On Wed, 28 Jul 2004 23:48:05 -0500, Ronn!Blankenship <[EMAIL PROTECTED]> wrote: > At 11:04 PM 7/28/04, Gary Nunn wrote: > > >Option #1 > >diameter =114mm, 1000mm focal length, f/8.8 > >5x24 viewfinder with bracket; three eyepieces (.965") - > >H25mm, H12.5mm and SR4mm for magnifications of 40X, 80X, > >250X; 3X Barlow lens > > > > > >Option #2 > >diameter =70mm, F=350mm, f/5 > >Includes two eyepieces (1.25") - H25mm and MA9mm for > >magnifications of 14X and 39X. > > > Assuming that the optics on both scopes are good (you really need someone > who knows telescopes to look through them at real stars at night, as well > as reading the manufacturer's claimed test results), get #1, replace the > eyepiece holder with a real 1.25" or better 1.25"/2.00" focuser, get some > good eyepieces (if you absolutely have to, get an .965" to 1.25" adapter > and use the ones that come with it until you can get some real > eyepieces). I'd also probably add a larger finder to find fainter > objects. Get a full-aperture solar filter, too, to look at the Sun. A > place to start looking for these items is <<www.telescope.com>>: I've had > good luck with them. If you need more help, e-mail me privately (unless > there's interest in keeping it on-list) and I'll make suggestions. > > All other things equal, always go for the biggest aperture (= > light-gathering ability) available. You can see fainter objects, brighter > objects will be even brighter, and you can see finer detail (resolution in > arc-seconds is inversely proportional to aperture). Only if it is so big > that the telescope requires you to buy a flatbed truck to move should you > consider any other factor more important than aperture . . .
Ronn, doesn't Option #2 have the bigger (wider) aperture (f/5 vs f/8.8)? (At
least in the camera world that's how it works - are telescopes different?) If
aperture is the most important factor, why recommend the telescope with
the smaller aperture?
The aperture of a telescope is the diameter of the objective: the main mirror or lens. 114 mm > 70mm. The light-gathering power is proportional to the square of the diameter, so scope #1 will gather 2.65 times as much light as scope #2, or, to put it another way, you will be able to see objects in scope #1 which are more than a whole magnitude fainter than the faintest ones you can see in scope #2.
The f-number of a telescope, as with a camera, is the ratio of the focal length of the objective to its diameter. To change the f-number of a camera lens, you use a variable aperture or iris to vary the diameter of the part of the lens you are using while the focal length of the lens remains constant. By stopping the lens down, you reduce the amount of light reaching the film or other detector. That's fine if you are taking a picture in bright sunlight. However, stars are already so faint that you want to use the full light-gathering area of the lens or mirror. So the f-number of a telescope gives the ratio of the (fixed) focal length of the objective to the full diameter of the objective, and is therefore a fixed number for any given telescope�. The magnification you get with a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece used. So as mentioned above, a 25mm eyepiece will give a magnification of 40x with scope #1 but only 14x with scope #2.
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�Okay, some big telescopes do allow you to change the optical configuration (frex Newtonian to Cassegrain) and hence perhaps change the focal length from one number to another (in that example, perhaps f/6 or f/8 to f/15 or f/20) by replacing one secondary mirror with another (it also puts the eyepiece in a different place). And there are some instances in which you want to stop down the aperture of the telescope, frex when using a large telescope to look at the Sun, particularly if you are projecting the image from the eyepiece onto a screen and not using a full-aperture solar filter. In that case, all of the Sun's radiation enters the telescope, and the big objective gathers so much radiation that when focused it heats the inside of the telescope so much that it causes convective currents in the air inside the telescope tube which ruin the seeing and may indeed damage the telescope. A full-aperture solar filter goes over the front end of the telescope and blocks over 99.9% of the radiation from ever entering the telescope, so you can safely observe the Sun with a large aperture, hence a high magnification and good resolution and see lots of detail. It is not safe to use any telescope over about 6" aperture to observe the Sun without a full-aperture filter.
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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