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Focusing using colour filters


Beardy Bob
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When taking a series of colour shots (e.g. R/G/B, Ha/O111/S11) is there much refocusing between wavelengths? I'd be interested to hear from people using v.expensive triplets (- to know how successfully the manufacturers have got the wavelengths all converging), through to completely uncorrected achromats (- to have a reference point of how fine an adjustment is needed to move between the wavelengths).

Edited by Beardy Bob
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You can buy sets of parfocal filters. These generally don't need refocussing provided your 'scope isn't too fast (so the light cone converges slowly, therefore focussing is less critical anyway). Although I'm not sure if you could get *all* the filters you mention in one single parfocal set.

Having said that, my LRGB filters are defintely not parfocal with my F/4 Hutech/Borg refractor

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Great question :D

I have the full Baader set of parfocal filters, L,R,G,B,Ha,OIII,SII.

In a doublet there is considerable difference in focus between blue and red, Ha and OIII. Big enough to warrant refocussing for sure. I dont yet have an expensive triplet to test though:p

In the SW MN190 there is a very slight difference in focus between blue/red requiring a tiny nudge of the focusser. I haven't tried in a plain newt yet, but there shouldn't be any shift there.

A great way to test, is to use a Bahtinov mask with a live view on screen, and rotate through the filters. This shows you instantly whether your colours are parfocal, and if not, by how much they are out, and which side of focus they fall. The movement of the central spike is a clear indicator of a shift in focus, and is very intuitive.

Olly Penrice has a meade triplet and also a Takahashi. Try and twist his arm into doing the exercise above and posting the pics for comparison :D

Cheers

Tim

Edited by Tim
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Good point Dennis, any test would have to be in a short space of time at the same temperature.

What causes the focus shift with temperature? Is it contraction/expansion of the tube itself?

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Although I have parfocal narrowband filters (Astronomik) I too have to re-focus between filter changes but I then I only have an ED doublet (SW ED 80) and I am pretty well convinced that the main culprit is the telescope, not the filters. A Bahtinov mask makes this a trivial issue for me to resolve.

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Personally, I refocus between each colour regardless of what scope I'm using. I think it's good practise to do it even if it's just to check.

From experience, the worst scope in terms of correction I've used is the ZS66. There's a noticeable change of focus point between Red & Blue on it while the best ones (ZS110, FLT98) the focus point barely moved between each colour.

Tony..

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Although I have parfocal narrowband filters (Astronomik) I too have to re-focus between filter changes but I then I only have an ED doublet (SW ED 80) and I am pretty well convinced that the main culprit is the telescope, not the filters. A Bahtinov mask makes this a trivial issue for me to resolve.

I have the same and find the same aswell...I was having focusing issues but it seems to have resolved itself, I think due to the cold - everything has tightend up a bit.

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