INSTRUMENTS
ASTRAL TELESCOPE
page 8
THE SUN AND THE MOON
THE SUN
This is the only star which tan be seen without difficulty during the day.. These
precautions include the use of the appropriately dark sun filter which can be attached to
the ocular (fig. 10).
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The sun filter is screwed to the rear of the ocular and allows you to look
directly at the solar photosphere (changing the original color from yellow to green) and
its details. It is also necessary, in order to avoid excessive overheating of the filter,
to observe the sun only when isis relatively low on the horizon, and to diaphragm the
aperture of the telescope, removing not the whole cap of the tube but only the small
lateral cap (if this is not provided, you can construct one at home). The resultant
aperture will be greatly reduced, as, likewise, will be the concentration of solar heat by
the objective on the sun filter. Use of the filter is not possible with all types of
telescope. |
In the case of certain models with a high degree of
luminosity, the intense heat of the sun provokes a deformation in the mirrors of
the telescope and these are not protected by the filter. Check in file list of available
accessories if the sun filter is available, or else the filter suitable for all apertures
which will be described later in this manual.
The safest sun filter, suitable for ail apertures, may be positioned in front of the
aperture. This system allows the minimum of light to penetrate the optics so eliminating
the risk to the eye and to the telescope. It will still be necessary, however, to check
before all observation, that there are no gaps or holes through which the sun could pass.
This type of accessory is not common in all models, nor do all models offer it as a
possible accessory.
Aiming at the sun is not easy because (which should be covered during
observation of the sun to avoid inadvertent damage to the eyes which must be protected
from the light projected) otherwise permanent damage can be caused to the eyesight. The
safest system consists in moving the telescope in such a way that the shadow cast by the
telescope is as small as possible (fig. 11).
With a little patience it will be possible to aim correctly
at the sun.
What can be seen on the sun? The most obvious details are the sun spots, which have
bizarre and very different shapes, characterized by the black area of shadow, surrounded
by those in partial shadow which are grey. Drawing these sun spots in a notebook is fun,
and you can then observe their position on the solar disc. It is important to take note of
the date and time of observation so as to compare these with other observations at
different times and dates. In this way we can keep a record of the evolution of particular
details. The sun is truly spectacular during an eclipse, whether partial (when the sun is
partially covered by the moon) or total (rare but wonderful). During a partial eclipse, or
the partial phases of a total eclipse, it is necessary to take ail of the precautions
already listed. Among the most beautiful features of a total sun eclipse are the
protuberances, jets of red gas which seem to come out from the irregular or jagged borders
caused by the covering of the sun by the moon, and also the magnificent corona, which is
seen as an irregular black halo, surrounding the sun.
THE MOON
The moon is the easiest subject to observe in the night sky. It is
almost always the first thing to be observed by beginners. Its apparent dimensions are
conspicuous and so small instruments with low magnification levels are sufficient to
observe most of its details. Its brightness depends on its phases, which are dependent on
its position in the orbit relative to Earth and Sun. The best observations can be made
when the moon is waxing or waning and NOT when it is full. The reason for this is that
when the moon is full, its craters, cracks, mountains and canyons are lit up in front and
so there are no shadows and so there is very little contrast.
As well as this, when the moon is full there is too much light and so a lunar filter is
necessary on the ocular, attached in the same way as a sun filter (fig. 10). Details of
the moon are very bright and highly contrasted, and soit is possible to use the telescope
to its top capacity (that is its biggest possible level of magnification).
Oculars of short focal length can be used and, if you so wish, the Barlow lens may be
used. This is an optic accessory which can be inserted be- tween the telescope and the
oculars and it has the effect of doubling the focal of the telescope, and, as a result,
the possible magnification is also doubled.
With a high level of magnification you will notice that the moon tends to escape from the
ocular fieId. The reason for this is the movement and rotation of the earth (besides that
of the moon). It will therefore be necessary to continuously adjust the position of the
telescope in order to keep the moon centered, or indeed to keep any other astronomical
object centered. At a Iow level of magnification the effect of ail this is relatively
small, but it become irritating once you go beyond magnification of 70 -80 and upwards.
The moon too is eclipsed just as the sun is and this phenomenon is quite frequent (once or
twice a year) though it is not as spectacular as an eclipse of the sun. It is the shadow
of the earth that eclipses the sun, and this is interposed between our satellite and the
sun. The moon therefore does not disappear, but its brightness is diminished and it
assumes a dark red color. The moon is only eclipsed when it is full and the phenomenon is
very suggestive when observed at a magnification of 30 to 40.
French