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Planetary versus DSO telescopes.


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Ok, so I'm learning here - can someone answer me what's probably a pretty stupid question please?:)

I've figured out that for planetary observing, a higher power 'scope is best and it can be a slower one with smaller aperture, whereas for DSO's I would be better off with a big wide aperture to get more light in, and lower magnification is ok.

So.... What would be wrong with getting a nice wide 'fast' telescope for DSO's, and then putting a higher mag EP on it if I wanted to observe some planets? That way I'd be covered for whatever took my fancy on the night with just the one 'scope.

I'm sure there must be a good reason, but I just don't know what it is!:)

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Julian, I'm not sure it's that simple. For the planetary scope, resolution is defined by the aperture, so the larger the aperture, the smaller the detail that can be resolved too. The problem is, there is no one scope for all jobs, they all do their own jobs well, and the others less so. The Newt appears to be the best all rounder, as you can get a pretty decent aperture, with decent focal length for far less money.

The downside of using a small focal length eyepiece, the eye relief (distance of eye from ep) and the exit pupil (the size of the image that comes out the ep) both reduce as the focal length comes down. I have a 5mm Ortho, and it's got a lovely view through it, but it's so hard to use (having eyelashes dirtying the exit glass of the ep is uncomfortable).

I think I've got that about right, but I'm sure someone will correct anything shortly.

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Nothing... Except smaller (read smaller and cheaper) eyepieces tend to have less eye relief so are less comfortable to use and if you are imaging, it's the "planetary" scopes which win out as they are easier to handle and get better images than the fast wide angled 'scope which tend to suffer from off-axis distortions. If you look in the planetary imaging section, have a look as some of the ones starman has done. He's imaging with a 14" SCT at F/67 via barlows etc. To get the same focal length with a 14" (which gives you resolution) newtonian, you'd need a massive mount to cope with the weight of the OTA and 2x the image magnification to get the focal length.

Basically, the focal length gives you image scale on the "planetary" 'scopes.

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think of the quality of the viewing as the comfort of a car, the focal length as the gearing and the eyepieces as the revs (exept a smaller eyepiece is more revs)

if you wan to go at 70 mph you can either do it at say 2500 rpm in 5th or 8000 rpm in 2nd, similarly if you want say 250x mag, you can either do it with a 2500 mm focal length and 10mm eyepiece or a 500 mm focal length and a 2mm eyepiece.

both get you to 70mph / 250X but whilst the 8000rpm solution may be fun for a short while you sure as hell wouldn't want to spend a night driving down the motorway like that.

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  • 2 years later...

There are scope designs that do deliver excellent wide views and excellent planetary views at high power:

- Large apochromats

- Maksutov-newtonians

Both are expensive designs though.

Edited by John
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