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

In summary, a high school student is interested in building a telescope with a 4 cm eyepiece and 5 cm objective lens to view the craters of the moon and the satellites of Jupiter. They are seeking tips for choosing the optical lenses and have considered purchasing lenses online. The recommended magnification for a telescope is the focal length of the objective divided by the focal length of the eyepiece. It is suggested to use a doublet lens to correct for chromatic aberration. Surplus Shed is recommended as a source for affordable optical components. The diameter and focal length are important factors to consider when choosing an objective lens. Links to resources for building a small refractor telescope and a Galilean-style telescope are provided. Edmund
  • #1
DrCy
2
0
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?
 
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  • #2
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.
 
  • #3
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.
 
  • #4
DrCy said:
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.
 
  • #5
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
 
Last edited by a moderator:

1. How do I choose the right optical lenses for my telescope?

Choosing the right optical lenses for your telescope depends on several factors, including the aperture of your telescope, the focal length, and the magnification you want to achieve. It's also important to consider the quality of the lenses and the type of coating they have. Consulting with a knowledgeable salesperson or doing research online can help you make an informed decision.

2. What is the difference between a refracting and reflecting telescope?

A refracting telescope uses lenses to gather and focus light, while a reflecting telescope uses mirrors. The main difference is in the way they gather and focus light, which can affect the quality of the image produced. Refracting telescopes are typically more compact and portable, while reflecting telescopes can have larger apertures and are better for viewing fainter objects.

3. How do I clean the optical lenses of my telescope?

Cleaning the optical lenses of your telescope requires a gentle touch and the right tools. Avoid using harsh chemicals or rough cloths, as this can damage the lenses. Instead, use a soft, lint-free cloth and a solution specifically designed for cleaning optical lenses. Gently wipe the lenses in a circular motion, being careful not to scratch the surface.

4. What is chromatic aberration and how does it affect the image quality?

Chromatic aberration is a type of distortion that can occur in refracting telescopes, caused by the different wavelengths of light being refracted at different angles. This can result in a blurry or colored image. To minimize chromatic aberration, lenses with special coatings can be used, or an achromatic or apochromatic lens design can be employed.

5. Can I upgrade the optical lenses of my telescope?

In most cases, the optical lenses of a telescope can be upgraded or replaced. However, it's important to make sure that the new lenses are compatible with your telescope and will provide the desired results. It's recommended to consult with a professional or do thorough research before making any upgrades to ensure the best performance from your telescope.

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