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Explaining eyepieces and barlows


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Hi all,

I've read through many of the primers and advice on this forum but am still having trouble grasping which eyepieces to use and when as a complete beginner.

If we assume that I am using a scope of length 1200mm and 10" aperture (so f/5), when would I use supplied eyepieces of 10mm and 25mm? Am I right in thinking that a smaller eyepiece would give more magnification and therefore be better for planets and the moon, whereas larger eyepieces would give better views of more distant objects? If so, using the example I've given what would be a decent collection of eyepiece sizes to have the biggest range of targets on any given night?

PS. What do Barlow lenses do????

As always - HUGE thanks in advance to those who reply, your expertise is truly invaluable and wholly appreciated.

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A 2x Barlow will double the focal length of the scope, or halve the focal length of the eyepiece. So a 10mm becomes a 5mm eyepiece.

If you divide the eyepiece size into the focal length you get the magnification. So for the scope you mention a 10mm ep will give 120x mag. Working on a practical maximum of 200x in UK seeing - get a range of ep's that give maybe 50x, 100x, 150x and circa 200x mag.

Bear in mind if you get just two ep's and a decent barlow that will give you effectively 4 eps. The two lower powers will want to be wide angle for larger dso's. I think you'll find the f-ratio is f4.8. This is quite fast so the eyepieces will want to be quite good quality and the scope will need accurate collimation.

This is only an example - I'm sure others will pop up with other good ideas for you ;)

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You are on the right track but a 10" F5 scope has a focal length of 1250mm. The magnification is obtained by dividing the scope focal length by the eye piece focal length, so the 25mm EP gives a magnification of 50x and the 10mm EP 125x. A Barlow increases the magnification by the value of the Barlow so a 2 times Barlow for the above will give 100x and 250x.

For a good over view of what EPs to use have a good look at Eyepieces - very least you need.

HTH

Beat me to it Brantuk, you obviously type faster than me.

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Thanks for the advice guys ;)

I'm thinking maybe to add a 6mm eyepiece and a 2x Barlow. By my (sketchy) math that would give magnifications of 200x (with the 6mm), 120x (with the 10mm), 96x (with the 25mm and the Barlow) and 48x (with the 25mm). Hopefully I've understood that right - Cyber humiliation if I haven't!

Do you think I'd be missing out on anything in particular by having such a big gap between 200x and 120x mag? I assume a 3x Barlow on the 25mm would fill that if needed?

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A 6mm with barlow equates to a 3mm eyepiece - in a 1200fl scope that's 400x magnification - conditions in the UK get that good only 2 or 3 times per year. I doubt you'd get much use out of that combo ;)

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