If we have a look at figure 7, we see the picture that you would get when you are looking through a beginner telescope with long
focal length (with the eyepiece removed). If the telescope is collimated the astronomer should see the concentric images of his
eye, the secondary mirror, the main mirror and the focuser.
However, your Explore Scientific dobsonian has a much faster f-ratio. For this
reason the secondary mirror has to be shifted from its centered position in two
directions
– away from the focuser and towards the main mirror (by the same
amount).
This offset is a few mm in both directions. You do not have to take care of this
–
we already did that at the factory. We are just mentioning it here in case you are
looking at the secondary cage from above and recognize the fact that the
secondary is off center. You can test the correct rough collimation like this:
1. Remove the eyepiece from the focuser and look through it. The secondary
should appear round as in figure 7. If the secondary does not appear round, tilt it
with the three collimation screws that are located on the back side of the
secondary holder.
2. Next check the reflection of the middle point of the main mirror in the
secondary. If the reflection of the mark on the main mirror is not in the center of the secondary, center it by using the three
collimation screws like you did in step 1.
Figure 8
Adjusting the main mirror
If the secondary appears round and the mark on the main mirror is centered in the secondary we will collimate the main mirror.
This has to be done when the reflection of your eye and the reflection of the secondary on the main mirror(5, figure 7) are n ot
concentric. Insert the collimating tool into one of the collimation screws that are located on top and on each side of the main
mirror. When you turn the collimation tool you will notice that the hexagonal head of the tool will lock into the hexagonal head of
the collimation screw. Now you can adjust the main mirror by turning the collimation tool (see figure 8). Adjust the screws until
the eye and the reflection of the secondary are centered in the main mirror. Practice this until you get a feeling what a turn at
Figure 7
