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Some technicalities explained
| Newtonian Reflector: |
| The optical layout of this
design is shown in the figure. Light from the object is reflected from the primary
concave |
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| mirror in a
convergent beam which is deflected side ways by a small flat secondary mirror to form
image outside the main tube. This image is magnified by the eyepiece. |
| Magnification Power: |
| It is the number of times an
object is seen enlarged diametrically. For example, two stars which subtend an angle of
0.5 degrees with the unaided eye will be seen 50 degrees apart through a |
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| telescope at
100X magnification power,(or simply power). Photographs alongside show a chimney as seen
with the naked eye (left) and as seen through our SV-125 model telescope at 42X power.
About 35X power is sufficient to see Moon's craters, Saturn's rings, Jupiter's 4 Galilean
satellites, phases of Venus or Mars as a reddish-yellow disc. Most deep-sky objects can
also be seen at this magnification but a large aperture telescope is needed to see them
with reasonable brightness. |
| Resolving Power: |
| It is the minimum angular separation between two point objects
seen distinctly as two. The Resolving Power of a normal unaided eye is about 4 are minutes
(or 240 seconds) |
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| Thus a telescope of 1 arc-second resolving power is 240 times
more sensitive than the naked eye and can be used at 240X power without distorting the
image. Figure shows two stars fully resolved in the left diagram but not resolved at the
same magnification in the right diagram. |
| Diffraction
Limited Optics - Mirror Surface Accuracy: |
| It is the departure of the
mirror surface from the ideal surface measured in terms of wavelength of green light. A
mirror polished to a 1/8 wave accuracy or better will perform to the theoretical limit of
resolution and is said to be diffraction limited. It is so called because further
improvement in resolution is restricted by the wave nature of light rather than the
surface accuracy. The propagation of light in waves causes the image of a star to be a
disc (called the diffraction disc) rather than a point. Larger the mirror, smaller the
disc and hence better the resolution and higher the useful magnification (at which the
image is undistorted). Also a telescope with poor optical quality will form larger and
deformed diffraction discs than that of the same size with diffraction limited optics. The
former will give a distorted image at higher magnifications while the latter will retain
sharpness to the theoretical limit. In general, a magnification of 25X per centimeter of
aperture is the maximum accepted limit for diffraction limited optics. It is clear that
any telescope can have this maximum magnification or even more but the key to its
performance is the ability to retain sharpness of the image at this high magnification. |
| Star
Magnitude: |
The brightness of stars is
measured in terms of magnitudes. The first magnitude stars are brighter than the second by
two and a half times. The second magnitude stars are 2½ times brighter than the third and
so on. Under good seeing conditions the naked eye can see up to 5th magnitude stars. The
big eye of a 125 mm telescope will show up to 12.5 magnitude stars. Most of the Messier
objects lie within this 5-12th magnitude range. The light gathering power of a telescope,
which is directly proportional to its aperture (or mirror size), is expressed in terms of
the magnitude of the faintest star than can be seen with it.
Altazimuth Mount :
This type of mount allows the movement of the telescope tube in two axes, one being
horizontal (or altitude), the other vertical (or azimuth). Thus the telescope can be
pointed in any direction. |
| Equatorial
Mount: |
| Here one movement is along
the polar axis of the Earth and the other perpendicular to it, the advantage being that
sky objects can be tracked by moving the telescope along one axis only. |
| Aluminising
Facility: |
We have our own aluminising
facility. All our mirrors are aluminised in our own vacuum coating plant. These highly
reflective, hard coated mirrors will retain their brightness for a number of years if
cared for properly. However should your mirrors require re-coating, send them back to us.
We will re-aluminise your old mirrors to make them as good as new and your telescope will
once again show images as bright as ever. If you have any other front surface aluminising
job, do let us know.
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