# Question on Refractor Telescope !

by willingtolearn
Tags: refractor, telescope
 P: 62 I am plan on building a small refractor telescope about 20 cm long, i need your guys help ! 1. What len is use for Objective len and what len is use for Eyepiece len ? (how is it look like) 2. How far should i place objective len and eyepiece len apart ? 3. If i make a 20 cm long refractor telescope, what is the diameter should the objective len and eyepiece len should be ? Thanks
 Sci Advisor HW Helper P: 8,953 The simplest refractor is a keplerian telescope (http://en.wikipedia.org/wiki/Optical_telescope) It just needs 2 convex lenses about 1" diamter for the objective and 1/2" for the eyepiece, the wiki link explains how to position them.
P: 62
 Quote by mgb_phys The simplest refractor is a keplerian telescope (http://en.wikipedia.org/wiki/Optical_telescope) It just needs 2 convex lenses about 1" diamter for the objective and 1/2" for the eyepiece, the wiki link explains how to position them.
Thanks ! But i couldn't find the answers for my questions from that. Help please ?

 Sci Advisor HW Helper P: 8,953 Question on Refractor Telescope ! If you want an overla tube length of 200mm you need the focal lengths of the two lenses to add to 200mm. So you need an objective lens of around 150mm focal length and roughly 25mm diamter with an eyepiece lens of 50mm focal lenght and roughly 12mm diamter. You can measure the actual focal lenghts by focusing an image of the sun onto paper and measuring the lens distance with a ruler. Then just put the two lenses in a tube separated by the sum of their focal lengths and you will have a rather crude telescope. It won't have a good image quality but it will show you what Galileo / Kepler saw when the first used one.
 P: 55 The other posts got most of the stuff pretty much. But to get the most magnification out of your telescope you want the focal point on the first lens to be the largest possible, and then the focal length on the eyepiece to be as short as possible. But making it too short causes aberrations and imperfections, so you need to find a balance between the two lenses. I just know what the mathematical stuff is but I have no idea what the practical measurements are so I would go with what the person above me said :)
P: 62
 Quote by mgb_phys If you want an overla tube length of 200mm you need the focal lengths of the two lenses to add to 200mm. So you need an objective lens of around 150mm focal length and roughly 25mm diamter with an eyepiece lens of 50mm focal lenght and roughly 12mm diamter. You can measure the actual focal lenghts by focusing an image of the sun onto paper and measuring the lens distance with a ruler. Then just put the two lenses in a tube separated by the sum of their focal lengths and you will have a rather crude telescope. It won't have a good image quality but it will show you what Galileo / Kepler saw when the first used one.
 Quote by Rib5 The other posts got most of the stuff pretty much. But to get the most magnification out of your telescope you want the focal point on the first lens to be the largest possible, and then the focal length on the eyepiece to be as short as possible. But making it too short causes aberrations and imperfections, so you need to find a balance between the two lenses. I just know what the mathematical stuff is but I have no idea what the practical measurements are so I would go with what the person above me said :)
Thanks all ! Now i'm start building it
 P: 62 And one last question, why the image is inverted ? which is up side down

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