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Question about filter sizes

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I asked before if it's possible to use a 2" filter with a 1.25" EP. I've seen adapters like this one but I don't think this could be used for this purpose? Reason for asking is that I'm trying to decide what to buy and thought it would seem logical to have all 2" filters if it's possible to use them with 1.25" EP's. Is this usually what people do, or am I looking too much in to it?

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The best way is to get 2" filters if you can. If that adapter is threaded for 2" filters (many are) then that will do the trick of allowing 1.25" eyepieces to be used with a 2" filter. If you are using a SCT or refractor, you can attach the filter to the end of your 2" diagonal as an alternative method.

Edited by John
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if you buy a 2" barlow they usually have an adapter like that included.

you can indeed use 2" filters with the threaded adapters as John says, most are threaded.

depending on the length of your 'nosepiece' (i.e. the bit of the eyepiece that goes in) you may find the filter is touched by the eyepiece - be careful of this. you can get around it by using a 14mm Baader Fine Tuning ring Baader Planetarium - Baader Hyperion Finetuning Ring screwed into the adapter first which increases the space available. prevents smashed or damaged filters in the dark if you forget!

Edited by Moonshane
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Cool, thanks for the advice. Would using one of those Fine Tuning Rings alter the magnification or focal ratio?

Only if it's used with a Hyperion eyepiece. The ring does not contain any optical elements - it's just a spacer.

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Ahh okay. I just wasn't sure if having the EP further away from the scope would affect the focus or magnification.


The fine tuning ring is screwed on to the 2" part of the 1.25" - 2" adaptor - it basically means that the thread, where you would screw a filter, is now a bit further away from the bottom of the eyepiece barrel. The eyepiece position is not affected.

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