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The joys (or otherwise) of RC collimation


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Perhaps someone can help me here.

Despite the horror stories about RC Collimation on the web, I have a 203mm f/8 Ritchey-Chretien.

Being a tinkerer, I do like to get things like collimation as close as possible to perfection. Something has been puzzling me about this scope: If I put the Cheshire eyepiece in the focuser, I can adjust the secondary in the traditional way to get the centre circle dead centre. All well and good. I haven't dared fiddle with the Primary mirror yet.

However, If I then do a star test I get the results shown below (excusing the effects of turbulence and heat currents). The left-hand pair of images shows the out-of-focus star after 'perfect' collimation with the Cheshire. Collimation is clearly off. So I then adjust the secondary till I get the images on the right which suggests that collimation is better with stars nearer a circular shape.

48974551637_28e8a998e8_b.jpg

However, if I then check with the Cheshire, I get the view shown below. The centre circle has moved off-centre suggesting collimation is out, even though the star test suggests that I've improved it.

48974370081_332ab3d319_b.jpg

So, do I ignore what the Cheshire is telling me and assume that the star test is more accurate? The word on the street is that you need a Howie Glatter collimator with circular laser attachment, but these aren't made any more so that's hopless advice!

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58 minutes ago, Ben the Ignorant said:

A star test is good only when the optics are placed well relative to each other. If there's a misalignment somewhere, it's not in the optics.

Note sure I really understand what you're saying here.

What I'm querying is what should I read into the conflicting results of the Cheshire collimation vs. the star test, and what I need to do to achieve optimum collimation if I can't trust either method?

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My first thought (Probably wrong) would be to check the alignment of the focuser, as the two mirrors could be in alignment, but if the focuser is skewed that would throw everything else out.

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In my mind I would get the mechanical axis using a laser pointer in the focuser down the throat of the secondary support. 

Then align the secondary to be concentric. Which should show in the Cheshire. 

Then align the primary on a star for concentric star rings . If you move the primary a lot, re do the secondary. Repeat until happy. 

A test for alignment is that as you through focus, the stellar central point will stay perfectly centred. 

How have you managed to control the primary being physically central ?

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What you don't show/mention is the quality of the in-focus image; if that's good after adjusting with the star test then why worry about the Cheshire?

Personally I would use mechanical alignment as a starting point and always fine tune collimation with star testing when possible; with an RC scope you can follow the procedure below for adjusting your primary and secondary with your imaging camera attached to get very accurate collimation. Since it also relies on images of (slightly) de-focused stars you don't need amazing seeing to get a good result.

https://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf

Paul

 

 

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