This is a very useful site, with formula and a quick calculator for working out the resulting f ratio and combined focal length when a reducer is added into the optical train for either a refractor or a SCT. Scroll down for the reducer formulae.

http://www.wilmslowastro.com/software/formulae.htm .

The thing I found hardest was to determine the Focal length of the reducer ... I've many different numbers published for the same reducer ! Best to measure it yourself.

The formulae can be used to either work out what f number you are getting with the air gap you have (often its not possible to get the 'ideal') but the reducer will still work with a wide variation of gaps, but deliver different f Numbers. The design aim of both the Celestron and Meade (and Antares which I have) is to deliver a f6.3 result.... but the reducer WILL work and deliver other f numbers with different gaps.

Or you can vary the gap in the calculator until you get the desired f number.. thus finding out the gap you need,

My Antares has a focal length of 220 mm, but the 'ideal' 95mm gap is not achievable with my setup .. so I use 105 mm which give me f 6.0 which is fine.

I get some vignetting, but appropriate flats eliminate it (along with the shadow of my Off Axis guider.. which is what stops me achieving a 95mm gap!)

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