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Evaluation of magnitude per area of Jupiter and
the problem to measure the star

Diameter of Jupiter: 49 arcsec
Magnitude: -2m7
Area of Jupiter π/4 d² = 1900 arcsec²
1 arcsec² = 1/1900 of Jupiter = + 8m2
1 arcsec² ~ as bright as a star with 5m5

Keep in mind, that this is only equivalent for visual magnitudes. And we have to take into account the limb darkening.
In any case, we have to do strong filtering, to enhance the contrast

Using the Methane Absorption Band to observe the occultation

If we observe in the same wavelengths, as the methane, which is in Jupiter's atmosphere, absorbes light, the contrast between the star and the surface of Jupiter can be strongly enhanced. As we can found in a publication of E. Karkoschka (1994, Icarus) an important and deep band are the wavelengths around 890nm with a width of about 20nm.

Therefore small band interference filters had been produced, to be put in front of the camera. They have the following optical data:

Central wavelength:......891nm
Maximum transmission:.~80%

One important disadvantage however is the loss of photons, because only the light of the filtered 17nm is transmitted in comparison to the full spectrum without the filter. But, as can be seen in the following images, the enhancement of contrast is quite large.

The following has been taken with a 12 inch instrument just recently. The Jupiter surface is reasonable fainter than the jovian satellites, which can be seen simultaneously. A WATEC 120 camera has been used.

© 2009 M. Busse, IOTA-ES

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