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Collimating using a colour filter?

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#1 Chutzpah

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Posted 28 October 2005 - 09:39 AM

I was reading one of the many collimation threads on these boards and quite a few beginners it seems are defeated by the fact that they can't make out the diffraction rings properly. They either rack too far in/out of focus or don't use high enough mags (I did both for a while). That set me to thinking...

The first time I remember seeing *really* clear and distinct diffraction rings was when I defocused on Venus one early morning and was shocked by how crisp and clear the rings were when compared to the rather less distinct pattern you often get using a star at night. That would have been helped by the clear steady seeing at 0530hrs I suppose, but maybe it was also because I was seeing a pattern formed by mostly red light and nothing else?

I don't know (I've had no clear skies for while to test this) but maybe using a colour filter on a bright star would help people see the diffraction pattern more clearly and help them judge collimation more easily than broadband light that's diffracted at many wavelengths? It's just a thought.... Has anyone tried this?
:question:
Clear skies! (and send some to the UK please!)

#2 Tim2723

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Posted 28 October 2005 - 10:32 AM

My guess, and it's only a guess, would be that the filter would dim the image and make the rings harder to see. Defocusing on Venus is one thing, it's very bright, but even the brighter stars are significantly dimmer to start out.

As far as focusing too far in or out and using the right magnification, well, those are things that require practice like everything else in the hobby. My guess there would be that a newcomer would benefit more from the struggle to learn than by anything else. Sometimes we just have to pay our dues and master the task.

CS, Tim

#3 sixela

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Posted 28 October 2005 - 03:24 PM

Well, given the Airy disc size *is* a function of wavelength, yes, a filter letting through only one frequency does help get clearer patterns - and you can compensate by using a brighter star.

OTOH, if you have e.g. a UHC with a red leak passband, things can get ugly when you have two nicely separated frequency bands...


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