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Viewing Telescopic Meteors
METEOR NETWORK ARTICLE #6
The following article was written by Malcolm J. Currie (England) and
published in IMO's December 1994 edition of the WGN Journal.
WHAT IS THE BEST TELESCOPE OR BINOCULAR FOR SEEING TELESCOPIC METEORS?
There is no single best-buy telescope or binocular. There is a wide
selection of suitable instruments; the choice will depend on the quality of
your observing site, your eyesight, observing goals, and how much you wish to
pay or what is already available. However, there are two main factors that
should influence your choice: the instrument should have a low power and a
wide apparent field of view. They both affect the number of meteors seen in
a given time. Let us consider these in more detail.
THE MAGNIFICATION PER UNIT APERTURE
You must have a low magnification for a given size of objective lens or
mirror. To put that into numbers, the magnification should be in the range
of 1.4 - 2.0 times the aperture in centimeters. So, for example, a 7 X 50
binocular has a magnification 1.4 times the aperture in centimeters, and a 10
X 50 has magnification twice the aperture. To explain how these numbers
arise here is a brief optics lesson. If you hold a telescope or binocular to
the light and away from your eye, you will see a small illuminated disk.
This is called the exit pupil. Its diameter is given by the telescope
aperture divided by the magnification. As this is just the inverse of our
factor, a given factor produces a certain sized exit pupil regardless of the
telescope's aperture. So returning to our specific limits, a factor of 1.4
times has a 7 - mm exit pupil and a 2.0 times has a 5 - mm beam. For normal
mortals, a 7 - mm beam is as much as the pupil of the dark-adapted eye can
handle, for older observers even this may prove to be too wide. Also, if you
are located at a site with some light pollution, a slightly higher
magnification will let you see more meteors as the contrast is improved.
Through the telescope, most meteors appear as lines rather than points, but
nevertheless, like for stars, you can still see fainter with additional
magnification. You can only take this so far. As the magnification is
increased, the true field of view is decreased, and the area of atmosphere
being viewed reduces as the inverse square of the magnification, and so the
observed rate falls. That is not all. Due to the increased magnification
the apparent speed of the meteors is accelerated, which reduces the apparent
brightness of meteors, and so more meteors will pass through the field
undetected. There comes a point where the improved visibility of faint
meteors is offset by the loss of area being viewed. This is approximately
twice the aperture in centimeters. Binoculars with 6 - mm exit pupils are
unfortunately much rarer than the standard 7 - mm ones, though it is getting
better. For example, Celestron produce a 7 X 42 and an 8 X 50. If sky
conditions are too bright, you can always stop down the objective lens to
give better contrast.
THE APPARENT FIELD OF VIEW
The apparent field of view is governed by the eyepiece design. You can
derive it from the product of the magnification and the true field of view.
So, for example, a 10 X 50 binocular, with a 6 degree true field, has an
apparent field of 60 degrees. A wide field of view will encompass more of
the sky, and hence you will see more meteors. The recommended range is 45 -
70 degrees, with 50 - 60 degrees being preferred. You may be wondering why
we set an upper limit. One of the principal reasons for observing telescopic
meteors is to investigate radiant properties by plotting meteor paths
accurately. As the apparent field of view enlarges, the average plotting
accuracy goes down. So ultra-wide fields (>65 deg) are best for determining
rates, and hence deriving the time of maximum for a shower; whereas for field
sizes around 50 degrees rates are still reasonable (because the eye perceives
only a fraction of the meteors in the outer 10 deg annulus) and accurate
positional data can be obtained. Given the choice between the two, you
should err on the side of the smaller apparent field as it offers more
flexibility and science. Also, ultra-wide eyepieces or binoculars are either
very expensive if they give pinpoint images across the entire field, or give
increasingly distorted images towards the periphery of the field. Below 50
deg the loss of sky coverage starts to be come important. If rates become
too low boredom and loss of concentration can soon set in.
BINOCULAR versus TELESCOPE
Binocular vision is the natural way to look, and since comfort is a critical
consideration for the telescopic observer, a binocular is preferred to a
(monocular) telescope. There has been debate in the literature by how much
it improves the limiting magnitude from nothing to about a magnitude. A
telescope with a star diagonal is more flexible for viewing fields close to
the zenith, and if you want a larger aperture, will be far less expensive.
Angled binoculars only seem to come with large apertures and even larger
price tags.
APERTURE
Aperture is less critical, and IMO observers' apertures range from 40mm to
300mm, though most are in the range 50-80mm. Certain showers like the
Perseids are progressively weaker towards fainter magnitudes and this
suggests a small aperture is best, say a 6X30. Increasing the aperture
increases the average meteor magnitude and so exaggerates any mass-sorting
within the stream, and will give improved plotting accuracy. The
intermediate apertures (50-80mm) look best.
OPTICAL QUALITY
The quality of the optics can make a big difference to the performance.
Remember that you will be observing for long periods and considerations like
accurate collimation and pinpoint images will reduce strain. This
consideration can outweigh some of those mentioned already. For example, a
quality 7X42 is going to let you see more meteors than a cheap 8X50.
In conclusion, an 8X50 or 10X60 binocular with a 55 degree apparent field
would be excellent for telescopic meteors. Many other similar combinations
will perform well too.