Care And Maintenance Of Your Telescope; Collimating A Newtonian; Aligning The Secondary Mirror; Aligning The Primary Mirror - Dörr DANUBIA DELTA 20 Notice D'utilisation

10 | CARE AND MAINTENANCE OF YOUR TELESCOPE

10.1

COLLIMATING A NEWTONIAN

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 fi eld 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 di raction rings. If the rings are symmetrical about the Airy
disc, the telescope's optics are correctly collimated (Fig.r).
If you do not have a collimating tool, we suggest that you make a "collimating cap" out of a plastic 35mm fi lm
canister (black with gray lid). Drill or punch a small pinhole in the exact center of the lid and cut o 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 o the
lens cap which covers the front of the telescope 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.s). The secondary mirror is aligned
by adjusting the central bolt behind it, (which moves the mirror up and down the tube), and the three smaller
screws surrounding the bolt, (which adjust the angle of the mirror). 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.t)
10.2

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
refl ected 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 refl ected 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.u), 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 or loosen one and then compensate for the slack by tigh-
tening the other two. Stop when you see all three mirror clips (Fig.v). Make sure that all three small alignment
screws are tightened to secure the secondary mirror in place.
10.3

ALIGNING THE PRIMARY MIRROR

Find the three locking screws at the back of your telescope and loosen them by a few turns.
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If you see 3 large nuts protruding from the back of your telescope and 3 small Phillip's-head screws besides
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them, the Phillip's-head screws are the locking screws and the large nuts are the adjusting screws.
If you see 6 Phillip's-head screws but 3 protruding from the back of your telescope, the 3 protruding screws
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are locking screws and the ones next to them are adjusting screws.
If you see 3 hex bolts and 3 Phillip's head screws, the hex bolts are the locking screws and the Phillip's-head
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screws are the adjusting screws. You will need an Allen wrench to adjust the locking screws.
Now run your hand around the front of your telescope keeping your eye to the focuser, you will see the re-
fl ected image of your hand. The idea here being to see which way the primary mirror is defected, you do this
by stopping at the point where the refl ected image of the secondary mirror is closest to the primary mirrors'
edge (Fig.w). When you get to that point, stop and keep your hand there while looking at the back end of your
telescope, is there a adjusting screw there? If there is you will want to loosen it (turn the screw to the left) to
bring the mirror away from that point. If there isn't a adjusting screw there, then go across to the other side
and tighten the adjusting screw on the other side. This will gradually bring the mirror into line until it looks like
Fig.x. (It helps to have a friend to help for primary mirror collimation. Have your partner adjust the adjusting
screws according to your directions while you look in the focuser.) After dark go out and point your telescope
at Polaris, the North Star. With an eyepiece in the focuser, take the image out of focus. You will see the same
image only now, it will be illuminated by starlight. If necessary, repeat the collimating process only keep the
star centered while tweaking the mirror
Fig.r
Fig.s
Fig.t
Fig.u
Fig.v
Adjusting screw
Fig.w
Secondary mirror
Primary mirror
Fig.x
Both mirrors aligned
with collimating cap in
24
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 refl ected
image for now
Primary mirror
Primary mirror clip
clip
Primary mirror clip
Locking screw Locking screw
Hex bolt
(Locking screw) Adjusting screw
stop and keep your
hand here
Both mirrors aligned
with eye looking in focuser
Adjusting screw