Technical photographs
Star trails
Figure
1: Corners of the photograph
of the Omega
nebula.
The
star images in the corners of this photograph are nearly point
symmetric. In the upper right corner one sees a small deformation
which obviously depend on the spectral type and can nearly be
compensated by the introduction of the Risley prism into the
optical path. The different spectral distribution of the incoming
light, sometimes are shorter sometimes are longer wavelength
dominant, is split by dispersion into small spectra of different
length.
NGC 2683 – the begin of our work in the field around refraction, dispersion and alignment of the hour axis
Figure
2: NGC 2683, exposure 70 minutes, d=
33.4o, local hour angle at the begin t=
3h10m.
The
star trails in this photograph of NGC2683
are different in length and direction. The star trails in region
above the center and in the upper right corner are acceptable
while in the other regions they are not.
Correction of the dispersion on a RGB photograph
Figure
3: Fomalhaut 10.6o above horizon. On the left upper
corner star image with a webcam and then clockwise the three
different color channels red, blue and green.
The
dispersion splits the star light into the spectral components.
Close to the horizon the dispersion has a large effect and can be
easily observed visually. If one takes a photograph with a webcam
or an RGB CCD then the dispersion can be partly compensated by a
relative movement of the three color channels.
Figure
4 Same as fig. 3 (left) and the moved color channels (right). The
resulting star image has the size of the image in the blue
channel. The remaining signal outside of the main image is due to
the IR and red part of the photons which pass the blue filter.
These parts can not be removed easily.
The
result is not overwhelming because Fomalhaut was photographed
for demonstration purposes at an elevation of about
10.6o above horizon. For shorter zenith distance the
correction work is far better.
Figure
5: Mars on August 28th, 2003 at 00:10 UTC. Raw Single
shot (left), with dispersion correction, with adjusted brightness
and unsharp masked (right).
Mars
was only about 27o
above the horizon when it culminated. That is the reason why the
opposite edges are colored blueish (upper) and red/yellowish
(lower). At the north edge of the South polar ice cap one can
observe another blueish edge. Like in the previous case one can
correct the dispersion by moving the three color channels. This
can be done manually (“by eye”) or one calculates the
number of pixels and move it according to that in the direction of
the parallactic angle towards the zenith. In this way the
photograph of Saturn has been processed.
Figure
6: Saturn at the zenith distance of 46o, not place for
high resolution photography. The dispersion is easily visible on
the edges of the ring and on the body of the planet itself. The
tilted straight line (from the lower part to upper left)
indicates the direction to the zenith.
In
fig. 6 the color channels are moved by the amount indicated by the
red, green and blue straight lines. The result
is still quite acceptable.
Figure
7: Like fig. 6, but 5 single shots were added together. In case
where one is a little bit generous one can recognize the gap
between the rings.
Comments, questions, corrections: markus.wildi@one-arcsec.org