Planet observation with
methane band filter 

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Projectmanager/Author: Silvia Kowollik
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Saturn is not the only planet with a ring in our solar system. On March 10th 1977 rings around the planet where discovered during the measurement of a star coverage by Uranus with the Kuiper airborne Observatory . If one wants to photograph the Uranusrings with telescopes from the ground, one must work with special filters, since the rings are extremely dark in comparison to the brightness of Uranus and are completely over radiated by the light of the planet disc.


Photographs of the Uranusrings with the
Keck II Telescope on Hawaii

Photographs of the Uranusrings with the
Hubble Space Telescope

For a successful observation of the Uranusrings with the 80 cm telescope of the Observatory extensive searches and tests are necessary in front of the observation.

- the selection of a suitable filter, which prevents that the weak rings are over-radiated by the light by Uranus
- the selection of a camera, which is sensitive in the planned wavelength coverage enough, in order to catch the weak light
- the determination of the necessary exposure time with the filter for a limit of 16 magnitudo.

The brightness of Uranus varies with the wavelength, in which it is observed. In the atmosphere of Uranus is methane. At 890 nm the sunlight is absorbed by gaseous methane in the atmosphere - Uranus appears darker in this wavelength. Its rings consist probably of ice, this reflect the sunlight without absorption and in this way they should appear brighter.

These wavelength was selected for the observation project, since most cameras are still sufficiently sensitive there.

  The search for a narrow-band filter, which exactly lets light through in this wavelength, and block on all wavelengths from ultraviolet light until the infrared spectrum at 1050 nm, became difficult.

With the filter XBPA890 of the Japanese company "Asahi Spectra" with FWHM = 12 nm an ideal filter for the observation project was found.
  With the selection of the camera, one has to make certain, that the camera contains a b/w chip, cameras with Colour-Chips are unsuitable.

With the DMK 21AF04.AS from the company "The Imaging SOURCE" was found a suitable camera. 640 x  480 square pixels with 5,6 x 5.6 my provide for a sufficient sensitivity, the selection rate of up to 60 pictures per sec. as well as the possibility from long-term exposure up to 60 minutes per picture are ideal for planet photography with various filters.

After the selection of filter and camera on April 24th 2008 a first test with an 8"Newtonian f=5 was done. The test served for the examination of the theoretical initial considerations.

Here we present you the first test photographs of the planet Saturn with the methane band filter.

rough raw pictures, cutout from the entire visual field:

Saturn with 1/15 sec. Saturn with 1/8 sec. Saturn with 1/4 sec. Saturn with 1/2 sec. Saturn with 1 sec.

The camera was placed with the filter directly at the focus of the 8", f = 5 Newtonian. We took up 450 pictures with exposure times between 1/15 and 1 sec. in each case.  The treatment of the pictures was divided into 3 steps:

- subpixelexact averaging
- contrast rise
- sharpening

Here the finished Saturnpictures with the methane band filter:


With the shortest exposure time at the monitor only the ring was visible, after the treatment of pictures the Saturndisc also appeared. 

  At an exposure time of 1/2 sec. already the brightest Saturnmoons Titan and Rhea appear weakly. Since the moon Titan has methane in its  atmosphere, Titan appears darker than within the visual range.
At an exposure time of 1 sec. the magnitudo limit rise up to 10.2 mag. So the Saturnmoon Tethys could be illustrated.

In order to avoid artefacts by Hotpixel, Darks were taken and subtracted during the treatment of the pictures.



By the rotation of the earth after the last attempt Saturn already stood too deep to the horizon. So attempts with longer exposure times could not be accomplished. On the early morning of April 25th 2008 however Jupiter could be taken up with the methane band filter.

This first test with the methane band filter at an amateur telescope with a 20 cm mirror showed that the filter fulfills all our expectations. The light of the planet atmosphere is effectively weakened.

With suitable weather conditions further tests are accomplished in the months May and June 2008.

In September 2008 Uranus will be seen at the night sky and we will observe Uranus during the following month also with the methane band filter.

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