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# hi , im making a refractor telescope , so can i use a objectivelens with focal lenght 2000mm and eyepiece as a 10mm lens Answered

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## 10 Replies

rickharris (author)2014-03-28

Steve will really be the man to answer this BUT:

Your calculation is correct - for a reasonable image you need to have a large diameter objective. This gathers more light and allows for stopping down to reduce chromatic aberrations which will appear unless you use a complex lens system.

Do you intend this for Astro use or terresterial?

steveastrouk (author)2014-03-28

Yes, you need a "doublet" for the objective ideally.

pnag1 (author)2014-03-28

sir , could you please post a plan on how to make this telescope

steveastrouk (author)2014-03-29

CabbitCastle has provided a most excellent link. I suggest you read it.

CabbitCastle (author)2014-03-28

Generally speaking, yes.

If you wish to build a refracting telescope made of two lenses, the bigger one should have higher magnification (2000mm) and the smaller one a smaller magnification (10mm).

Nasa has a page explaining how to build such a telescope. It also states that you need a certain type of lenses, namely both have to be converging/convex.

American Museum of Natural History (you can use these instructions for a simple assembly to test your telescope)

Roger Ceragioli 's Refractor Construction Page (very detailed and skimming over it, a bit complicated)

pnag1 (author)2014-03-28

so , can it give me a magnification power of 200x

CabbitCastle (author)2014-03-28

According to this site, yes.

The formula for magnification of distand objects using a refraction telescope goes as this: Magnification = Objective lens focal length / Eyepiece lens focal length.

I do not know how the size of the lenses plays into this though, I have seen some information suggesting it factors in as well.

steveastrouk (author)2014-03-30

The resolving power of the scope (and the image brightness) is "controlled" by the diameter of the objective lens - we reckon a good telescope, in the best seeing can probably manage 50X per inch of aperture, so to get 150X from a scope, you need 3 inches minimum.

pnag1 (author)2014-03-28

i want the telescope for astronomical use , so the lenght of telescope would be 2000mm +10mm that is 2.1 metres

steveastrouk (author)2014-03-29

2.01, not 2.1 long. Much depends on what diameter the lens you have is.