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Collimators


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4 minutes ago, Andy67 said:

"I'm looking to buy my first collimator and I'm not sure which would be best, a laser or a Cheshire?  What are the pros and cons of each? I have an XT8."

Hi Andy,

XT8. According to the internet that's a 20cm Dobsonian.

In that case I would go for the laser collimator.

Hope that helps,

 Regards,

Chaxastro

"Humour is reason gone mad" Grouch Marx

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As an owner of an 8” Dobsonian I would without a doubt go for the Cheshire. No nonsense, easy to use and most importantly extremely accurate. Once collimation becomes second nature to you, then you can add a laser for a quick check of your primary before viewing.

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Whenever the subject of collimation comes up, there's always the point made that a laser collimator might need collimating.  :)

I've never used a Cheshire type, although I can see the principle, but is there a possibility that a Cheshire might not be aligned correctly?

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I think a laser is a lot more intuitive to use for aligning the secondary-primary axis since you can see it from the back of the telescope. Or if its very off you can just look in the tube and see where the spot is in the primary.

I use a TS concenter eyepiece to center the secondary mirror straight under the focuser, then use a laser to align the 2 mirrors. Then the concenter goes back in to check how the secondary moved during the laser adjustments and re-center. Rince and repeat until its good. Once its well aligned, either the concenter or a laser can be used to check collimation very quickly. I prefer the laser for checking since its easy to do in the dark. I personally think the concenter eyepiece does everything a cheshire does but much, much better. If you plan on imaging the secondary being exactly centered below the focuser will become a critical issue, where as in visual use maybe not so much.

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5 hours ago, Capt Slog said:

Whenever the subject of collimation comes up, there's always the point made that a laser collimator might need collimating.  :)

I've never used a Cheshire type, although I can see the principle, but is there a possibility that a Cheshire might not be aligned correctly?

No I have never heard of a Cheshire collimator being misaligned. 

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37 minutes ago, wookie1965 said:

I can see the principle, but is there a possibility that a Cheshire might not be aligned correctly?

No, it’s a metal tube with a cross on the bottom. There’s nothing to collimate in a Cheshire 

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14 hours ago, bosun21 said:

No, it’s a metal tube with a cross on the bottom. There’s nothing to collimate in a Cheshire 

There's a "mirror"  (piece of glass, I know)  at 45 degrees, too.   If this mirror wasn't at 45 degrees, or at 45 but 'off' in the other axis, would the collimator still be accurate and working correctly?

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21 minutes ago, Capt Slog said:

There's a "mirror"  (piece of glass, I know)  at 45 degrees, too.   If this mirror wasn't at 45 degrees, or at 45 but 'off' in the other axis, would the collimator still be accurate and working correctly?

It’s not a mirror it’s a piece of polished metal and is securely fixed in place and can’t move 

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It isn't just that laser collimators may not point to dead centre, even if they are perfectly super collimated themselves, the focuser has to be perfectly aligned and centred and able to hold the collimator centred, and the secondary must be absolutely square to the optical axis, and in practice is is quite easy with a laser to get an apparently perfect result when in actual fact there is considerable misalignment or twist happening. A star test afterwards will demonstrate this, but doesn't really help solve the issue.

That doesn't happen with a chesire so easily, and even if the focuser, secondary and primary are all a little offset within the tube itself, you can still get perfect collimation.

For a much more detailed discussion of the above, check out New Perspectives on Newtonian Collimation by Vic Menard, it includes ray diagams to illustrate what can happen. It also recommends chesire eyepieces, specifically the Cats Eye system.

With practice just looking at a slightly defocused star will tell you if something is amiss, and good collimation makes the biggest difference when observing planets.

HTH

Tim

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I don’t think the angle of the shiny surface has any bearing here. It’s only job is to let light into what would otherwise be a dark tube. The precision of the sight tube, pupil and cross hairs certainly do make a difference. That said I am happy with the quality of the Celestron Cheshire eyepiece and sight tube combination tool.

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