# Telescopes

## Introduction

Commonly when folks retire, they decide to buy a telescope.

The goal of this lab is to introduce you to the important parts and components of a telescope, so when you purchase your own, you're well informed.

## Part 1: The Galileoscope

The Galileoscope was originally developed for the 2009 International Year of Astronomy - to get telescopes into as many hands as possible.

The Galileoscope is a simple, high quality telescope that can be used to see the moons of Jupiter, the rings of Saturn, the lunar highlands of the Moon.

There are 5 important parts to a telescope:

#### 1 - Its Aperture

This is the size of its "light bucket", the larger the aperture, the more light it captures, and the dimmer the object it can see.

#### 2 - Focal Length

The focal length is the distance from the telescope's lens to where it creates a focused image. The longer the focal length, the longer the telescope.

Let's say that your telescope has a focal length of 600 mm. If you put an eyepiece into your telescope with a 25 mm focal length, then the magnification will be:

**Magnification = Telescope Focal Length / Eyepiece focal length**

**M = 600mm/25mm **

**M = 24x**

Most astronomers have a set of eyepieces with different eyepiece focal lengths to get them various magnifications.

The focal length is determined by the Lens Maker's Equation:

**1/focal length = 1/object distance + 1/image distance**

Where the object distance is how far away the object that you're looking at is from the lens, and the image distance is how far from lens the image is created by the lens.

For astronomical objects, the object distance is so far that 1/object distance is very close to zero.

Then the Lens Maker's equation becomes:

**1/focal length = 1/image distance**

**Or**

**focal length = image distance**

This means that the image is created at the focal length of the lens.

So if you wanted to determine the focal length of a lens, simply:

1 - look at an object far away through the lens

2 - relax your eye

3 - Move the lens away from your eye until the image is clear

4 - Once the image is clear, measure the distance from your eye to the lens.

#### 3 - Magnification

As previously mentioned, the magnification is F / f, where big F is the focal length of the telescope, and little f is the focal length of the eyepiece you're using.

A general rule - if you measure the aperture of your telescope in inches, the maximum usable magnification before the image gets too dim is 50x the aperture in inches. So if your telescope is the Galileoscope, what is the maximum usable magnification?

#### 4 - Focal Ratio

The focal ratio is the focal length / objective lens diameter.

The bigger the number, the higher the magnification - great for observing planets and other bright objects. Planets are best observed with telescopes that have ratios of f/10.

For wide-field objects like galaxies and star clusters, you'll want f/7 or smaller.

#### 5 - Resolving Power

The resolving power of a telescope tells you the smallest angle it can see. The better the resolving power, the more you can distinguish between double stars, for example. Resolving power is simply:

RP = 116/(Aperture in mm) [arc seconds]

As you can see, you simply divide by the aperture, and the answer is in units of arc seconds. This tells you the smallest angle you can see in units of arc seconds. What is the resolving power of the Galileoscope? Be sure to convert your aperture to mm!

#### Galileoscope Specifications

http://galileoscope.org/specifications/

## Part 2: Ray Diagrams

Ray diagrams are schematic drawings that shows the path light takes through a telescope.

It helps people understand how a telescope works, and can be used to calculate the numbers we talked about in the first part.

For this part of the lab, try to look up the corresponding ray diagrams for the telescope types listed in the lab manual.

## Part 3: The Real Deal

In lab we have a Meade LX200, a very nice Cassegrain reflector telescope.

https://www.bhphotovideo.com/lit_files/514162.pdf

Take a look at its properties from the specification page of the Meade website. Use this to answer information in the lab manual.