Focal Ratio

[mikeyggg]

Sorry to ask what must seem like a dumb question, but I'm still confused about focal ratios.Is a higher focal ratio (eg f/10) better or worse for photography that a lower one.And does higher mean fast , or lower.I'm thinking of getting a Baader Zoom eyepiece and read somewhere that its not so good on the faster focal ratios.My six inch celestron dob is an F/8 i think, so is this fast or slow?

[John]

Short focal ratio = "fast"

Long focal ratio = "slow"

F/8 is slowish, F/10 slower still. F/6 and below is fast, lower than F/5 very fast. I'm not an imager so I can't comment on that. Faster scopes are more fussy when it comes to eyepieces.

Hope that helps a bit,

John

[Andrew*]

I reckon the Hyperion zoom will perform admirably in your f/8 (slowish) scope. It did fine (IMHO) in my f/5 scope.

Fast scopes are generally more beneficial for long-exposure photography as they capture the light "faster" if you like. In fact, an f/10 requires four times the exposure length that an f/5 scope would require. I think that's right anyway!

But as John says, fast scopes are more fussy about eyepiece quality. Right snobs.

Andrew

[mikeyggg]

Ah, that makes more sense now, thanks very much.So basically, the faster the scope, the less exposure time required but the more need for quality gear...Good to hear the Hyperion wiil work well with my dob.Think I could probably get rid of my bog standard celestron eyepieces and just have the one!.Mike..