reach a good focus and stop the motor This position will be your new “zero”. During regular
use, reach perfect focus with the focuser.
CASE B: you only have the “regular” focuser and no BFL motor.
Same as case A, but you
correct the longitudinal position of the secondary mirror
manually. See photo at left. Bring the focal plane indicator
(on your camera body) to the design BFL and than focus
your real star moving the secondary mirror. You shouldn't
move it more than 1 mm (one turn of the screw).
If you don't have a “regular” focuser
(an unusual solution
needed for some telescope / camera combination) obviously
you have always to use the BFL motor, because as a matter
of fact you always change focus changing M1M2.
IN ANY CASE, A, B or C,
if you need to move your
secondary mirror more than 1 mm, to reach focus when the
camera is at the nominal BFL, then your primary mirror is
probably in the wrong (longitudinal) position, i.e. it is too
close or too far from the secondary. It is quite unusually that
this may happen, but it is not impossible. If this is your
situation, here is the
procedure to solve the
problem:
1. Set the secondary mirror position to factory zero, if
you have the BFL motor.
2. Focus your camera on a real star and measure the
BFL.
3. If the focus is too far
inside
the scope (i.e. you
measure a back focus length
shorter
than nominal)
you need to move the primary mirror
toward
the secondary mirror. Loose safety screws, if
present, and rotate the three collimation screws of the primary mirror counterclockwise, all
by the same amount. As a matter of fact, you are changing focus moving the primary
mirror. Remember that each complete turn of the screws will move the mirror 1 mm, and
this will move the focal plane by 4 to 9 mm, depending on optical scheme. So, if your focal
plane is too "short" by 8 mm, you will have to move the primary mirror by 1-2 full turns of
the screws. Try ½ turn first, then another ¼ turn, and so on.
What is important is that
you move each screw by the same amount (and of course in the same direction!).
After each movement, refocus the star (using the focuser, not the secondary mirror
movement) and return to point 2.
4. If the focus is too far
away
from the scope (measured back focus
longer
than nominal)
follow the procedure described at point 3, only changing the action on the primary mirror
screws (you
tight
them i.e. turn them
clockwise
, to pull the mirror toward the backplate).
5. Just a note: when you will “fine tune” the tip-tilt position of the primary mirror (sections 6.4
and 6.5) you will not change its longitudinal position significantly.
6.3) Secondary mirror tip-tilt regulation.
1. Identify the collimation screws of the secondary mirror (3 screws 120° apart, visible looking
the telescope from "the sky"). See photos next page.
2. IMPORTANT: if the secondary mirror has an electric motor to change the BFL (Back Focus
Length),
turn on the scope electronic control and set it to the "factory zero".
The
22
How to move the secondary mirror
manually. Loose the safety screw (red
arrow) and rotate the big frontal screw
(green arrow) using the same tool
provided for the primary mirror screws.
One turn of this screw does move the
mirror 1 mm. When done, tight again
(gently!) the safety screw.
