Collimating, or optically aligning, a F/3.0 telescope is easy. Perhaps it's because at f/3, it's obvious when it's not collimated.

It's important to understand the single requirement of collimation, namely, that the eyepiece axis and primary mirror axis be co-incident, or on top of each other.

Achieve the single requirement with a two step process:
1. Center the laser collimator dot in the exact center of the primary.
2. Align the primary axis by centering the return dot on the laser collimator target.

That's it - nothing more than one simple requirement!



What not to worry about:
1. Diagonal placement (the so-called 'offset' controversy: the optical and mechanical axes may be slightly mis-aligned, but it's not a factor for the vast majority of amateurs).
2. Diagonal angle (doesn't have to be exactly 90 degrees).
3. Off-axis illumination (it's difficult to position the focuser and diagonal to cause a noticeable difference in illumination from one side of the field of view to the other side.

If you want to additionally center the off-axis illumination and make the optical axis co-incident to the optical tube assembly axis, then go to the bottom of the article.

Tools and prerequisites needed for optically aligning a f/3.0:
1. High quality focuser
2. High quality laser collimator that slides tightly into the focuser and who's laser beam is precisely aligned with the focuser axis.
3. Notebook ring on primary centered to 1 mm (1/32 inch). I use the two string method.
4. Rigid mount so that the laser dot stays centered in the notebook ring whether the scope is pointed horizontally or vertically.

The view on-axis through the focuser, showing optical alignment.


Closeup of laser centered in notebook ring that's carefully placed at the mirror's exact center.


Closeup of centered laser return.


How to center the off-axis illumination:
1. Center the laser collimator dot in the exact center of the primary (this is the same step #1 above).
2. Using a centering tool to position your eye in the exact center of the focuser (the tool might be an old film cannister or a Cheshire tool), verify that the reflection of the primary mirror is centered just inside the diagonal's rim.
3. Most likely this won't be the case. Move the diagonal by adjusting the spider's tensioning bolts and by sliding the diagonal holder back and forth in the spider hub.
4. Repeat steps #1 to 3 until satisfied.

This is what a properly centered primary mirror reflection in the diagonal looks like. Note the primary mirror rim's reflection centered just inside the diagonal's rim.


How to align the optical axis and mechanical axis (more precisely, the optical axis with the axis formed by the primary and the upper ring):
1. Stop down the upper end to the mirror diameter.
2. Follow the steps above, paying attention to the primary mirror's reflection and the stopped down upper end. They should fit concentrically just inside the diagonal.


To calculate the field illumination profile, see my diagonal calculator. For advanced telescope builders, see the analysis of diagonal offsets and why a focuser offset may be desired at my diagonal offset analysis page.


Nils Olaf Carlin's collimation page http://w1.411.telia.com/~u41105032/kolli/kolli.html
Bryan Greer's Adventures in Collimation http://www.fpi-protostar.com/bgreer/collim.htm

Mel Bartels, 2010- 2012