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jamieren

Newtonian collimation ..again.

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I was helping a friend collimate his 10" celestron and we were having a real tough time getting it to respond. After going through all the steps a couple of times we noticed that the set screws for the primary mirror would bottom out and still not quite reach the backing plate. After thinking about this we first tried putting shims so the screws would make contact, but the view it gave was "ghosted", there would be a main image with a couple of more transparent images flanking it. I was ready to give up at that point when he suggested we back the set screws way off and tighten the adjustment screws a couple of turns, causing the mirror to come closer to the backing plate, and 'extending' the focal length. After doing so and re - collimating the view was far sharper ( it was dusk and cloudy so all we could test it on was a tree).

Here's the question: why would a difference of 5 mm in the focal length of the scope cause such a major malfunction? -Keep in mind we still have to star test it, but from what I saw it was as good as I've seen in my scope. Anyone have an explanation?

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Uh, guys? None of you tech savvy telescopers have the explanation for this? Where's Rus on this one?!

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I'm no expert but it's my uinderstanding that when the primary and secondary mirrors are made they are grinded in such as way that they make a certain focal ratio which is turned into a focal llength determined by where the focuser is positioned.

In your case you have you have moved the primary further towards the secondry hence the focal lengths been changed and the collimation is out. As you found out you have to start again by moving back the primmary to it's original position.

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I've no experience of this but I've had a few thoughts. May all be rubbish but here we go.

If it is a classic Newtonian you have a parabolic primary and small flat secondary at 45 deg. Moving the primary along the optical axis will over or under fill the secondary with the primary 'cone'. The optimum will be when the cone just fits the secondary. If the primary is too close it will overfill, and some of that overfill may reflect off some other structure to give multiple images. Moving the primary back corrects this.

Tilt of the primary would distort the images, and may cause other effects from structure. Rotation or tilt of the secondary may also cause refelections in the eyepiece tube section. Moving the primary back would reduce the cone size prior to the eyepiece and may reduce this.

If it is a Newtonian with some form of corrector plate at the entrance aperture of the scope, then the exact separation of the corrector plate - primary may be very important. Likewise if the secondary is not flat but another curved mirror.

End of musing.

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I much appreciate the opinions. A star test soon will give the final say on whether the problem is truly fixed.

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The focal length is fixed and cannot be changed. Moving the mirror towards or away from the secondary will not affect it - all it does is change the focus position at the eyepiece. Secondaries are usually slightly oversized to ensure the full slice of the cone of light hits it and is the reason why you can see the whole primary concentric and within the outline of the secondary when using a colli-cap or cheshire eyepiece in the drawtube. If the outside diameter or edge of the cone of light misses the secondary ie. you can't see the primary edge (and mirror clips) within the secondary outline, you're effectively reducing the aperture of the 'scope. This is also why the outline of the secondary has to be concentric within the bottom edge diameter of the focuser drawtube to ensure full-field illumination - one of the first steps in collimation.

I think it's simply the mechanics of the cell. You're supposed to have the primary mirror adjustments fully tightened so the springs are fully compressed and back them off a turn or so before starting collimation. Fully back off the lock screws before doing this.

This ensures the springs have enough compression on them to prevent the primary from slopping around in the cell and losing collimation. Once collimated, re-tighten the lock screws.

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I agree with Astronut and found in no time at all, the best way to start your collimation, is to screw down the primary as far as it will go and then back off all 3 screws by 1 turn. Then (when it comes to it) collimate the primary by adjusting just 2 of the screws.

Moving the primary down the tube has the secondary benefit that the cone of light will easily fit inside the area of the secondary.Align the tilt to be square onto your focuser tube and then you can start aligning the secondary in the normal manner...

Works well, every time.

Good luck !

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