The planets
Imaging Mercury
Mercury is a tricky planet to image because it never strays that far from the
Sun. Evening appearances are better in the spring, while morning ones are
better in the autumn, the planet appearing higher above the horizon at these
times. Even then, catching sight of Mercury is limited to a few days before and
after greatest elongation, when it appears furthest from the Sun.
Mercury’s brightness fluctuates considerably when it is visible, because
of its rapidly changing distance from us and the fact that it shows a phase
(the changing illuminated portion of the planet that we see). Orbiting the Sun
every 88 days, Mercury appears a bit like a cosmic moth, flying endlessly
around the intensely bright Sun.
One of the best ways to capture Mercury with a camera is to use a
standard or telephoto lens. Appearing as a dot on the final image, there’s
something very satisfying about grabbing a permanent record of this elusive
little world.
The camera should ideally be mounted on a tripod to keep things nice and
steady. If you’re using a simple point-and-shoot camera, try to record the
scene in fully automatic mode first. The chances are that the sky will record
too brightly and Mercury won’t be seen your using this method, but it’s worth
a shot.
If auto doesn’t work then you’ll have to resort to manual settings. If you’re
not sure what your camera can offer here, the best advice is to look up
"manual settings" in the camera’s documentation. The main settings you’ll
need to get to grips with are focus, ISO, lens f-ratio and exposure.
Focus needs to be set to infinity. Here, our Moon can help if it's visible.
Turn auto-focus on, point the camera at the Moon and half press the shutter
button to focus on it. Once in focus, turn auto-focus off and the camera
should now be properly focused. If the Moon's not up, focus on a bright star
or planet as accurately as you can. If your camera has a live view option, use
that for greater accurary. The ISO setting you’ll use will be determined by
the brightness of the scene. As Mercury is typically located against a bright
twilight background, a low ISO of 100–200 is probably the best starting point.
The lens f-ratio is the setting which controls how open the
lens aperture is; in other words the control that dictates how much light is
let into the camera. This should be set fairly wide (low f-ratio number) but it
is best to avoid the absolute lowest setting as this can sometimes release
aberrations inherent in some lenses.
For the exposure, start with a shutter speed of 1 second. Take a shot
and view it on the camera’s review screen. If the shot is too dark, increase
the exposure time. Over a certain exposure length, Mercury will no longer
appear as a dot but will instead appear to trail. This can be countered by
increasing the ISO and decreasing the exposure.
It’s very difficult to give absolute settings for different camera set-ups
when attempting to shoot against a twilight sky. Cloud cover coupled with
ever-changing light levels means that you typically have to adjust and adapt
your settings to the conditions. Here the review screen is perfect because it
can be used as a feedback device to let you know if your settings are off. If
the image looks too dark, increase the exposure and/or the ISO setting. If the
image looks too light, decrease these values.
Many cameras will let you know if the image contains areas of white
indicating over-exposure which should be avoided. Methods may be different
between camera models but a common way a camera shows this is to
flash the areas of pure white in the review image. Again, if you’re not sure
whether your camera can do this, check its documentation. If you do have
regions of pure white in your shots, reduce the exposure time and/or ISOMercury
Mercury is the innermost planet and the smallest. It is only 3000 miles in
diameter, much less than the 7900 miles for the Earth, and its force of gravity
is much lower. The escape velocity of a planet is the velocity needed to
escape from the surface with no extra impetus. The value for Earth is 7 miles
per second. The particles making up our atmosphere cannot move as fast as
this, so we hold on to our atmosphere. Mercury’s escape velocity is 1.5 miles
per second, and if there once was a dense atmosphere there it has now
leaked into space, leaving Mercury virtually airless. Without air there can be
no water and no life of our kind. Mercury is unfriendly in other ways too. It is
so near the Sun that during the day the hottest part of the surface is so torrid
that life could not exist. At night, on the other hand, the temperature sinks
very low because there is no atmosphere to blanket in the heat.
Because Mercury is lit by the Sun and has an orbit around the Sun that is
inside Earth's, it shows phases or changes of shape; its orbital period is 88
days. With the naked eye Mercury is well seen either in the west after sunset
or in the east before dawn. It is never seen against a really dark sky and most
people have never seen it at all. When you do find it, it appears surprisingly
bright – often brighter than every star in the sky. A small telescope will show
patches, once regarded as seas, but we now know there has never been
water there. There are mountains and craters, so in some ways Mercury is
not unlike the Moon though here there are important differences in detail.
Very large telescopes show much more of the surface, though by now
we depend upon the results sent back by spacecraft that have flown past
Mercury, and one probe, known as MESSENGER, that has been put into orbit
around the planet.
It cannot be said that Mercury is a promising subject for the amateur
observer, and normally there is little to see apart from its phase. Occasionally
the planet will transit across the face of the Sun, appearing as a small
black disc against its surface. Transits are interesting to watch but not
astronomically important.
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