# Building a Telescope for Physics Class: Choosing Lenses & Mirrors

• Dgolverk
In summary, the person is looking for advice on how to build a telescope for a physics class using 3 lenses and 1 mirror. They are considering an achromatic design and are unsure of whether to use one or two lenses for the eyepiece. The speaker suggests looking at existing telescope and eyepiece designs and mentions the possibility of using a Newtonian design with an extra mirror. They also mention the variables involved in designing the telescope, such as surface shape, spacing, and glass types.

#### Dgolverk

Hi,
I need to build a telescope for my physics class.
I need to use any 3 lenses(converging or diverging) and 1 mirror.
I was thinking of using an achromatic telescope design for that.
However I am not sure what kind of lenses to use and whether I should make the eyepiece from two lenses or just one.
Should I use 3 converging lenses? 2 for eyepiece then one as an objective lens?

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There's several proven telescope and eyepiece designs out there, no need to re-invent something.

Must you only use one mirror? That rules out a lot of designs. Can your mirror be curved? that gives you an extra surface with optical power. Are you free to pick and design the lenses, or must you use lenses pre-chosen? 3 lenses plus one mirror gives 13 variables- each surface shape (7), the spacings (3) and the thicknesses (3). Glass types are another variable, but I suspect you are stuck with BK7 or the like.

I suggest you poke around and see what designs are out there.

Also, is something like a Newtonian design allowed? Besides the primary mirror, Newtonians have a second flat mirror whose sole purpose is to redirect the light out to the side. This 2nd mirror has nothing to do with the focusing properties of the telescope, so does this constitute one-too-many mirrors to qualify for this project?

## 1. How do I determine the size of the lenses and mirrors needed for my telescope?

The size of the lenses and mirrors needed for a telescope depends on the desired magnification and focal length. The magnification can be calculated by dividing the focal length of the objective lens by the focal length of the eyepiece. The focal length of the objective lens should be at least three times the focal length of the eyepiece for a clear image. Additionally, the diameter of the objective lens or mirror should be at least 10 times the diameter of the eyepiece for optimal light gathering.

## 2. What type of lenses and mirrors should I use for my telescope?

The type of lenses and mirrors used for a telescope will depend on personal preference and budget. For a basic telescope, a spherical lens or mirror can be used, but for higher quality images, an aspherical lens or parabolic mirror is recommended. It is also important to use lenses and mirrors that have a high degree of clarity and are free from imperfections.

## 3. How do I determine the focal length of my lenses and mirrors?

The focal length of a lens or mirror can be determined by measuring the distance between the center of the lens or mirror and the point at which the light converges to a single point. This is known as the focal point. It is important to note that the focal length may vary depending on the curvature of the lens or mirror.

## 4. What materials should I use for my telescope's lenses and mirrors?

The materials used for the lenses and mirrors will depend on the type of telescope being built. For a simple telescope, glass lenses and mirrors are commonly used. However, for more advanced telescopes, materials such as quartz or specialized coatings may be used to improve image quality. It is important to choose materials that are durable and have a high degree of optical clarity.

## 5. What are the key considerations when choosing lenses and mirrors for a telescope?

When choosing lenses and mirrors for a telescope, there are a few key considerations to keep in mind. These include the desired magnification, focal length, and type of lens or mirror. It is also important to consider the materials used and their level of optical clarity. Finally, budget and personal preference should also be taken into account when making a decision.