Why you should use an IR-cut filter with the ASI224MC

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 :)


Planetary Astronomy, the book: first announcement!

Have you ever wondered if there was a complete and recent book for observing and imaging planets? … Now there will be one!
In 2015 me and six skillful amateurs published a book that has won a great success in France (… and even outside France!): Astronomie Planétaire. I am […] Continue Reading…


The JUNO workshop in Nice, May 12th-13th 2016

Getting ready for the arrival of the JUNO spacecraft in orbit around Jupiter, a workshop was held this month in Nice, France, around scientists and amateurs from various countries in the world. High quality informations now available for everyone!

Some thirty astronomers from the entire world met for this two-days workshop […] Continue Reading…


The sampling of the ASI224MC camera

I’m beginning a serie of articles dedicated to this little imaging bomb that is the ASI224MC camera from ZWO. It’s quite different from the PLA-Mx that I have been using this past years, first because of its pixel size… Let’s take things by the beginning: the setting of the […] Continue Reading…


JUNO needs amateurs!

In 2016, a major event in the Solar system exploration is going to take place: the arrival of a new space proble around Jupiter : JUNO. This should take place during the summer of 2016 and it will be the first since the Galileo orbiter (1996-2003). Scientists ask amateurs for […] Continue Reading…


Meteorological conditions at AstroQueyras (3): the onset of good conditions

After the mediocre to fair nights described in (1) and (2), here are my observations during the next two following nights (August 31th to September 1st and 1st to 2nd). Those two nights have been somewhat cloudy, but with a much better seeing. Meteorological conditions had changed too !
During […] Continue Reading…


Meteorological conditions at AstroQueyras (2): a strange wind at night

After a general review of the AstroQueyras observatory context, here are some of my notes related to the observed weather and night sky quality during my stay. On the first two nights (August 29/30 and 30/31) the  sky was clear but seeing was quite mediocre. I have note a particular […] Continue Reading…


Meteorological conditions at AstroQueyras (1): the context

AstroQueyras is an observatory located at an altitude of 2930 meters in the deep french Alpes mountains, very close to the italian border. It welcomes one-week amateur missions as well as one-night public stays. It’s equiped with a 620 mm F/15 Cassegrain, a 500 mm F/8 Ritchey-Chrétien and a 200 mm […] Continue Reading…


The Martian terminator “plumes” in Nature magazine

During the 2012 and 2014 apparitions of mars, we talked a lot about a very curious detail imaged by several amateurs: an object projected beyond the terminator, that could only be at a very high altitude. Seven scientists and six amateurs co-sign this month a paper about it in […] Continue Reading…


Some better conditions on Jupiter, why ?

In my last article Jupiter under the jet-stream, what’s the result? I described the effect of one the worst meteorological conditions for the seeing. During my last observing night of January 17th, images were noticeably better (although still not really good…). It is possible to find explanations?

Here are some Jupiter images […] Continue Reading…