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I am looking for a high F ration scope to get some image scale from Saturn, my current scope is a 200mm F5 reflector. I know Seben have a really bad name but is that for all of there scopes of just the 'big boss' types? anyways here is the link http://cgi.ebay.co.uk/1200-90-New-Seben-Comet-Telescope-Spotting-Scope_W0QQitemZ260071447131QQihZ016QQcategoryZ3636QQrdZ1QQcmdZViewItem

whats everybody's view on it?

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Funny you should ask, I have one of these which I use for my other hobby of plane spotting :D to be honest its ok for looking across a well lit airfield and for a nice view of the moon, but not really for anything else. It all feels ok to handle, focusing is a little hit and miss, but for the money it makes a reasonable spotting scope. Now to be really honest, I dont spot with it anymore, i went back to using binoculars.

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You could get a really good 2x barlow for that money (Tele Vue) - that would give you a 2000mm equivilent focal length and retain the image detail and light grasp of the 8 inch scope.


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if you want f/30 then just double stack your barlows. Use your 3x and add a 2x as well. With quality barlows you wont loose much light and your main problem will be keeping the image on the chip!

I thnk this is the wrong way of looking at a Barlow. It doesn't change the cone of light from the mirror, except at the focal plane. The scope is still f/5, even though the magnification it is giving you is equivalent to and f/10 scope and that particular ep, it is the effective focal length of the ep that is changed, not the effective focal length of the scope.

The magnification achieved with an f.5 scope and a 10mm ep and a 2x Barlow may be equivalent to that 10mm ep with an f/10 scope, but it is also equivalent to that f/5 scope with a 5mm ep, and I think this is the more valid comparison, particularly as far as contrast and clarity are concerned.

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The f/ number thing again! Why oh why do astronomers do it this way?

The image scale comes from the focal length, end of. That's why photographers measure telephoto lenses by focal length all the time. You can get an 18mm f/4 wideangle lens or a 200mm f/4 telephoto lens and they're no way the same piece of kit or used for the same thing. You need super close in hyper telephoto for planets, so go for focal length when doing the sums.

That means that an 200mm 'scope at f/5 will have a focal length of 1000mm (200 X 5)

Lets try the frac that I have, 120mm f/8.3, guess what? 1000mm.

Both these 'scopes have 1000mm focal length and so give the same image scale.

If I go for f/30 by adding Barlow lenses I get a focal length of 6000mm for the Newt and 3600mm for the Frac. But the image scale is twice as big (ish) with the Newt. because the focal length is almost twice as long.

Again working out the f/30 thing for an ED80, you would only get 2400mm focal length.

For Saturn, anything over 8000mm focal length and the planet will barely fit on the chip, so f/30 is good for the Newt., for my Frac, I'd need f/50, and for the ED80, f/75

6000mm is about the maximum I'd go for as getting the critter on the chip is the hard bit and keeping it there is the harder bit.

Bottom line is that you don't need as high an f/ number if you have enough aperture, you get the required focal length at lower f/ numbers.


I know its confusing, but getting a bigger aperture really has all sorts of advantages as you can get close in without needing huge f/ numbers and that means more light on the chip, brighter image on the screen = YAY!

Captain Chaos

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Yes, CC, but to reduce my argument to a nutshell, the Barlow doesn't alter the focal length or f/number of the scope, it reduces the effective focal length of the eyepiece. If you want to alter the f/number of the scope, put an aperture mask on it! If I pop the cap off the aperture mask on my Newt, I've suddenly got a 50mm scope at f/15, with no obstruction.

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Eyepiece? What's one of them for? :D

Don't use 'em myself usually, in fact the camera stays in the focuser all the time, that's why I don't relate to them.

If I get the theory, which I suspect I might not, then the eyepiece is used to inspect an aerial image produced by the rest of the optics. The aerial image is produced downstream of the Barlow lens and (obviously) upstream of the eyepiece. So if this is the case, the aerial image must change because the eyepiece is merely used to view it. With my setup the camera sensor is placed where the aerial image forms at the point of focus of the mirror (or lens in the case of a refractor).

On thin ice now,

The Barlow lens intercepts the cone of light before it comes to a point and slows down the rate at which it comes together so that the light cone has a change in slope and the cone narrows more slowly thus making the point of the cone further out towards the viewer so the focus point is further out. A focal reducer does the opposite, blunting the point of the cone and moving the point of the cone away from the viewer and needing more in focus travel. I think.

Captain Chaos

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  • 3 weeks later...

Camera.... wot one of those?

Scope f/ratio=Scope focal length/scope aperture

Magnification=scope focallength/eyepiece focal length

Field of view=eyepiece effective field of view (e.g. 82degrees for a Nagler)/magnification.

For high power planetary observation, see what magnification other people use... work out what eyepiece focal length that means for you and if the image is too dim them buy a larger (aperture) scope. If you need a very short focal length eyepiece and don't fancy losing the eye relief then go for a barlow (never used one myself as my scopes do not track and so I do not use crazy magnifications).



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