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sorry for a proper newb question but if my scope is a 130mm with a 1000mm focal length does that mean my focal ratio is f7?

just curious to know if im doing my calculation correctly.:glasses2:

edit: and if this is an f7 does that make it a slow scope?

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Hi ecosse

That's pretty much correct, focal ratio is focal length / aperture so 1000/130 = f7.7

Magnification is then focal length of scope / fl of eyepiece so a 25mm ep will give 1000/25 = x 40 mag

Hope that helps

Cheers

Stu

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Yes, nice phone, and free rather than paying for an iPhone :-)

ecosse, I guess f7.7 is neither fast nor slow but somewhere inbetween! Imaging is probably best done with f4 or 5, not that I know much about it! Should still be manageable, perhaps with a focal reducer?

My scope is f20, now that really is slow!

Stu

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Agreed - my TAL is an f8 and I consider it a great all-rounder. It's fantastic on planets, lunar and other bright objects and other people have found it to be a capable DSO imager, though you need a lot more patience and longer exposures than with a faster scope.

J.

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Magnification is then focal length of scope / fl of eyepiece so a 25mm ep will give 1000/25 = x 40 mag

How do you calculate the maximum mag the scope can handle?

Once I know that, that's 3 basic pieces of info gained :glasses2:

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Generally x50 per inch of aperture for a decent scope so possibly x200 or maybe x250 for a 5" though that may be pushing it a bit. My scope is 8" but I stick to between 150 and 250 normally

The atmosphere tends to limit things quite alot, often x250 is only achievable with very clear, stable skies. Most people prefer a smaller sharper image, maybe x150 ish.

The moon generally takes quite high mags well, whereas things like larger galaxies need low magnification to fit them in the field of view

Stu

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Generally x50 per inch of aperture for a decent scope so possibly x200 or maybe x250 for a 5" though that may be pushing it a bit. My scope is 8" but I stick to between 150 and 250 normally

Agree with this entirely. With a 6" mirror the TAL's max usable mag by this calculation is 300. I have a 6mm EP and a 2x Barlow. The 6mm gives 1200 / 6 = 200x which is pretty decent in most conditions though I still prefer lower powers quite often. With the barlow, the mag increases to 400x which I have never yet found to be usable. Incidentally, I use the barlow with other EPs; I should probably have gone for a 8mm Planetary I could barlow to 4mm (300x) but there you go :glasses2:

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