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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. |
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» 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. |
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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. |
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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. |
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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. |
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« 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.. |
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« 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. |
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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. |
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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. |
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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. |
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» 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. |
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» 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. |
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» 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. |
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» The ADC system and the Celestron SkyRis
camera in focus of the C14 |
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Conclusions
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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All Images and all Content are © by Franz Hofmann
+ Wolfgang Paech |