Object Signal - Electrons created by the sensor in response to photons from the actual astronomical object. The object signal grows linearly with exposure time.

Mouse Over

Hold your mouse cursor over the image to see the effect of collecting photons for 30 seconds vs 300 seconds on M16 at ISO 1600 at f/6 .

There is a lot of light, and a lot to photons, for normal daytime photographs, so exposure times are short.

For deep-sky astronomical objects the opposite is usually true. Light intensity from these types of objects is very faint. This means we need to use long exposures to record the scarce photons from them. The more photons we collect, the better we can see the object and separate it from the noise that is present in all digital cameras.

The photons from the object create the signal that we are interested in. It is recorded along with various other signals that are not as interesting, along with noise. Increasing the signal to noise ratio, and removing signals that we are not interested in, are the ways to improve our pictures.

Normally the way to increase the signal is to simply collect more photons. This is done by increasing the exposure.

In the example images above, we can see the effect of a 30 second exposure and a 300 second exposure. The 300 second exposure has recorded 10 times as many photons and the object is much more easily visible because there is more signal.

Sky Signal - The sky signal comes from electrons created in the sensor in response to photons from the sky that get detected along with the object photons. Many of these photons originate in the sky "foreground", which is basically the Earth's lower and upper atmosphere. The lower atmosphere can contain particulate matter and moisture that is illuminated by man-made light pollution. The upper atmosphere contains light sources such as the atmospheric airglow and other sources such as auroral activity. Airglow is the brightest component and is caused by oxygen atoms glowing in the upper atmosphere which are excited by solar ultraviolet radiation. Airglow gets worse at solar maximum. There is also a blue component to the sky foreground signal that is caused by starlight scattered in the atmosphere which has the same cause as the blue daytime sky (scattered light from the Sun). Sky signal also grows linearly with exposure time.

Thermal Signal (Dark Current) - When photons of light from the sky strike the CMOS sensor in our cameras, electrons are produced that are stored and later counted. Other electrons, called "thermal signal" are also generated in the silicon substrate material by heat. The number of these non-sky electrons increases as the temperature goes up. Sometimes these heat-generated electrons can get into a photo-site where they are counted along with electrons generated by the photons from the sky. This happens even when the sensor is not exposed to light, hence the name "dark current". This signal is more properly called thermal signal because it is present in all frames, light and dark. Thermal signal grows linearly with exposure time.

Mouse Over

Hold your mouse cursor over the image to compare the thermal signal in a dark frame to the thermal signal in a light frame. The hot pixels and color splotches are in exactly the same place. This is the thermal signal. It can be removed from a light frame by subtracting a dark frame from it.

Thermal signal is not noise, even though it looks like it is. Because it is signal, it does not vary randomly, but repeats consistently.

The thermal signal, or dark current, can be seen in the example above as hot pixels, clumps of hot pixels, and larger areas of red, green and blue splotches.

This thermal signal is present in this exact form in all long exposures, it only varies in intensity depending on the temperature, length of exposure and ISO.

Thermal signal is repeatable for images taken with the same exposure length and temperature. It is related to the temperature, generally doubling for each increase of six to ten degrees Celsius. This is why astronomical-dedicated CCD cameras have built-in coolers. DSLRs are usually not cooled, but thermal signal will be less at lower ambient temperatures. Thermal signal is linearly related to the length of the exposure, with a doubling of the exposure resulting in a doubling of the thermal signal.

Each individual pixel in the sensor has its own thermal signal which stays constant for that pixel at a given exposure and temperature. However the amount of thermal signal varies from pixel to pixel, just as the sensitivity to light varies from pixel to pixel. This constancy from pixel to pixel results in what is called "fixed pattern noise". It can be seen as a snow-like appearance in an image, and as clumps of colored red, green and blue pixels.

Some pixels have dark current that is very high. These are called "hot pixels" and show up as white pixels in the image. Other pixels are dead and do not respond to light at all. These are called "dead pixels". They show up as black pixels in the image

All of these manifestations of thermal signal can be calibrated out by subtracting a "dark frame" (an exposure where no light reaches the sensor) taken at the same temperature and exposure length as the light frame.

Bias (Offset) Signal - The bias signal comes from electrons from current that is applied to the sensor to get it charged and ready to collect electrons generated by the detection of photons from the sky. Bias adds an offset or pedestal to the voltage presented to the Analog/Digital converter.

The bias signal is constant in every frame taken at the same temperature. It is essentially a picture of the constant voltage offset, the amplifier readout noise, and camera electronic noise.

Cosmic Rays - Real detections of high-energy particles from the sky that appear as hot pixels or short streaks in the image.

Amp Glow - Amp glow is a red glow that can be found usually on one edge or corner of the frame for long exposures. It originates from electrons in the metal-oxide semiconductor field-effect transistors (MOSFET) that are in the readout amplifier. Amp glow is not caused by heat, it is caused by electroluminescence. It can be controlled by turning off the amplifier during the exposure, as is done in most of the recent models of DSLR cameras.

Mouse Over

Hold your mouse cursor over the image to see a comparison between a light frame with amp glow and a dark frame with amp glow. Image taken with a Nikon D2H DSLR Camera.

For cameras that do not turn off the amplifier, amp glow can be reduced by the subtraction of a dark frame. However, if the exposure is long enough so that the amp glow saturates any pixels, these pixels cannot record any data from the object being photographed.

Miscellaneous Electronics Signals - Because a DSLR is a complicated device consisting many different pieces of electronics, repeatable signals may be generated from various amplifiers and electronic circuits. Manufacturers do an excellent job of controlling these signals, but they may still show up in an image. If the signal is repeatable, it can usually be removed later in calibration.