LRGB Imaging

Beautiful deep sky images can be obtained with a modern modified webcam (B/W or color). Many astro imager's use a webcam with a B/W CCD because they are much more sensitive than a color CCD. Disadvantage of a B/W CCD is that only grey scale images can be made unless we use color filters. Although highly sensitive for the visual spectrum most CCD's are extremely sensitive for IR. Its an advantage for luminance imaging but this IR signal will corrupt the color data and therefore the purity of the color image. Therefore its important to use an IR rejection filter in combination with each color filter.

Before we can start "true color" imaging with a B/W CCD camera we have to take care of two important issues. First we need to know the RGB response of the B/W chip in the camera and we need to know the influence of atmospheric extinction.

RGB response of the ICX424 B/W chip

The response of a CCD throughout the spectrum is not everywhere the same (see figure below).

CCD response for a ICX424 CCD chip

If the ICX424 CCD chip is combined with the True Technology LRGB type 2 filterset the response for each color channel is shown in below graph.

CCD response for a ICX424 CCD chip combined with my True Technology RGB type 2 filterset

In order to make "True color" astro images we need to balance the three color channels. This can be done by determining the weight factor for each channel. These weighting factors can be calculated from the response of the CCD chip for each color channel on a pure white light source. Spectral type G2 class V stars are the ideal objects in the sky since they emit pure white light. The measured brightness of such a star through my color filter set tells me how the ICX424 CCD respond to pure white light as passed by each color filter. The CCD response to white starlight, no filters used, will be set as 100%. Due to uneven spectral sensitivity of the CCD (see above) the response through each color filter will be different and only a fraction of the CCD response without any filter, resulting in the following typical color weighting factors for my CCD/Color filter setup.

R : G : B = 1,00 : 0,74 : 0,77

For deep-sky imaging these weighting factors can be used to determine the exposure times in red, green and blue. Simply multiply the exposure time used for the red channel by the weighting factors for green and blue to get the needed exposure times for the green and blue channel. Such a set of RGB images have "balanced" pixel values so that a white star produces a white image on the computer screen.

The influence of Extinction

The term "extinction" means the loss of light in the atmosphere from a beam of starlight. Because the extinction is generally larger at short (blue) wavelengths the effect is much stronger in blue light than in red light. At the zenith the influence is minor but it increases with a decreasing altitude of an object in the sky because at lower altitudes light has to travel through more air (see figure below).

Wavelengths from 400nm to 700nm are transmitted with approximately equal efficiency at the zenith, but as the angle of elevation of an object approaches the horizon, blue transmission is extinguished much more than red. Green extinction is somewhere in between. For "True color" astro imaging this effect have to be taken in account, specially at lower altitudes.

True color imaging

For true color imaging the above mentioned factors have to be taken into account.

Example : We want to make a true color image of an object at 30° altitude.
Example : Which RGB weight i needed for this true color image?

CCD response is R : G : B = 1,00 : 0,74 : 0,77
Extinction correction is R : G : B = 1,00 : 1,08 : 1,15 (see graph above)

The total RGB weight for this image will be R : G : B = 1,00 : 0,80 : 0,83

Therefore a set of exposure times could be R : G : B = 60 : 48 : 50 seconds.

Example image

Three different color filters, each in combination with an IR rejection filter, are used to obtain an image that has a limited but pure spectral range (RGB image). The filtered color image is often grainy and low in contrast, or in astro imager's language "it has a low S/N (signal/noise ratio)", and therefore unappealing (see figure 3).

Figure 3, typical RGB image

Of course this problem can be solved with large aperture telescopes and highly sensitive CCD camera's using very long exposures but for many amateur astro imager's these solutions are not affordable. An other solution to overcome this RGB hurdle is LRGB imaging. Beside the low contrast and grainy color images, which still have to be made, a fourth image has to be obtained. This needs to be an unfiltered high resolution grey scale image (see figure 4).

Figure 4, Luminance (L) image

This image will be used as a Luminance (L) layer which will compensate the signal and detail lost in the filtered RGB images. In practice this means a very long exposure (or stacked shorter exposures) image will be used for the luminance layer. Since the color data will have a minor effect on the sharpness normal low contrast RGB images can be used. This doesn't mean that short exposures should be used because spending more exposure time on the RGB images will result in a richer and more saturated end result. The result should be an aesthetically pleasing high contrast color image (see figure 3).

Figure 5, Final LRGB image

Here you can download a R:G:B weight calculator for you RGB filterset.
The file includes lots of G2V stars to choose from, an extinction factor table and an RGB exposure set calculator.

Download RGB weight calculator (excel file)

Find a Tutorial for this calculator here.

© Copyright Rob Kantelberg
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