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The atmospheric dispersion corrector with the ASI224MC

After the IR-cut article of last time, here is another unavoidable accessory with this color cam (…and every other!): the atmospheric dispersion corrector, or ADC.

The atmospheric dispersion corrector is an accessory that allows one to correct the dispersion of light emitted by planets (or stars…) when it passes through our own atmosphere, that acts just like a glass prism would do. We have been talking about the ADC for many years now so I will not detail why and how it works ; you can refer to the original page by Jean-Pierre Prost or a most recent and excellent one by Martin Lewis. I will present here a few tests made with the Pierro-Astro ADC (thanks to Astrograph for lending me one)

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This model is of the highest quality both for mechanic and optic (refer to the constructor’s pages for details). In comparison with the former ADC’s generation (that is no longer on the market) one of its main interest is the possibility to adjust the levers all night long without the need to unset the camera (the direction of correction must always be vertical). Another advantage of the Pierro-Astro ADC is its excellent UV transmission (following Martin Lewis’s page quoted above, 25% more than some other brand!) and this is seriously fine for Venus imaging.

The idea there was to perform some tests with/without the ADC to see if it’s as unavoidable as many claims :) (and this includes me!)

The ASI224MC, with or without the ADC

Setting the ADC is very simple with a color camera just like the ASI224MC as the chromatic dispersion can be seen right on the laptop screen (you need of course to debayer the image for this). Just move the lever so as to make disappear the red at south, and the blue at north. Before the results it’s necessary to recall the main objection, or observation, some use to say during the early times of the ADC: when making digital imaging of a planet in color, it’s always possible to re-align during post-processing the three RGB color channels. But is the correction as good as you will get with the ADC?

 adc_colorAt left is a comparison of the result on Mars with the ASI224MC. (A) with the Pierro-Astro ADC ; (B) without the ADC ; (C) without the ADC but with color channels aligned during post-processing.

At the very low altitude where Mars was during the observation (21°), the result is clear: only the ADC is able to reach full correction. As an exemple, the edge of the south polar cap visible as a (relatively) sharp white edge at the bottom (A) is a blurry, reddish edge on (C). This result is even remarkable as the seeing was poor, this showing that the ADC is able to seriously improve images even when conditions are unfavorable.

The reason why the informatic processing alone is not able to correct the aberration is well-known: the dispersion of colors does not exist only between the three RGB components ; but as well inside each color bands. This is why the SPC’s edge is not well visible on (C), the green and above all blue layers being still affected by the dispersion. And this problem is impossible to fix during processing: you will have to do with it!

Let’s note the absolute interest that there is to use the ADC when one observes visually at the eyepiece: of course, the human eye alone is not able to make the RGB re-alignment…

Testing with blue filter!

To end the demonstration I have made another comparison with/without the PA ADC this time using the blue filter (and a b&w camera). Here again the interest of using it is absolutely clear. With the ADC (1) the image is noticeably sharper than without (2).

adc_bleu

I advice you as well the reading of the following pages that present some coherent testing:

Piotr Malinski

John Boudreau

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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 […] Continue Reading…

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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…

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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…

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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…

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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…

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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…

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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…

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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…

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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…