How to Collimate an RC – the Williamson Method

Many of you, Ritchey-Chretien owners would confirm that aligning an RC is not always the biggest fun. One of our customers. Peter Williamson provided a great and comparatively simple method to align an RC. He used it on an 8″ RC, but read on and you will see that he also added few notes for larger RCs…

“Hi Lads,

As you have all pointed out, collimating the cheaper RCs, with the focuser attached to the Primary mirror cell, ain`t easy.

You have to include a tipping plate, but even then it is an iterative process that can drive you mad.

Not now! The Williamson Method has arrived.

As you know, if you set up a light source 2x focal lengths from the primary and exactly on the optical axis, then an image will be superimposed on the source.

So, I removed the secondary mirror and primary baffle. The light source was just a pinhole in aluminium foil in front of a torch.
It`s handy to have the scope on the mount, so you can use the slow motion controls to get the reflected image spot on.

At the same time I have a laser attached to the focuser. This shines through the secondary holder hole left by removing the secondary bolt.

Initial setup will look like this.

The source is in the cross hairs, reflection image lower left and laser obvious.
Initially adjust the slow motion to bring the reflection image on top of the source, easy.
Next, adjust the tipping plate (tilting plate of the focuser), so the laser is on top of both.

Now the focuser is co-aligned with the primary optical axis.

Reattach the secondary, get laser spot central on secondary doughnut, using the primary adjustment screws.

Get the reflection off the secondary superimposed on the laser target in the focuser using the secondary adjustment screws.

Job done, replace primary baffle. Only reason for removing this is to make it easier to see the laser on the secondary doughnut. (i.e. with the baffle tube removed, you can see the secondary reflected in the primary, but if not, then you could simply place a small hand mirror down the tube to see the face of the secondary in the reflection.)

Please pass this on if you know anyone who might be interested.”

Notes for owners of larger RCs:

With the 12RC, it is a similar setup, but…

1. Use laser to centre focuser on secondary.

2. Remove secondary, mark the position first as per rotation.

3. Using the light source and screen, adjust the primary so the source, image and laser are coincident.
     This requires the source position adjusting to be coincident with the laser. Then switch the laser off to make it easier to see the image.

4. Replace the secondary. If the laser is still centred on the secondary then job done.

Courtesy of Peter Williamson.

We haven’t yet tried the above method, but once we have done will ad our own notes and recommendations.

Zoltan Trenovszki, 365Astronomy

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Posted on May 22nd, 2019.