# Optics: Focal length of Koenig eyepiece

• xspook
In summary, the Koenig eyepiece has a focal length of 100mm and is constructed using three lenses. The first lens is made of 755276 glass, 10mm thick on axis, with a left hand surface radius of -225mm and a right hand surface radius of 83.6mm. The second lens shares its left hand surface with the first lens and is made of 623569 glass, 50mm thick on axis, with a right hand surface radius of -102mm. The third lens, made of 607567 glass, is located 0.7mm from the right hand surface of the second lens and has a thickness of 33.3mm on axis, with a left hand
xspook

## Homework Statement

A Koenig eyepiece with a focal length of 100mm is constructed in the following way:
The first lens is made of 755276 glass which is 10 mm thick on axis. The radius of curvature of its left hand surface is -225mm, the radius of curvature of it right hand surface is 83.6 mm. The eyepiece is 130mm in diameter.

The second lens shares its left hand surface with the right hand first lens.
It has an axial thickness of 50 mm, and the radius of curvature of its right hand surface is (-102mm). The glass in the second lens is 623569.

The face of the third lens is located 0.7 mm from the right hand surface of the second lens. The gap is filled with air.

The third lens is 33.3 mm thick on axis. The radius of curvature of its left hand surface is 110 mm, the radius of curvature of its right hand surface is -458mm, and it is made of 607567 glass.

## Homework Equations

$\frac{1}{f}$=($n_{1}$-1)[$\frac{1}{R_{1}}$-$\frac{1}{R_{2}}$+$\frac{(n_{1}-1)d_{1}}{n_{1}R_{1}R_{2}}$]

## The Attempt at a Solution

It is stated the eyepiece has a focal length of 100mm, so I assumed that using the equation above and plugging in numbers that I would get the focal length given to us. However I am unable to obtain 100mm. I tried to obtain a focal length for each lens and add them together, but I think the first and second lens need a different equation (since it has glass on one side and air on the other).

Attached is the system that I created from the given information

This is the third question on our homework, so the writing on it is from the first two questions.

#### Attachments

• unnamed.jpg
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## What is the focal length of a Koenig eyepiece?

The focal length of a Koenig eyepiece is typically around 25mm, although it can vary slightly depending on the specific design. This is the distance between the eyepiece and the point where light rays converge to form a clear image.

## How does the focal length of a Koenig eyepiece affect magnification?

The focal length of a Koenig eyepiece is directly related to its magnification power. The shorter the focal length, the higher the magnification. For example, a 10mm Koenig eyepiece will provide a higher magnification than a 25mm Koenig eyepiece.

## Can the focal length of a Koenig eyepiece be changed?

No, the focal length of a Koenig eyepiece is determined during the manufacturing process and cannot be changed. However, you can achieve different magnifications by pairing the eyepiece with different telescope focal lengths.

## What is the purpose of the focal length in a Koenig eyepiece?

The focal length in a Koenig eyepiece is an important factor in determining the magnification and overall performance of the eyepiece. It helps to focus the light from the telescope and produce a clear, magnified image for the viewer.

## What other factors should be considered when choosing a Koenig eyepiece?

In addition to the focal length, other important factors to consider when choosing a Koenig eyepiece include the eyepiece's field of view, eye relief, and overall quality of construction. It is also important to consider the specific needs of your telescope and the type of observation you will be doing.

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