Collimating a Tak CN212

I have finally managed to collimate my CN212.

Both primary and secondaries were out of collimation.

First I built a low cradle to sit the tube on. Made this from 3 pieces of agricultural ply with two semicircular cut-outs. Cushioned the tube with a strip of foam rubber over each semi-circular saddle.

I first fitted the Newtonian fittings. i.e the focuser and eye-piece holders on the side of the tube. The focuser base can be tilted. Mine was square to the base . If it hadn’t have been I would have made it so.

Next I measured the radial distance from the side of of the knurled secondary holder knob to inner wall of tube. This was along each spider arm. Used a dial type vernier callipers. This was actually spot on as far as I could measure.

The secondary mirror for the Newtonian is actually physically displaced on the backing plate by quite an appreciable distance. Away from the eyepiece. The secondary mirror offset recommended for Newtonians is away from the eye-piece / focuser and towards the primary mirror.

Positioning the secondary mirror is possible in all 3 planes.

The first one is rotating the secondary mirror so that it faces straight out the focuser tube.

The second is the distance the secondary is positioned away from the primary mirror. The 3rd. is the mirror's tilt to the scope centre line.

There is a long threaded rod (screw) attached to the approx centre of the back of the secondary. A threaded collar with a screw-driver slot pulls this up the centre of the tapered mirror mounting sleeve.

Using a Cheshire eyepiece I positioned the mirror so that the marked central dot on it was exactly under the crosshairs of the Cheshire. This takes a fair while.

To lift the secondary mirror away from the primary (as I had to do) it means loosening all 3 radial adjustment screws and then tightening the central screw. As soon as you loosen the adjustment screws the whole secondary can (and does) rotate.

I found it far simpler to remove the secondary mirror assembly and do it on the bench. I used the dial micrometer to measure the distance between the rear of the mirror holder and the steel backing plate to get it the same at all 3 adjustment screws. The Cheshire type eyepiece is invaluable with this and to a fair extent the next step also.

When I was happy with the secondary placement I moved on to the "good" part.

The primary mirror assembly is held in place in the main tube with just 3 screws. Remove it.

I found it best to remove the finder scope assembly also. This is held in very tightly with 2 metric socket head cap screws.

Now remove the hexagonal nut locking the focuser to the arm on the rear of the primary mirror.

Unscrew the front half of the central baffle tube. This is a very fine thread.

While doing this I sat the assembly in the mouth of a large poly end cap for plastic sewage type pipe. This has a thread on one end and the other is glued to the pipe. (As used to carry plans, tools etc on some service vehicles.)

You can now lift the primary mirror and it's attached sleeve off the central baffle tube and housing.

There is considerable data on the web with regard to collimating Newtonian scopes. I spent a full day adjusting. Assembling, checking etc. before I thought I had it pretty close. This was using the Cheshire eyepiece and standard Newtonian collimation methods. I then inserted the Cassegrain secondary and collimated the secondary in the standard manner.

This wasn’t quite right though as collimation changed from inside focus to outside focus. I then took some guided 300 sec images of a star field with my ST8E CCD camera. Much better than before but the stars were still not quite round especially away from the centre.

Second Attempt.

This time I used feeler gauges between the steel backing plate on the mirror and the metal plate it is held against / away from.

In the third photo you can see there are 3 quite small socket head grub screws (metric) vertical to the plane of the metal back plate. These are the adjustment screws. Slightly offset and below them are 3 more socket head grub screws screwed into the side of the metal plate. These are the 3 lock screws. The latter must be loosened before and adjustments are made with the 3 top screws. The 3 larger Phillips head screws are a bit of a mystery. I couldn’t work out what they did. I suspect they help hold the whole lot together. I loosened them for adjustments but don’t think they had any bearing on the matter.

It is possible to reach the 3 lock screws without dis-assembling the mirror/tube but impossible to use the 3 adjustment screws. Using the feeler gauges I got the two metal plates as close to parallel as I could measure.

Re-assembled, re-collimated the secondary and star tested. Still not quite right.

The Correct Method

Saw a reply to a query on one of the Yahoo Astronomy groups by Robin Casady.

This is at  www.robincasady.com/Astro/myimages/collim.html

Download this page, burn it on to CD and give a copy to your favourite grandkids.

This is definitely the way to do it.  

Make up a simple frame to hold a sheet of plywood or similar 6 feet or so high.

Drill a hole in the middle of the board width at eye-height. This hole should be 18-25mm in diameter.

If you can only arrange a shorter piece a board you can make the hole at eye-height while sitting.

I found it easier to mount the scope on my GEM for this method.

Point the scope accurately at the hole in the board. A laser collimator is handy for this.

Remove the secondary mirror.

 At about twice the focal length of the scope look towards the primary.

Looking through the hole in the board at the primary mirror make sure the 3 spider arms and the mirror 

support are all centered over their reflection while the cassegrain exit hole is centered in the secondary mirror mount aperture.

Adjust until this is right. This is purely a once off option and is only necessary for primary mis-collimation. 

Tighten all the primary mount lock screws and remount the primary mirror fully and screw down.

Now replace the secondary (cassegrain) mirror.

The distance you are in front of the scope is pretty critical to get the relative sizes or the rings right. 

Best to have the background (behind the scope) shaded or dark.

The scope must be facing out towards the light.  

I tried doing this without the board with a hole in it but found it impossible to hold steady enough.

The hole must be large enough to allow you to position yourself in exactly the right position.

That is you have to center the spider arms again exactly over their reflections then staying in that exact position

check the concentricity of the mirror reflections and the central aperture.

Any misalignment is now relatively easy to see.

I made a small "T" handled metric allen key to do the adjustment.

This is the most "sensitive" scope to collimate I have seen.

It only requires very small adjustments to move it quite a lot.

I allowed the CN212 to cool down for about an hour that night.

It is the first time in my life I have actually clearly seen "crepe rings" around a star.

I have only seen illustrations in books before. None of my SCT's ever stopped shimmering with heat enough to show crepe rings with any semblance of clarity.