[Blue Skies (Jacquie)]

Peter, I think you have it back to front.

Put in the Cheshire.

Look at the secondary and adjust the tilt so you can see all three primary clips and make them as even as possible around the edge.

Now adjust the primary mirror screws so that the ring on the primary matches with the bright donut of the cheshire. This should be it. Stop here.

When you put in the little cap you are being shown the effect of the secondary mirror needing to be slightly offset - BUT DON'T TRY TO COLLIMATE IT AGAIN with that cap. There is nothing wrong per se, it is something that happens (or is needed) with the modern fast focal length telescopes, called offset.

Here's an excerpt from a little something I prepared years ago:

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So what is offset? What we find is that the physical centre of the secondary mirror does not sit in the centre of the optical axis of the telescope in a correctly collimated Newtonian. This is a tricky concept to come to grips with, but trust that it is well proven that this is so! It may help to draw the light cone from the primary mirror out on paper if you would like to nut out how it happens. To place the secondary correctly we need to move it in two directions; away from the focuser and towards the primary mirror. In long focal length telescopes (as was common in the past) this offset distance was minute and you really didn't have to worry about it. But since most of us now have short focal lengths and bigger telescopes it's a concept that is worth at least being familiar with, even if you don't make the necessary adjustments in your personal instrument.

People who use a sight tube and Cheshire eyepiece to collimate solve half of this problem automatically without even realising it. They get the correct offset towards the primary mirror using a sight tube. After using the Cheshire you should notice that the reflection of the centre dot from the primary mirror lies towards the front of the telescope in the secondary reflection. Now, since most books say you must have everything centered nicely, including reflections, you might think they've done something wrong. Well, you haven't! As long as the centre dot from the Primary matches the centre ring of the Cheshire you have done the job correctly.

Please note: If you are purely using a sight tube and Cheshire to collimate, there is no reason to dot your secondary mirror at all. As we've already mentioned, the sight tube automatically gives you offset towards the primary without you even realising it.

.... it needs to be emphasised that your telescope can still be collimated even if you don’t apply any offset to the secondary. So don't panic. Continue to keep your spider mounted symmetrically in the telescope tube if you want. The telescope will still work well without offset being applied.
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So there you have it. Remember secondary first, primary second and don't panic when you see the ring slightly off centre once you finish with the cheshire.