Hi, Just need some clarification when I try to collimate my 8" collapsable, I have a cheshire and the first step, using the secondary screws, is to center the secondary so the reflected image of the donut on the primary is centered under the fuzzy cross hairs, right?, then using the primary screws center the reflected dot of the cheshire also under the cross hairs, right?.
Now if I do the first step with cheshire then swap it with a simple collimation cap the reflection of the primary mirror clips in the secondary is off, if I re-adjust so the reflected image of the primary mirror clips are nice and even in the secondary using the cap then the reflected image of the donut is off when I use the cheshire again.
As far as I can tell the cross hairs are nicely centered in the cheshire and the donut is centered on the primary.
If some one could clear things up I'd appreciate it.
Thanks Peter.
I haven't used a Cheshire eyepiece in a little while, but what you discribe sounds like the secondary mirror might need to be adjusted with a twist to get it perpendicular to the axis of the focuser. Maybe even in the up and down direction too to align the secondary. This is what will centre the images of the primary mirror within the secondary. Then you make the adjustments with the collimation screws of the secondary and the primary mirrors.
If you are feeling REALLY adventurous, carefully remove the secondary from the spider and look at marking its centre with a permanent marker. There are a few web sites that can take you through the necessary steps. Most commercial reflectors don't have their secondary mirrors 'off set', so centre marking is fine. It really makes adds to the accuracy, and hair pulling, of collimation.
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.
Last edited by Blue Skies; 13-03-2010 at 08:40 PM.
Reason: Trying to get it to make sense
Thanks I'll have another go today, and see what happens, I'm getting headaches trying to view through the chehire and working out where the center is, those cross hairs are just fuzzy.
Thanks for the link PeterM and yes Jacquie it appears I was doing it backwards, after making sure the secondary was nicely centered up and down and looked round under the focuser, I then used my cap to get the primary mirror clips even using the secondary screws, then used the cheshire to center the donut using the primary screws and bingo everything looks lined up with both cheshire and cap.
