packages.

Using of NIR Preflash brings also other disadvantages in addition to increased delays among exposures. The charge accumulated in the chip substrate slowly dissipates and partially moves into image pixels as added noise. This added signal behaves very similarly to dark current, so we can say that the dark current increases after IR Preflash, when the substrate is saturated with electrons, compared to dark current of the detector with “empty” substrate. Even after the dark frame is subtracted, higher dark current always means higher background noise. CCD signal RMS corresponds to square root of the signal level, so higher background signal also means higher deviations, which are not removed by calibration. The solution is the same like in the case of natural dark current. If NIR Preflash is used, CCD must be cooled to limit dark current as much as possible. This is why not using of NIR Preflash when it is possible and e.g. to adopt the observing program is a better option.

Notes:

NIR Preflash control is implemented in SIPS v2.1 and newer versions. Older versions do not support NIR Preflash.

NIR Preflash support in drivers for other software packages is stated in the particular driver documentation in the History of changes section. It is recommended to download the latest version of the used driver from this www site.

RBI overview is well described in Richard Crisp article “Residual Bulk Image: Cause and Cure”, published in Sky and Telescope, May 2011, pages 72-75.

We thank to Richard Crisp for advices and consultations regarding RBI effects.