How Do I Choose the Right Optical Lenses for Building a Telescope?

[DrCy]

Hi. I am new to PF. I am an incoming high schooler with an interest in astrophysics, and a friend and myself are working on building a telescope (not a very sophisticated one, just one that can reveal the craters of the moon and the satellites of Jupiter). I am curious to know if any amateur (or not-so amateur) astronomers have any tips for optical lenses of the telescope. The eyepiece for the telescope will be 4 cm, and the objective lens will be 5 cm. I have considered going online to order eyeglasses, though most sites do not sell the circular, uncut glass without a frame. If I could purchase such a lens, I would not know how to choose a prescription. I do not know how one chooses the desired level of magnification. Does anyone on here have any advice on optical lenses?

 

[Drakkith]

The magnification of a telescope is the focal length of the objective divided by the focal length of the eyepiece. For example, your 40mm eyepiece has a focal length of 40 mm. (You were talking about the focal length when saying it was 4 cm weren't you?) If your objective has a focal length of 400 mm then the magnification is 10x.

A refractor telescope with one lens is going to have massive amounts of chromatic aberration, which just means that the different colors focus at different distances from the lens. This results in a very colorful, but blurry image. Practically all refractors use a minimum of two lenses, which is referred to as a doublet. You MIGHT be able to get by with a single lens if you make your focal length very very long.

If you visit Surplus Shed you can find used optical components for very very cheap. Here's a link to the achromats section: http://www.surplusshed.com/pages/category/achromats_1.html (Achromat just means that they correct for the chromatic aberration by making the red and blue ends of the spectrum come to focus at the same spot. This greatly improves the quality of the image.)

You should be able to find a small objective that should work fine for a very good price. Your main factors for choosing an objective should be your diameter and focal length. The longer the focal length the more magnification you will get from your eyepiece, and the less the aberrations will be. But too long of a focal length will result in a very very long telescope that is unwieldy and has more magnification than your objective can handle, and images will be blurry.

 

[DrCy]

Thank you very much! When I said 4 cm and 5 cm, I was referring to the diameter, but I am pretty sure I know what to from here anyway.

 

[Drakkith]

Thank you very much! When I said 4 cm and 5 cm, I was referring to the diameter, but I am pretty sure I know what to from here anyway.

That's a pretty big eyepiece, it's nearly as big as your objective! But then again, I've never measured the diameter of my eyepieces, so maybe it's not so big lol.

 

[Bobbywhy]

DrCy, Welcome to Physics Forums. Here we have professional astronomers, amateurs, and those with tons of experience in astronomy and astrophysics. All are ready and willing to assist you and your friend on your way to new knowledge.

This article will show you the complete process for making a small refractor telescope.
http://www.stormthecastle.com/how-to-make-a/how-to-make-a-small-telescope.htm

Here's another example of how to build a Galilean-style refractor:
"We, the first ever astronomy group of History 333, were given the task of recreating Galileo's observations using replicas of the Galilean-style telescope. On a limited budget, our group had to design and build a set of these telescopes. This link discusses the design that was created by our group."
http://galileo.rice.edu/lib/student_work/astronomy95/telescope_design.html

For the source of lenses, this is my favorite supplier: Edmund Optical. Be sure to check out their application notes.
http://www.edmundoptics.com/
http://www.edmundoptics.com/optics/optical-lenses/
http://www.edmundoptics.com/learning-and-support/technical/learning-center/application-notes/

Let us know about your observing progress!
Cheers,
Bobbywhy