6.
On many scopes it is not possible to have the spot
and
the circles in focus at the same
time, but it is easy to see if the spot is concentric to the circles.
7. Using the collimation screws of the secondary mirror center the black spot with the circles.
It is much easier if you have someone who can assist you.
Take your time, and try to be
as precise as you can.
The telescope from behind with the Tak inserted in the conical
flange, completely “inside”. You see the flashlight we used to
illuminate the white plastic window (1), the TC itself (2) and only a
out-of-focus spot of light (3)
Here you see the users' fingers (1) pulling out the collimation tool.
You see also the flashlight (2). After 1-3 centimeters of
movement, you will see the black dot, that is
embedded
in the
Tak (3).
Extracting the Tak another centimeter or so, the black dot will go
out of focus (1), while the ring printed on the secondary mirror will
come into focus (2). In this image it is clear the black dot and the
ring are not concentric. Please note that
this image (spot and
circles) does not change
even if you “shake” the Tak. Try that!
This image was taken after the tip-tilt regulation of the secondary
mirror. You see the flashlight (1), the users' fingers (2) and the
black dot and the circle on the mirror, now concentric.
Just to give you an idea, a single collimation screw was rotated about 45° (i.e. just 1/8 of a turn!) to change the image from “perfect”
(left) to “horrible” (right). Please note that you MUST reach “perfection”. The image in the middle is NOT ACCEPTABLE even if the
black dot is within the innermost circle. It must be as concentric as possible. Note that in the “perfect” image only the innermost circle
painted on the secondary mirror is visible, while in the other two also part of the second circle is illuminated. Usually you need to work
on more than one collimation screw to reach “perfection”. The TC is very sensitive, and clearly shows a tilt of the secondary mirror
smaller than one arcmin from the correct position.
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