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shanmac

focal ratio/field of view

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Something thats bugging me on choosing my first serious telescope is the focal ratio/field of view aspect.......

Im starting to get my head around it but could someone explain!

What would be the difference be say when looking at the moon with an F/5 scope compared to an F/12 ?

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None at the same magnification in the centre of the view.

But if they both have a 2" focuser, the maximum field of view you could obtain using e.g. a 30mm 82° eyepiece is a lot larger in the f/5 scope. And, of course, if it's a large scope like a 300mm aperture scope, the f/5 scope is a practical one and the f/12 scope is impractical unless it's a scope with folded light path.

The drawback is that you'll need much more expensive wide field eyepieces in the f/5 scope, that there will be more off-axis aberrations, and that you'll need shorter eyepieces or a barlow to get to higher magnifications. It's also going to be harder to focus unless you have a dual speed focuser.

Edited by sixela

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Generally speaking, you'll get better contrast in a "slower" scope like a F12

Also, nearly all classic eyepiece designs struggle to cope properly at F5 and show distortions near the edge of the view.

Hope that helps.

Edited by great_bear

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Using the same eyepiece on say an F/5 Newtonian dobsoinian scope would give you a wider field of view (i.e you would fit more into the eyepiece) than say an F12 Meade LX200 maksutov cassegrain. The magnification for any given eyepiece would also be less in a F5 than in a F12.

In general long focal lengths which are capable of large magnification are considered excellent for lunar and planetary work as well as small objects like the whirlpool or pinwheel galaxiy. F5 scopes would be good for wide field work such as large nebulas etc. In my lx200 I would normally need to fit a focal reducer to bring it down to F6.3 or F3.3 from F10 to view the pleides but leave at F10 to view Jupiter or Saturn.

So the moon should appear larger in a F12 than a F5 because the magnifition would be higher using the same eyepiece.

Paul

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Generally speaking, you'll get better contrast in a "slower" scope like a F12

Certainly not in general. What conditions were you thinking of? It's certainly true if they are both achromatic refractors, but not necessarily if you compare scopes with different designs, and an f/5 scope is likely be of a different design compared to an f/12 of the same aperture.

Also, nearly all classic eyepiece designs struggle to cope properly at F5 and show distortions near the edge of the view.

If they're wide field eyepieces. For good Plössls and orthos, f/5 is still relatively benign.

Edited by sixela

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One thing to add is that an F12 scope will be a lot more forgiving on mediocre or cheaper eyepieces than an F4 or F5.

In my F10 I can use 2" moonfish eyepieces that are otherwise poor in my F4 scope.

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So the moon should appear larger in a F12 than a F5 because the magnifition would be higher using the same eyepiece.

Paul

But you usually wouldn't be using the same eyepiece.

You usually switch eyepieces to frame the object well or to tune the magnification to detect objects optimally or match the seeing, so you'd tend to compensate dot the different f/ratio by picking another eyepiece or by using a barlow.

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OK ,thank you all for your help...Im getting my head around it now!

Think ile maybe go for the skymax 127 ... F/11.8

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As I think I've said before, if (in a hypothetical world) I was only allowed one scope, the Sky-Watcher Skymax 127 would be it :)

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The f number does not come directly into magnification.

Magnification = (Telescope Focal Length) / (Eyepiece Focal Length)

So a 10" f/5 scope gives the same magnification as a 5" f/10 scope. Same magnification, different f number.

Field of view is the eyepiece field of view divided by the magnification, so:

FoV = (EP Field of View) / (Magnification)

So your field of view is inversely proportional to magnification, it gets less as you go for magnification, and varies directly by the type of eyepiece. Standard plossls have 50-52 degres field of view usually, WA's are around the 60-70 degree field of view.

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The f number does not come directly into magnification.

Well it kinda does, really...

Stick a camera on the back of two scopes identical in all but F-number and you'll see magnification differences in the images. You can't get a more direct relationship than that...

In the same way, if you've got two scopes effectively identical in all but F number (for example Sky-Watcher 150PL vs. Sky-Watcher 150P), the scope with the higher F-number will produce greater magnification with any specific eyepiece compared to the other scope.

Similarly, the minimum magnification that the scope is capable of, is directly related to the F-number.

Edited by great_bear

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