Which Eyepieces Do I Need?

Which Eyepieces Do I Need?

Meade Super Plossl 

Having bought a telescope the next purchase is usually some eyepieces. However, this can be an equally bewildering endeavour to buying a telescope. There are so many different kinds, sizes and the price range can be enormous. Which are the best eyepieces to purchase. This article will hopefully address the problem of which sizes to get. Future articles will look at the different types.

How Do I Know Which Size Eyepieces to Get?

First off eyepieces come in two main sizes. These are the 1.25 inch and the 2 inch models. Your telescope will usually come accepting either 1.25 inch or 2 inch diameter eyepieces. Most beginner telescopes are 1.25 inch.

Once you know which size eyepiece fits your telescope you'll want to buy a range of lens sizes. The size of the lens in general terms determines the magnification of the view you get down the telescope. The higher the number the lower the magnification. So, a 25mm lens will give a low magnification compared to a 6mm lens, for instance. 

So, Why Not Just Buy the Largest and the Smallest Lens Sizes? 

This comes down to your telescope. You need to work out what the maximum magnification your telescope can manage, without the image down the eyepiece just appearing blurry. Too much magnification for your telescope will result in you trying to find the focus point, but the image will never attain a good, crisp focus. 

To work out your telescope's maximum useful magnification you need to know the aperture of the telescope. A good rule of thumb is 50x magnification for each inch of aperture. So, a 5 inch telescope could magnify to 250x. This rule of thumb is good for perfect observing conditions. Unfortunately, the air in the sky is constantly moving and produces turbulence to the image. This turbulence is termed the "seeing". A better rule of thumb to account for the seeing conditions on most nights is to work to about 30x per inch aperture.

How Do I Calculate How Much Magnification I'm Using?

To work out how much magnification you have down the telescope you simply have to divide the focal length of the telescope by the focal length of the eyepiece. The focal length of the telescope is usually printed on it somewhere.

finding a telescope’s focal length

For example:

My 5 inch telescope has a focal length of 1500mm. So if I'm using a 25mm eyepiece my magnification is 1500 / 25 = 60x magnification.

To work out what eyepiece I would need to reach my maximum useful magnification of 30x per inch of aperture, which would be 150x for my 5 inch telescope. It is simply a matter of dividing the telescope focal length (1500mm in this case) by the maximum magnification (150x). This equals 10mm. So my highest power eyepiece should be 10mm. 

So Which Eyepieces Do I Need?

Armed with this new knowledge of which eyepiece would give you the maximum useful magnification you can purchase an eyepiece of this size in mm. The next best eyepiece would be a low power eyepiece, anything from 25 - 32 mm is usually recommended. These give a bright, wide field of view down the telescope which really helps to find space objects and see open star clusters. Then if you have enough funds an eyepiece mid way between these would really be a nice addition.

After purchasing these you might want to consider a 2x Barlow lens which would double your eyepiece range by doubling the magnification of each eyepiece you have. Alternatively, a good wide-field eyepiece like the excellent Televue 24mm Panoptic would make a worthwhile purchase. You can find a guide on Barlow lenses here and a review on the Televue 24mm Panoptic here.

Related Posts:  Equipment

Back to Beginners guide to Astronomy

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Part 1. Getting Started in Amateur Astronomy

Part 2. Binoculars or a telescope, which should I buy first?

Part 3. 5 Things you Need to Know Before Buying a Beginner's Astronomy Telescope

Part 4. Goto or Not Goto? That is the Question

Part 5. How to Set-up an Astronomy Telescope

Part 6. 8 Tips for Making Your Goto Telescope More Accurate

Part 7. 10 Easy Astronomical Objects to see From the City

Part 8. 5 Things to do on a Cloudy Night

Part 9. Which Eyepieces Do I Need?

Part 10. 10 Useful Astronomy Accessories

Part 11: How Can I See Deep Space Objects Better?

Part 12: How Can I See More Detail On Planets?

Part 13: How to Dress for Astronomical Success

Extras: Beginner Astronomy Telescopes

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