second layer. Increase the saturation of the upper layer by a small amount,
say 10–20%. Keep doing this until the colour is teased out of the image. If
the coloured regions look noisy and garish, apply a Gaussian blur (a blurring
widely used in graphics software) to this layer. Finally, set the blend mode
of the layer to colour and merge back with the bottom layer. This last step
brings back all of the luminance data – the detail – of the lunar surface,
combining it with the colour that’s just been teased out.Going in Close
The techniques described above are generally useful for shots which cover
the whole disc of the Moon or at least a large area of its surface. Increasing
the focal length increases the magnification of the camera’s view but will also
reveal a limitation of still cameras, namely due to that imposed by planet
Earth’s atmosphere.
As light passes through the atmosphere its path gets refracted or bent
by fast-moving blobs of air of differing temperatures and densities. The
net result is a view of the Moon which appears to shimmer and shake.
The stability of the view is described as atmospheric seeing and is a huge
limitation to getting really high-magnification images of the Moon, filtered
Sun and brighter planets. Fortunately though, there is a partial solution
which can greatly reduce the effects of seeing – the high-frame-rate camera.
Unless the atmosphere is very unstable, it’s normally possible to see brief
periods of stability when looking through the eyepiece. The human brain
is very good at ignoring the wobbly views and concentrating on the good
stuff, but this is one area where a stills camera fails. Take a shot with a stillscamera and unless you’re very lucky indeed, the fine detail in your image will
be distorted and blurred by the atmosphere.
The trick is to take lots of very short exposure still images in rapid
succession in the hope that some of them record the less distorted views.
Then, by pulling the good frames out of the pack, aligning them together
and averaging the shots, the end result should be a relatively noise-and-
distortion-free representation of the Moon.
The task of taking fast stills is handled by a high frame-rate-camera,
something that can range from a PC webcam through to a high-end
specialized planetary camera. These will typically capture still images at
rates of between 5 and 120 uncompressed frames per second. It’s common
to store the resulting frame in a movie file format such as AVI.
The arduous process of pulling the good frames out of the collection is
thankfully made significantly easier by a number of software applications,
some of which are available to download and use completely free of charge.
These programs can measure the quality of the still frames automatically,
sifting them out for alignment (registration) and averaging (stacking). One
popular freeware program to do this is called RegiStax.
By using a high-frame-rate camera connected to a telescope, it’s
possible to create some amazingly sharp images of the Moon. Increasing
the effective focal length of the scope by adding a Barlow lens, it’s possible
to magnify quite small regions of the lunar surface and pull out incredible
detail. Barlows, remember, are optical amplifiers: optical systems which
increase the effective focal length of your scope by the power of the Barlow.
For optimum work, it’s worth keeping the focal ratio, that’s the effective focal
length divided by the aperture of the scope using the same units, between
f/25 and f/40, the upper range only being feasible if the seeing is really good
to excellent.
If using a mono high-frame-rate camera, the seeing can be steadied
further by fitting a red or infra-red filter over the nose-piece of the camera.
Longer wavelengths (red light having a longer wavelength than blue) tend
to fare better during their passage through the atmosphere, and using one1312
Astronomer Book