Using The Optional Barlow Lens; Focusing; Polar Alignment; Tracking Celestial Objects - Dörr DANUBIA DELTA 20 Notice D'utilisation

07.4

USING THE OPTIONAL BARLOW LENS

A Barlow is a negative lens which increases the magnifying power of an eyepiece, while reducing the fi eld
of view. It expands the cone of the focussed light before it reaches the focal point, so that the telescope's
focal length appears longer to the eyepiece. The Barlow is inserted between the focuser and the eyepiece
in a refl ector, and usually between the diagonal and the eyepiece in a refractor or a catadioptric (Fig.f). With
some telescopes, it can also be inserted between the focuser and the diagonal, and in this position it gives
even greater magnifi cation. For example, a 2x Barlow when inserted after the diagonal can become 3x when
placed in front of the diagonal. In addition to increasing magnifi cation, the benefi ts of using a Barlow lens
include improved eye relief, and reduced spherical aberration in the eyepiece. For this reason, a Barlow plus
a lens often outperform a single lens producing the same magnifi cation. However, its greatest value may be
that a Barlow can potentially double the number of eyepiece in your collection.
07.5

FOCUSING

Slowly turn the focus knobs under the focuser, one way or the other, until the image in the eyepiece is sharp
(Fig.g). The image usually has to be fi nely refocused over time, due to small variations caused by temperature
changes, fl exures, etc. This often happens with short focal ratio telescopes, particularly when they haven't
yet reached outside temperature. Refocusing is almost always necessary when you change an eyepiece or
add or remove a Barlow lens.
07.6

POLAR ALIGNMENT

In order for your telescope to track objects in the sky you have to align your mount. This means tilting the
head over so that it points to the North (or South) celestial pole. For people in the Northern Hemisphere this is
rather easy as there is a bright star very near the spot Polaris. For casual observing, rough polar alignment is
adequate. Make sure your equatorial mount is in level and the red dot fi nder or fi nderscope is aligned with the
telescope before beginning. Look up your latitude on a map, road maps are good for this purpose. Now look
at the side of your mount head, there you will see a scale running from 0-90 degrees. Unlock the hinge of the
mount by gently pulling on the lock lever counter-clockwise. At the bottom of the head is a screw that pushes
on a tongue under the hinge, changing the angle. Spin this until your latitude is shown on the scale by the indi-
cator pin, then lock the hinge (Fig.h). "Pole Star" is less than one degree from the North Celestial Pole (NCP).
Because it is not exactly at the NCP, Polaris appears to trace a small circle around it as the Earth rotates.
Polaris is o set from the NCP, toward Cassiopeia and away from the end of the handle of the Big Dipper (Fig.i).
EQ1 Delta 20, Wega 900
Unlock the DEC. lock knob and rotate the telescope tube until the pointer on the setting circle reads 90°.
Retighten the DEC. lock knob. At the top of the main shaft is a white line with "R"and"A" on either side of it.
Loosen the a zimuth lock knob and rotate the mount until the white line points roughly at Polaris. Retighten the
azimuth lock knob. Look through the fi nderscope and centre Polaris on the crosshairs by adjusting the azimuth
and latitude settings if a more accurate polar alignment is desired.
EQ2 Delta 30, Saturn 50:
Unlock the DEC. lock knob and rotate the telescope tube until the pointer on the setting circle reads 90°.
Retighten the DEC. lock knob. At the top of the main shaft is a white line with "R"and"A" on either side of it.
Loosen the a zimuth lock knob and rotate the mount until the white line points roughly at Polaris. Retighten the
azimuth lock knob. Look through the fi nderscope and centre Polaris on the crosshairs by adjusting the azimuth
and latitude settings if a more accurate polar alignment is desired. After a while you will notice your target
drifting slowly North or South depending on the direction of the pole relative to Polaris. To keep the target in
the center of the view, turn only the R.A. slow-motion cable. After your telescope is polar aligned, no further
adjustments in the azimuth and latitude of the mount should be made in the observing session, nor should you
move the tripod. Only movements in R.A. and DEC. axis should be made in order to keep an object in the fi eld.
Southern Hemisphere
In the Southern Hemisphere you must align the mount to the SCP by locating it's position with star patterns,
without the convenience of a nearby bright star. The closest star is the faint 5.5-mag. Sigma Octanis which is
about one degree away. Two sets of pointers which help to locate the SCP are alpha and beta Crucis (in the
Southern Cross) and a pointer running at a right angle to a line connecting alpha and beta Centauri (Fig.j).
07.7

TRACKING CELESTIAL OBJECTS

When observing through a telescope, astronomical objects appear to move slowly through the telescope's
fi eld of view. When the mount is correctly polar aligned, you only need to turn the R.A. slow-motion to follow
or track objects as they move through the fi eld. The DEC. slow-motion control is not needed for tracking. A
R.A. motor drive can be added to automatically track celestial objects by counteracting the rotation of the
Earth. The rotation speed of the R.A. drive matches the Earth's rotation rate for stars to appear stationary in
the telescope eyepiece. Di erent tracking speeds are also available in some models. A second drive can be
added to give DEC. control which is very useful for doing astrophotography.
Fig.f
Fig.g
Fig.h
Fig.i
Fig.j
20
Eyepiece
Barlow
Refl ector Telescopes
Eyepiece
Barlow
Diagonal
Refractor Telescopes
Big Dipper
Little Dipper