http://www.skyandtelescope.com.au 63ECLIPSE: SEAN WALKER /
S&T;
HORSEHEAD
: KEN CRAWFORD
limit to how far we can push the colour saturation before it
becomes garish and detracts from the final image. One way to
avoid over-saturating your images is to monitor the individual
colour channels in your favourite image-processing program.
For instance, when increasing the saturation of an emission
nebula, pay close attention to the red channel. Make sure
that detail is still apparent in the brightest areas and doesn’t
appear overexposed after your adjustments.Composition
Although composing an astronomical subject in an image can
be tricky, it’s still an important aspect of the best astrophotos.
Composition in astrophotography is tough because most
things in the night sky are so far away they are essentially
motionless, at least on a human timescale. You can’t simply
move two star clusters closer together for that family portrait!
So take care when composing deep sky astrophotos. A
perfectly tracked, well-processed image of a galaxy still looks
odd if the subject is placed too close to one side of the picture
frame. And a tiny planetary nebula in the middle of an image
surrounded by a nondescript star field might have a better
impact if it were shot at an adequate image scale that resolves
lots of small-scale detail.Spend time matching your target to your chosen
equipment. There isn’t much point spending an entire
evening shooting a secluded, distant galaxy just 30
arcseconds across if your telescope and camera can only
resolve 5-arcsecond objects or larger. That galaxy will only be
about 6 pixels across in the final image!
Another aspect of composition is how your target is placed
in the picture plane. A spiral galaxy like M104, the Sombrero
Galaxy, might present a more dynamic composition if you
rotated your camera slightly so that the long-axis of the target
cuts across the frame diagonally rather than straight across.
These considerations also apply when imaging the bright
planets. In particular, Jupiter offers more compositional
choices than the other planets due to its size, extremely
dynamic atmosphere, and retinue of Galilean moons, which
can add up to four additional smaller detailed objects to a
single composition.
Composing multiple adjacent targets in the same field
presents its own challenges. Just because two objects can fit
in the opposite corners of your camera’s field doesn’t mean
it makes for a good composition. Each object in your image
should have a comfortable space between it and the edge of
the frame whenever possible.SOLAR ANGLE Composing interesting images of the Sun, Moon
and planets can be challenging, since all are simply round objects
surrounded by a blank sky. But special events can offer unique
opportunities. When photographing last August’s total solar
eclipse, the author planned everything long in advance, though
in the excitement of the moment, camera orientation was left
to chance. Fortunately, the error produced a better result than
originally planned.T RICH COLOUR When increasing the saturation in your pictures,
take care to monitor each colour channel and avoid over-saturating
the result, which presents problems when printing your results. This
detailed image of the Horsehead Nebula, Barnard 33, displays richly
saturated colour without going overboard. Here north is to the left.