Both visual and photographic lunar- and planetary observations demand a lot of the observer and his instruments. The optics must be excellent as well as the necessary accessories like filters, Barlow and eyepieces. The more experienced the amateur astronomer is in observing and webcamming, the better the results he will achieve, both visually and photographically, these issues we can keep under control.
Some issues cannot be kept under control, like the continuous instability of the atmosphere and the refraction when planets or the moon are at a low position with regard to the horizon, which is the case at the moment for the planets Saturn (20°) and Mars (30°). We cannot influence the seeing conditions, but the refraction can certainly be kept under control; this is not well known in he amateur astronomy society. With this article we wish to change this.
Seeing must be monitored continuously and the best moments must be used for doing a lunar or planetary observation. The refraction can be adjusted with an instrument that is known as an atmospheric dispersion corrector. Only a limited group of planetary observers used this in the past to improve their photographic results.
What is atmospheric dispersion? For the incoming solar, planetary or starlight our atmosphere is one large prism. The light rays that enter our atmosphere are bent by this. The refraction increases when the planet or star is nearer to the horizon. The refractive character of the light is also dependent on the wavelength. This natural phenomenon is called atmospheric dispersion, what in fact results in the vertical spreading of a point-shaped light source. An Atmospheric Dispersion Corrector (ADC) is a device that corrects the atmospheric dispersion, this results in a sharper image with higher resolution of the planet or star than without this corrector.
In most cases a combination of two round-shaped, thin, multi-coated prisms of 2° or 4° are used. This concept is used by the Dutch company Astro Systems Holland (ASH). The levers that are connected to the prisms allow the dispersion to be corrected for all possible declinations of the object under observation. A scale on the ADC allows for a simple and accurate marking of the correct position.
How to use the ADC in practice? You can do the adjustment with a color webcam or just visually behind the eyepiece. This last method looks the most practical to me. The ADC "calibration" for a certain telescope has to be done only once. For every 10° (calculated in height above the horizon) you observe a relatively bright star at a high power (300x or more), preferably under good seeing conditions. After exact focusing you will notice that the star shows color fringes: red on top and blue on the bottom. This is the effect of the atmospheric dispersion or smearing of the light over a vertical line.
By adjusting the levers of the ADC from the zero-point outwards the dispersion is corrected for. Always make sure that the distance of both levers remain the same with regard to the center, or zero-point. The levers remain in position, very handy. It is important that you position the zero-point of the ADC parallel to the horizon. From this starting situation you move both levers slowly and parallel from the center point outwards until the red- and blue shade around the star or planet is almost not visible anymore. Note the exact position on the scale for every 10° elevation.
Because the celestial objects continue moving on the firmament the ADC zero-point must be set parallel to the horizon as good as possible every 30 minutes. This procedure should be enough to allow you to make quality pictures of your beloved observation objects, the moon, planets, double stars etc. When you want to adjust the ADC very precisely, you can best choose a bright star that is almost on the same elevation as where your observations of the planet or moon start. Correct the dispersion visually or with a color webcam and after that start the observations under a ideally corrected dispersion situation.
What also must be said is that the current latest ADC version of the company Astro Systems Holland has a number of useful accessories. The basic adapter is fitted with a female T2 tread, via a T2 male adapter, a male C-mount or a female 1.25" adapter 4 possible connections can be used.
Do you fear light-loss at the telescope while using an ADC? The answer is: no, no noticeable light-loss could be determined.
Can you get away with it by using RGB filters instead of buying the pricey ADC? Unfortunately not. When you want optimal corrections the ADC is a must for larger telescope apertures, with a lower limit of about 200mm. Smaller telescopes, 150mm or less, seem to have no benefits with this instrument, but when you are in the 200-350mm telescope aperture ranges the benefit in image quality becomes larger with increasing telescope aperture.
When I want to want to work with my Celestron 14" (0"25 resolution power) without atmospheric dispersion to enable, when the seeing permits, the use of the full resolving power, the observation object must reach a minimum height of 77° (UV/IR luminance filter), 72° (blue filter), 52° (green filter) and 52° (red filter). For a 20cm telescope these values are 66° (blue), 42° (green) and 33° (red) respectively. What does that mean in practice for the planetary observer in the coming decade? Mars sinks further to a culmination height of 48° in 2012 to 13° in 2018 (16° in 2016). Jupiter maintains the higher declinations for a while, coming from 60° in 2012, to 22° in 2018 (54° in 2015). Saturn is much worse: 31° in 2012, further decreasing to 16° in 2018 (20° in 2015). All these numbers say that an ADC is an absolute necessity for larger amateur telescopes, when you want to visually observe and webcam these objects. The smaller telescopes, below 15 cm aperture, can save them the financial effort of buying this useful accessory. The purchase of an ADC, as an accessory to the telescope, is an expensive investment, but an absolute necessity when you take lunar- or planetary observing seriously.
The combination "color camera-ADC" offers the planetary webcammers outstanding possibilities and strongly improved results. Luminance pictures of the planets with weaker surface brightnesses like Saturn and Uranus benefit fully from the ADC. When comparing pictures with or without the ADC the results are convincingly in favor of ADC use. Think of the possibilities this offers for a L-RGB combination at planetary webcamming.
At last a very practical example of a very recent visual observation (beginning of April 2014) of the planet Mars that reached a culmination height of about 30°: "The air was very calm and the planet reached her culmination point. A few short test, with and without ADC, and a direct processing in AS2, showed clearly the benefit of the ADC: the sharpness remarkably increased with regard to the observations without it. Because of good seeing conditions a lot of webcamming was done to make good use of this situation. The big surprise came when I started visually observing with the Celestron 14. Very nice for webcamming, but I found that this telescope performs poorly with visual observations, even with good seeing: a clear, but contrast-less air-image of Mars. Atmospheric activity was clearly observed, but the albedo-formations only showed up feebly. The brighter edges were colorful, as green-blue stains, the polar-cap gray-white. After fitting the ADC, with adequate attention for the correct adjustment to the scale and the zero-point placed parallel to the horizon, I got remarkable sharp contrast-rich images of the planet. The atmospheric activity was stripped of its colors and was clearly white, just like the polar-cap. Structure in the dark albedo-formations was now present. Wat a difference!
The story of the atmospheric dispersion corrector is one with a great positive look at the instrument. This must-have-gadget must be present in the standard equipment of every serious lunar- and planetary amateur-astronomer. Better an ADC instead of on extra eyepiece.