Special Projects
  • Test of a "Atmospheric Dispersion Correction system" (ADC)
In spring 2015 we were asked by the company Gutekunst optical systems wheather we are interested to test an ADC system with our telescopes in Namibia.

ADC stands for "atmospheric dispersion corrector". It is an optical system to correct the atmospheric dispersion and is is mainly interesting for lunar, planetary and binary star observations.
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Theoretical overview

Atmospheric dispersion means the splitting of light of an object through the atmosphere into the spectral colors. The atmosphere acts like a prism here. The dispersion is only zero in the zenith. With lower elevations of the observed object above the horizon, the path of the light through the atmosphere becomes longer and the dispersion increases. In practice, this means that e.g. the image of a star is displayed as a small spectrum in the telescope. It is clear that this results in a decreased contrast and sharpness.

The refraction of light is wavelength dependent, blue light is refracted more than red light. The image of a star is vertically smeared with respect to the horizon. The red image is closer to the horizon than the blue part.
» The right image shows the atmospheric dispersion on Ganymede. Taken with a Canon EOS 60Da in the focus of a C 14 on July 7, 2015 at an altitude of approximately 15 degrees. The images are upscaled by a factor of 2. Left without ADC, right with ADC.
The ADC system of Gutekunst has 2 prism pairs, each are constructed as plane-parallel plates. By rotating the prism pairs against each other, the dispersion compensation can be very sensitively adjusted. For more informations and a detailed description of the theory and practice please visit the website of Gutekunst optics.
We have tested the module at the Zeiss APQ refractor (f / 8) and at the Celestron 14.

« The left image shows the ADC (Module Compact) at the C14 with a Celestron SkyRis video module.

The use is simple. The ADC is fixed in the eyepiece holder of the telescope and the knob for adjusting the prism pairs should be approximately parallel aligned to the horizon. The ADC works with prisms, so the observation object remains stationary in the visual field of the telescope.

In the "life view" mode of the camera the dispersion between red and blue can be minimized ty turning the knob. First observations of double stars (Alpha Centauri and Acrux) and the planet Jupiter immediately showed that the effect of the ADC strongly depends on the seeing.
Poor seeing conditions outweigh the dispersion correction of the ADC. With an estimated Seeing of 1-2 arcseconds the image enhancement by the ADC system is now clearly visible.
The binary system Alpha Centauri has currently a separation of about 4 arcseconds and is not a big problem for the 6 inch Zeiss APQ to resolve it. Nevertheless, the image appears more "crisp" with the ADC at the same magnification. The same applies to Acrux. Acrux is a triple system of two approximately equal bright (1.3 mag and 1.7 mag), very blue stars at a distance of 4 arcseconds. In addition, a 4.8 mag star is located at a distance of 90 arcseconds. The visual image is comparable with the observation of Alpha Centauri.

Visual observations are difficult to communicate, so we have done some photographic test observations (video and SLR).

The two planets Venus and Jupiter were used as test objects in July 2015. They were visible in close conjunction in the late dusk at this time. » The image shows the two planets just above the 4m dome of Onjala.
« At Venus and an altitude of approximately 25 degrees the image improvement is dramatical when the ADC is used. The images were taken on July 23 with a color video module at the C14.

Click here to load a large image..
« The same improvement on Jupiter and its moon Io at an altitude of 30 degrees above the horizon. The Images were taken on July 7 around 18:00 with a Canon EOS 60Da.

Click here to load a large image.

The ADC-system is also ideal for the visual or photographic observations of the narrow, young lunar crescent because the Moon is always close to the horizon at this phase.

» Test shots with a Canon EOS 60Da in the focus of the 6 inch Zeiss APQ refractor, on the top without ADS, below with ADS. Note the loss of detail due to the chromatic dispersion compared to the image below

» » On the far right the images are converted to monochrome. The superposition of the blue and red image is clearly seen as an image blurring.
Also observations of the ashen moonlight show that the images taken with the ADC system are much more neutral in color and sharper.

Click on the thumbnails to load larger images.
A small useful tool to optimally adjust the ADC is the Schott filter BG 38. This filter only transmits the red and blue part of the observed object.

It is best used in a filter slider so that the filter can be inserted in the beam path without disassembling the ADC system. If the ADC system should be disassembled after the adjustment, the radial position in the eyepiece holder must be marked so that it can be installed in the correct position afterwards.

As a final example, we want to show some images from Saturn.
» Unsharpened summed image of Saturn, taken on September 3 in the focus of the C14 and with BG 38 filter with unadjusted ADC system.

» » Unsharpened summed image of Saturn, taken on September 3 in the focus of the C14 and with BG 38 filter with adjusted ADC system.

It is clearly visible that the blue and red Saturn image are now congruent with each other.
» Unsharpened summed image of Saturn, taken on September 3 in the focus of the C14 without ADC system.

» » Unsharpened summed image of Saturn, taken on September 3 in the focus of the C14 with ADC system.

A clear neutral-colored and sharper image.
» Wavelet sharpened image of Saturn, taken on September 3 in the focus of the C14 without ADC system.

» » Wavelet sharpened image of Saturn, taken on September 3 in the focus of the C14 with ADC system.

The image enhancement of the ADC system from Gutekunst is clearly visible.

Celestron color video module, image addition of 300 frames (out of 3000 frames), captured in the focus of the C14 at f = 3.900 mm and f/11.
» The ADC system and the Celestron SkyRis camera in focus of the C14

The ADC system of Gutekunst optics is extremely simple to use and offers a significant image enhancement in the visual and photographic observation at altitudes below 50 degrees above the horizon (good seeing needed). According to the manufacturer, the system operates diffraction limited (not tested by us).

The only drawback is the high price of the ADC system (Module Compact), which is about 25% of a apochromatic 6-7 inch refractor. So the system is probably just for a few specialized observers of the Moon, planets and double stars within reach.
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