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Telescopes Part 4: Telescope Accessories


The magnification or power of a telescope is determined by the eyepiece that is used with it. All telescopes require an eyepiece to work. These come in different sizes, which will change the magnification that the eyepiece provides.

The term "magnification" gets used a lot with telescopes, but how do you actually calculate it?

The first thing to understand is the telescope's focal length. This is the distance between the object lens or primary mirror, and the point where the light focuses - unsurprisingly called the focal point of your telescope. Longer focal lengths result in higher magnifications.

To calculate the power of your eyepiece and telescope combination, you need to take the telescope's focal length and divide this by the size of the eyepiece. In other words:

Eyepiece Magnification

For example, the StarView 150EQ has a focal length of 750mm. If you wanted to use a 6mm eyepiece with it, you simply divide 750 by 6 to give a magnification of 125x.

"So, which eyepieces are best for me? Should I just get the smallest eyepiece to give the highest magnification?"

This is not necessarily the case! Higher magnifications mean that you will get a lower field of view, so you will be able to see a smaller portion of the object or group of objects, and you will start to lose sharpness and detail. As such, we recommend that each telescope has a maximum useful magnification, approximately calculated by multiplying the aperture (in millimetres) by about 2 or 2.5. The Earth's atmosphere will also limit your magnification to about 300x.


These useful accessories are attached to an eyepiece before it is inserted into a telescope and filter specific wavelengths of light. This shows different details that may not be easy to see under normal circumstances.

The most common filter is the Moon filter. The Moon reflects an incredible amount of light from the Sun, so it can be difficult to see lunar details during its larger phases and can even be uncomfortable to view for extended periods of time. The Moon filter enhances crater details and will make lunar viewing much easier on the eyes!

Other filters include coloured planetary filters. As a good rule of thumb, it is usually best to use the coloured filter that is the opposite colour of the object you are viewing, as this will give you the best contrast to see specific details. For example, Jupiter - a red-brown colour planet - will be best viewed with a blue planetary filter.

Guides and other useful accessories

While having a telescope is fantastic by itself, its use becomes very limited if you don't know what you're looking at! Australian Geographic sells a variety of books, guides and charts that are fantastic for getting the most out of your telescope. Two great products in this category are the Astronomy Australia annual book and The Night Sky Planisphere.

The Astronomy Australia book features a month-by-month breakdown of celestial events for that year, including the positions of planets and even comets. The book also includes a fantastic explanation on astronomy and using a telescope, making it a worthwhile addition to any budding astronomer!

Similarly, The Night Sky Planisphere maps out stars and constellations and their positions in the sky each night. Using this amazing guide, you can quickly and easily learn the shapes of the constellations, which will help you find just about anything in the night sky.

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