After dealing with the sampling of the ASI224MC, here is a second article that talks about an essential accessory: the IR-cut filter.
Digital cameras, whether they use CCD or CMOS chips are highly sensitive in the near infrared. This wavelength domain, that the human eye can not see, is very interesting for planetary imaging, but there is one situation where we absolutely don’t want it: when we use a color camera.
Color is not a subjective notion: it is due to the physical properties of the observed objects, to their capacity to absorb or reflect some wavelengths more than others. Its perception depends on the human eye, this why it can make sense only in relation to the wavelengths that this eye can see. For this reason alone, we must use an IR-cut filter with a color camera, otherwise the wavelengths that the camera will record will differ from that of our eye. And for a correct reproduction of the original colors of any objects, both must meet.
That said, what are in concrete the effects of the infrared on a color planetary image if it’s present? I have made a comparison on Mars, that is currently well visible. Mars is interesting there because this is a planet whose albedo is very important in the near IR. Differences are self-speaking…
The big difference in colors are immediately visible. The colors obtained with the IR-cut filter are largely coherent with those I have seen at the eyepiece: an orange planet with warm hues. The view without any filter presents a much colder pink tint. What allows me to conclude that the left image’s colors are correct? The clue here can be found on the water vapour clouds of Mars: while they are not prominent because this is the season (late northern summer) when they are becoming to get rare, they are still more visible on the IR-cut image, and close to their normal color that should be white (the planet is too low in my sky to pretend to a better result).
The middle image taken without any filter looks almost monochrome, and the rare perceptible clouds are almost as pink as the deserts. If we split the RGB component we understand why:
With the IR-cut filter, the differences between the color components are important, especially between red and blue: these albedo differences are the reason why they translate into colors to the eye. Without filter, we are seeing almost exactly the same details with the same albedos! In particular, the dark markings of the ground are still perfectly visible on the “blue” component, which is completely abnormal. This comparison, along with the IR740 image above, helps us understand that without an IR-cut filter, a “color” camera is not producing “color” images but infrared images polluted with visible light… this absence of albedo differenciation also shows that it would be vain to try to fix the problem while adjusting the color balance!
Finally, we can notice that the resolution of the image taken without the IR-cut filter is slightly lower. This is logical as the optical resolution is lower than in visible light. A supplementary reason to use it!
On the next article: the use of the atmospheric dispersion corrector