1. The gray circle is your field of view through the eyepiece. The white circle is the defocused star
and the black spot is the position of the secondary mirror's shadow. A collimated scope will
look like "A", above, at any magnification.
2. If the shadow si not in the middle (in our example, "B", is at the "8 o'clock" position) then move
the telescope, using the mount motors, so the image goes in the same direction, so the
defocused star goes near the edge of the field of view at the "8 o'clock" position, like in "C".
3. Try to tight or loose a bit one of the collimation screws. Discover which screw (or screws) move
the star image in the direction of the center of the field of view (like the black arrow in "C").
When the image will go back to the middle of the field of view, it will be more centered.
4. Repeat step 2-3 until you reach collimation (image "A", above).
5. Use a higher magnification and repeat.
Some notes about this operation:
It is very unlikely you have to move any collimation screw more than one turn – probably you
will reach collimation with less than half a turn.
Have someone to help you while you look at the eyepiece – or use a camera so you can see
the image while you work on the screws. It is a good idea to label the three collimation "A", "B",
and "C", and memorize the direction of the movement given by each screw. Obviously the star
will move in the opposite direction along a line when you turn screw "A" clockwise or
counterclockwise. Movements due to screws "B" and "C" will be 120° apart.
It is a good idea to mark the initial positions of collimation screws with a felt tip pen and keep
track of what you do (like “screw A 45° CW – screw B CCW 30°...” and so on). Doing so, you
will always be able to “undo” a wrong action, and, if needed, to go back to the original position.
In all
Officina Stellare
telescopes the collimation screws are spring-loaded, so if you tight one
screw there is no need to loose the other two.
And, very important: KEEP YOUR TIME, AND START WITH SMALL MOVEMENTS (1/8 or
1/16 of a turn).
When you have a perfectly symmetrical figure at any magnification (i.e. when the dark shadow is
always in the middle of the de-focused star) you are done, and you can use your scope for any
reasonable purpose. Go to section 6.5 and 6.6 for fine collimation and focuser tip-tilt.
5.4) And if I'm not able to reach collimation?
It is quite unlikely, but it is possible that your secondary mirror moved a bit during transport. If this
is the case, you will not be able reach a good collimation using only the primary mirror tip-tilt
control. You have to refer to Chapter 6, Collimation “from scratch”.
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