Choosing The Best Time To Observe; Aligning The Secondary Mirror - Dorr DANUBIA ATLAS 2000 Bedienungsanleitung

11.3

CHOOSING THE BEST TIME TO OBSERVE

The best conditions will have still air, and obviously, a clear view of the sky. It is not necessary that the sky
be cloud-free. Often broken cloud conditions provide excellent seeing. Do not view immediately after sunset.
After the sun goes down, the Earth is still cooling, causing air turbulence. As the night goes on, not only will
seeing improve, but air pollution and ground lights will often diminish. Some of the best observing time is often
in the early morning hours. Objects are best observed as they cross the meridian, which is an imaginary line
that runs through the Zenith, due North-South. This is the point at which objects reach their highest points in
the sky. Observing at this time reduces bad atmospheric effects. When observing near the horizon, you look
through lots of atmosphere, complete with turbulence, dust particles and increased light pollution.
11.4 COOLING THE TELESCOPE & ADAPTING YOUR EYES
Telescopes require at least 10 to 30 minutes to cool down to outside air temperature. This may take longer if
there is a big difference between the temperature of the telescope and the outside air. This minimizes heat
wave distortion inside telescope tube (tube currents). Allow a longer cooling time for larger optics. If you are
using an equatorial mount, use this time for polar alignment.
Do not expose your eyes to anything except red light for 30 minutes prior to observing. This allows your pupils
to expand to their maximum diameter and build up the levels of optical pigments, which are rapidly lost if ex-
posed to bright light. It is important to observe with both eyes open. This avoids fatigue at the eyepiece. If you
find this too distracting, cover the non-used eye with your hand or an eye patch. Use averted vision on faint
objects: The center of your eye is the least sensitive to low light levels. When viewing a faint object, don't look
directly at it. Instead, look slightly to the side, and the object will appear brighter.
12 | CARE AND MAINTENANCE FOR YOUR TELSCOPE
12.1 COLLIMATING A NEWTONIAN REFLECTOR
Collimation is the process of aligning the mirrors of your telescope so that they work in concert with each
other to deliver properly focused light to your eyepiece. By observing out-of-focus star images, you can test
whether your telescope's optics are aligned. Place a star in the centre of the field of view and move the focu-
ser so that the image is slightly out of focus. If the seeing conditions are good, you will see a central circle of
light (the Airy disc) surrounded by a number of diffraction rings. If the rings are symmetrical about the Airy
disc, the telescope's optics are correctly collimated (Fig.s).
If you do not have a collimating tool, we suggest that you make a "collimating cap" out of a plastic 35mm film
canister (black with gray lid). Drill or punch a small pinhole in the exact center of the lid and cut off the bottom
of the canister. This device will keep your eye centered of the focuser tube. Insert the collimating cap into the
focuser in place of a regular eyepiece.
Collimation is a painless process and works like this: Pull off the lens cap which covers the front of the tele-
scope and look down the optical tube. At the bottom you will see the primary mirror held in place by three clips
120º apart, and at the top the small oval secondary mirror held in a support and tilted 45º toward the focuser
outside the tube wall (Fig.s1). The secondary mirror is aligned by adjusting the three smaller screws surroun-
ding the central bolt. The primary mirror is adjusted by the three adjusting screws at the back of your scope.
The three locking screws beside them serve to hold the mirror in place after collimation. (Fig.s2).
12.2

ALIGNING THE SECONDARY MIRROR

Point the telescope at a lit wall and insert the collimating cap into the focuser in place of a regular eyepiece.
Look into the focuser through your collimating cap. You may have to twist the focus knob a few turns until the
reflected image of the focuser is out of your view. Note: keep your eye against the back of the focus tube if
collimating without a collimating cap. Ignore the reflected image of the collimating cap or your eye for now,
instead look for the three clips holding the primary mirror in place. If you can't see them (Fig.s3), it means that
you will have to adjust the three bolts on the top of the secondary mirror holder, with possibly an Allen wrench
or Phillip's screwdriver. You will have to alternately loosen one and then compensate for the slack by tighte-
ning the other two. Stop when you see all three mirror clips (Fig.s4). Make sure that all three small alignment
screws are tightened to secure the secondary mirror in place
Fig.s
Fig.s1
Fig.s2
Fig.s3
Fig.s4
26
Correctly aligned Needs collimation
Focuser
Support for
secondary minor
Primary mirror Secondary mirror
Primary mirror
Mirror cell
Locking screw Adjusting screw
Primary mirror
clip
Ignore the reflected
image for now
Primary mirror
Primary mirror clip
clip
Primary mirror clip