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Old 17-12-2009, 05:31 PM
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Miaplacidus (Brian)
He used to cut the grass.

Miaplacidus is offline
 
Join Date: Jul 2005
Location: Hobart
Posts: 1,235
Quote:
Originally Posted by that_guy View Post
Just read the articles, all wonderful and real helpful advice for the noobiest noob around such as my self... If I do buy a 6" dob, then what eye piece should i get with it? and how do track planets with a dob. even though its not recommended?

Thanks,
Tony
Hi Tony,
I think most new dobs nowadays come with a couple of plossl type eyepieces, often appropriately around the 25 mm and 10 mm focal length. These are usually of sufficient quality to give decent enough views. So you really should get started with them before thinking of outlaying a fortune on fancy glass. (I certainly ended up with too many eyepieces, which is a common trap for beginners, owning I'm sure at one point what seemed like one of every imaginable focal length. Since then I've systematically reduced them to, guess what, 30 mm, 10 mm, and a rarely used 5 mm.)

The magnification you will obtain will depend on the focal length of the telescope. Divide the focal length of the telescope by the focal length of the eyepiece, and that is how many times the object is magnified. (You probably already know that.)

People also talk about exit pupil, which is the size of the disk of light that comes out of the eyelens of the eyepiece. I think you can calculate this by dividing the focal ratio of the telescope (which equals focal length of the telescope / aperture) by the focal length of eyepiece (in mm). They say if an eyepiece has an exit pupil bigger than the size of your own pupil (5-7 mm, depending on whether you're old or young, have dark irises or pale), then you are wasting light. This is a technical thing which I strongly suspect was invented to bamboozle beginners and is quite unnecessary for most of us to worry about, especially early on. (Mind you, if the exit pupil is too small, it can be a bit awkward to see properly, and you're more likely to see annoying artifacts like "floaters" and other junk that float about in your eyeball.)

Higher magnification (if the sky's steadiness allows) is preferred if you want to see detail on planets, so the 10 mm eyepiece is for that. Honestly, unless you live in an exceptional climate, shorter focal length eyepieces seldom get much use.

I never meant to imply that it is not recommended to track planets with a dob. As I'm sure you know, all celestial objects will appear to move when viewed through a telescope because of the earth's rotation, and you have to compensate for this by constantly moving the telescope. This is true of all telescopes that don't have a motor drive. You just have to nudge the dob each time the planet is drifting to the edge of the field of view, but it isn't a big deal and is something that is quickly learned.

Welcome aboard, and I hope you enjoy the views.

Merry Christmas, too.

Brian.
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