One day, when I was adjusting optical axis of Newtonian telescope, an idea came up to me.
Why do we adjust secondary mirror first?
It is the primary mirror that directly collects starlight and reflect it to the secondary mirror.
Adjusting primary mirror should come first.
Secondary mirror should be adjusted to the (properly adjusted)primary mirror.
This was the start of the story.
How can we adjust primary mirror first?
Newtonian telescope is in a shape of a cylinder.
What if the cylinder axis of Newtonian telescope and axis of the primary mirror came together?
How can I make the two axes together?
How about this?
You remove the secondary mirror from the Newtonian telescope.
You look inside the hole of the center screw of secondary mirror.
Looking straight down into the center of the primary mirror through the hole, you can see the center mark of the primary mirror in front of you.
If you see your eye in the center of the center mark of primary mirror, two axes should be together.
First you remove the secondary mirror.
The view from the hole of the center screw of secondary mirror.
I made my Newtonian telescope look upward and put a camera on the center hole.
Actually, I used OCAL electronic collimator camera.
The first view of the camera.
You can see that the height of four fastener at the edges of the spider are not the same.
This means the two axes are not together.
(Two axes. One is cylinder axis of newtonian telescope and another is primary mirror axis)
First thing you should do is to show the blue circle (using controller panel of OCAL electronic collimator) which its diameter is the same with the primary mirror.
Second, you adjust the camera position with your hand to make the blue circle and the contour of the primary mirror match.
Then the camera is looking straight forward to the primary mirror.
In the photo below, blue circle which overlaps the edge of the primary mirror is a little bit difficult to see, but believe me the blue circle is there.
Then adjust the screws behind the primary mirror to get a view like this.
Then the two axes are together.
In the above picture, the intersection of purple cross shows the center of the camera view.
It is also the center of the blue circle.
As you can see, the intersection of the purple cross is not in the center of the center mark of primary mirror.
The reason is that the center mark is not in the center of the primary mirror.
It is me myself who placed the center mark with my hand, so I'm sure about that (^_^;)
Sorry if you are confused about that.
Now the primary mirror is in a proper position.
So, in the subsequent steps, you should not touch primary mirror.
Because the primary mirror is already in the right position.
In the next step, you adjust the secondary mirror.
You set the secondary mirror to its position.
The final goal looks like this.
(1) Intersection of the purple cross and the camera lens (in the view of secondary mirror) matches.
(2) Blue circle and the edge of primary mirror (in the view of secondary mirror) matches.
The first view after I set the secondary mirror was like this.
You can see the red OCAL electronic collimator camera in the center.
In the center of OCAL electronic collimator, there is a black dot which is the camera lens of OCAL electronic collimator.
You can see that the camera lens is inside center mark of primary mirror.
The camera lens is above the intersection of purple cross.
This means you have to rotate clockwise the center screw of the secondary mirror to move the secondary mirror upward (In this photo, downward).
This photo shows the view after I rotated clockwise the center screw of secondary mirror.
The camera lens is under the intersection of purple cross, that means I rotated clockwise the center screw too much.
Now I have to rotate a little bit the center screw counter-clockwise.
Now the camera lens and the intersection of the purple cross matched.
And the blue circle and the contour of the primary mirror (in the view of secondary mirror) matched.
Close up view.
Intersection of the purple cross and the center of red OCAL electronic collimator matched.
Ideally, Intersection of the purple cross and center of red OCAL electronic collimator should be in the center of the center mark of primary mirror.
But as I already mentioned, center mark of my Newtonian telescope is not in the center of primary mirror.
Finished
How do you think about adjusting primary mirror first in Optical axis adjustment of a Newtonian telescope?
Remove your eyepiece (or camera) and look inside your newtonian telescope to see the spider in the view of your secondary mirror.
Look at the four edge of the spider.
Heights of four fasteners on edge of the spider is not the same.
If you got this view, optical axis of your newtonian telescope is not in good shape.
I think this method is worthwhile trying.
