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Averaging 9 separate images increases the signal to noise ratio of the final image, allowing the
faint nebulosity in M78 to become visible and smoothing the black sky background.
Adding more frames would further improve the results although you do end up in a situation of
diminishing returns. Combining 18 frames will not yield a final image twice as good as
combining 9 frames.
Also note that in some cases, doing a “median combine” rather than an “average combine” may
yield better results. A median combine is recommended if several of the individual frames have
unique anomalies such as bright pixels caused by cosmic rays, satellites, airplanes, etc. With at
least 5 images, a median combine completely eliminates extreme pixel values that occur in
individual frames.
Colour images
Unless you’re using a single-shot colour camera such as the QSI 683c, producing colour
images requires taking separate exposures through different coloured filters and then
electronically combining the separate colour channels. The most common method used by
amateur astronomers for colour imaging is called LRGB, where separate colour images are
taken through red, green and blue filters and combined with a set of “luminance” images taken
through a luminance filter. The luminance filter is required because CCDs are generally
responsive to frequencies of light that can’t be seen by the human eye. The luminance filter
blocks the infrared (IR) and ultraviolet (UV) frequencies that fall outside the range of human
vision.
The luminance filter transmits most of the visible light coming from the object. Because the
individual frames taken through the red, green and blue filters block roughly ⅔ of the total visible
light, the luminance image will often reveal subtle details not apparent in the individual colour
frames. This actually works out quite well since the human eye is much more sensitive to
changes in brightness than it is to changes in colour. Combining a colour- balanced RGB image
with a luminance image will yield an LRBG image that the human eye would perceive as being
very close to the true colours of the object with more fine detail than is present in the RGB
image on its own.
Your processing software can be used with your QSI 600 Series camera to collect and
catalogue the various filtered images that you’ll need to create LRGB images. As with any
images, you’ll want to collect multiple frames through each filter and then calibrate and combine
them in order to reduce the major sources of noise. After calibration, you’ll have a master
luminance image plus master red, green and blue images. Those master colour frames are
combined into your final image. In addition to the colour image tools in your software you can do
further processing of your images in Adobe Photoshop or similar photo manipulation programs
to yield impressive final results.
