possible to get a high elevation
shot so you will have to take
more video to compensate.
Exposing the image
Exposure for the video can be a
little tricky. With capturing the Moon
it is a little easier as there is more of
the moon in the frame than dark
sky, but with planets the majority
of the image will be dark sky. The
trick here is to lower the exposure
control and possibly raise the gain
until the detail of the planet is visible.
If you are capturing Jupiter
then when you have the exposure
correct for the planet surface you
will find that the Jovian moons
have disappeared. This is because
to correctly expose the planet it
will underexpose the moons. To
overcome this I take two videos:
one for the planet; and one for
the moons. The video with the
moons correctly exposed will leave
the planet hugely overexposed.
I then process the two images
separately and then merge them
to create a composite picture
with all parts exposed correctly.
It can be a good compromise
to use a higher gain and a
corresponding higher frame
rate. This is especially true if the
atmosphere is very turbulent during
your imaging session. Increasing gain
increases the grain and decreases
image quality. Increasing frame
rates is like shortening the shutter
speed on a camera, the faster the
frame rate, the less movement is
captured. This can be effective for
countering atmospheric effects.
Capturing the video
The video is now ready to be
recorded. I normally ensure that
I take around 2000 frames. If the
atmosphere is particularly unstable
then the more frames you take the
better. It is easy to drop bad frames,
but impossible to get additional
ones later. In fact that is a good rule
of thumb: Take as many frames as
possible. The only thing to watch
out for is excessively big files.
One final thing to bear in mind,
particularly with Jupiter, is that the
All set up and ready for a night of lunar imaging. Credit Mike Barrett
planets not only move through the
sky but also revolve around their
axis. This means that you can only
really use content from the same
video to create an image as features
on the planet will have revolved.
Conclusion
Planetary Imaging is all about
compromise.
The main factor that affects
the quality of the images is the
Earth’s atmosphere. Being able
to image through as little of the
atmosphere as possible and on
good stable days will vastly improve
the quality of the videos, which in
turn will lead to better images.
In the next issue the processing
of the video into a single image
will show how a wobbly and
blurred video can generate
a stunning final result.
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