Alignment of a Newtonian secondary mirror - my way!

October 10, 2020

Hi All,
As part of getting my gear ready, I have updated various components of my 8” Newtonian Astrograph, the main upgrade was a robotic focuser replacing the dual speed Crayford it was shipped with. This was fairly straight forward but threw up a problem with my secondary. Even with the secondary housing almost touching the spider, it was still 6mm too close to the primary.

So, out with the spider and secondary, cut 7mm off the back of the secondary mount (it was way longer than needed) extended and re-tapped the M6 mounting hole (drilled out the hole to about 5mm off the back of the mirror) Cleaned up and ready to re-fit.

Now, How to correctly re-align the secondary?

I had a good look about (you-tube and other forums) but every method I found was relying on bits of paper, plastic tie-wraps, or just ‘that’s about right’. This got me wondering, there must be a better way.

Starting with the fact that this is an optical system, there must be a way to optically align the secondary (Not primary collimation, that’s a separate task)

A laser dot collimator is of no use here, as you can have the secondary mirror twisted (round the central axis of the OTA) and still adjust the secondary to put the dot in the centre of the primary. Looks fine but is going to produce weird and misshapen images.

So in order to check the secondary ‘twist’ I’ll need a line not a dot, but to align the line, I’ll need a second line at right angles to the first line….
OK, so a laser cross module from ebay (<£4) will do the trick. The one I got was focusable so I set it at about the focal length of my scope (near enough will do), and made a little adjustable housing that will mimic a 1.25” eyepiece (I used Aluminium as I had some and it’s easy to machine), ordered a M42 to 1.25” adapter (£2 off ebay again) and using a quick and dirty jig, adjusted my new laser to be exactly 90Deg to the thread on the front of the fitting.
To do this I used the M42 camera rotator I already had so I could hold it solid and rotate the laser, when the laser cross didn’t move on the wall (5m away) while turning it 360Deg, it was spot on down the centre of my M42 fitting.

So now to put it on the OTA (at about the focal point, but an inch either way won’t matter) we get a laser cross inside the OTA directly opposite the focuser. Using the rotator, twist the laser until one of the lines points directly down the OTA and at the centre of the primary, then lock the rotator.

Now put in the secondary and you get a laser cross on the primary. Now twist the secondary and the line on the primary twists twice as much (that’s very useful, a 5Deg twist on the secondary twists the laser line 10Deg)
As my spider vanes were lined up exactly with my focuser, I simply adjusted my secondary so the cross lined up with the centre point on my primary, and the laser lines lined up with my spider.

And presto the secondary is perfectly square to the focuser, and directing the laser at the exact centre of the primary.