22
The exposures which are required for photographing the star fields
are tenths of minutes without hindrance of the street lighting. Therefore,
for this period of time one should see to it that the camera follows the
sky precisely in its diurnal rotation. Near the centre of the field of view
of the camera one chooses the bright star to which the telescope is
pointed. To keep the star on the reticle cross-hairs of the finderscope is
the problem for an observer for the whole period of exposure. As the
cross-hairs of the finderscope is not illuminated, the image of the guide
stair should be slightly defocused in order to cross a light circle of the
unsharp image of the star by the cross-hairs and to keep the star in
this position for the period of exposure.
One applies minor corrections for a clock-work drive with the
help of handwheel 9 (fig. 5) keeping the star on the cross-hairs for the
whole period of exposure. One corrects the position of the guide star
with the micrometer screw of the declination axis if necessary. To obtain
the minimum corrections in declination the polar axis should be set as
precisely as possible to the celestial pole. One should remember that
if the polar axis is set incorrectly, even in the case when the star image
is kept on the cross-hairs, the images of the stars at the edges of the
field of view appear as dashes.
6.3. Telescope and atmospheric conditions
At high magnifications together with the increase of the visible
dimensions of the object the disturbances due to atmosphere are
increased. They are expressed in great blurring of the images of the distant
objects, in scintillation and blurring of the star images.
The observation in the cold seasons are possible when all telescope
parts acquire the ambient temperature. For air circulation inside the
telescope tube there is a hole plugged with a stopper in the cell of
the primary mirror. In operation the stopper should be unscrewed from
the cell.
But in observations in the street the great disturbance of atmosphere
may take place at nights, that leads to bad images of the celestial
objects.
It is quite possible that at those nights the observations of the fine
details of the planets and of the Moon are unsuccessful.
It is apparent that the observations with the aid of the telescope through
the window are senseless as the rough surfaces of the window glasses
distore the images.
In case of precipitation and stopping of operation the telescope
together with supply unit must be covered.
