Half-hours with the Telescope: Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction.Proctor, Richard A. (Richard Anthony)
Science
Half-hours with the Telescope: Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction.
Proctor, Richard A. (Richard Anthony)
Astronomy; Telescopes
We look first for the Great Bear towards the north-east, as already
described, and thence find the Pole-star; turning towards which we see,
towards the right and downwards, the two guardians of the pole ([beta]
and [gamma] Ursae Minoris). Immediately under the Pole-star is the
Dragon's Head, a conspicuous diamond of stars. Just on the horizon is
Vega, scintillating brilliantly. Overhead is the brilliant Capella, near
which the Milky Way is seen passing down to the horizon on either side
towards the quarters S.S.E. and N.N.W.
For the present our business is with the southern heavens, however.
Facing the south, we see a brilliant array of stars, Sirius
unmistakeably overshining the rest. Orion is shining in full glory, his
leading brilliant, Betelgeuse[2] being almost exactly on the meridian,
and also almost exactly half way between the horizon and the zenith. In
Plate 2 is given a map of this constellation and its neighbourhood.
Let us first turn the tube on Sirius. It is easy to get him in the field
without the aid of a finder. The search will serve to illustrate a
method often useful when a telescope has no finder. Having taking out
the eye-piece--a low-power one, suppose--direct the tube nearly towards
Sirius. On looking through it, a glare of light will be seen within the
tube. Now, if the tube be slightly moved about, the light will be seen
to wax and wane, according as the tube is more or less accurately
directed. Following these indications, it will be found easy to direct
the tube, so that the object-glass shall appear _full of light_. When
this is done, insert the eye-piece, and the star will be seen in the
field.
But the telescope is out of focus, therefore we must turn the small
focussing screw. Observe the charming chromatic changes--green, and
red, and blue light, purer than the hues of the rainbow, scintillating
and coruscating with wonderful brilliancy. As we get the focus, the
excursions of these light flashes diminish until--if the weather is
favourable--the star is seen, still scintillating, and much brighter
than to the naked eye, but reduced to a small disc of light, surrounded
(in the case of so bright a star as Sirius) with a slight glare. If
after obtaining the focus the focussing rack work be still turned, we
see a coruscating image as before. In the case of a very brilliant star
these coruscations are so charming that we may be excused for calling
the observer's attention to them. The subject is not without interest
and difficulty as an optical one. But the astronomer's object is to get
rid of all these flames and sprays of coloured light, so that he has
very little sympathy with the admiration which Wordsworth is said to
have expressed for out-of-focus views of the stars.
Public-domain text, read in full here on John Shaqi.
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