Eyepieces are one of the parts of a telescope most frequently overlooked by beginners. Hopefully you got some with your telescope, but over time you might want to expand your selection, or at least understand best way to use the eyepieces you already have.
The first thing to understand about eyepieces is the focal length and effect this has on the view, particularly the magnification. Magnification is something we touched on in our telescope guide but we need to look at it in more depth here.
Many beginners think that the more magnification you have the better, however it is not that straightforward. For some objects less magnification is actually better - star clusters or large nebulae are often more impressive when you can view the entire object. For others such as the planets and the moon you may want more magnification, allowing you to see more detail on the objects.
Regardless of what you are viewing your telescope will also affect the amount of magnification you can use. Somewhere in the manual or on the internet you will probably find a published 'maximum magnification' value. This is the absolute maximum you would be able to use in perfect conditions to see a very bright object.
These are the main factors that may limit the maximum magnification you can use with your telescope.
Any vibrations or movement in your mount will be magnified by the same amount as the image you are trying to view. The more stable your telescope and mount the higher magnification you will be able to use.
Often called 'seeing' by astronomers, the weather and air conditions in the atmosphere can affect the light entering your telescope. This is the cause of the twinkling of many stars and the 'shimmer' you may see when looking through your telescope. These disturbances are magnified when looking through your telescope so that at high magnifications your target may be coming completely blurred.
More magnification means that a smaller amount of light is being spread across a bigger relative area, which means that you can magnify brighter objects far more than dimmer ones. Less magnification may mean the objects in view are much smaller but they will be considerably brighter.
The relationship between the focal length of your telescope and the focal length of your eyepiece determine the magnification you get. Slightly counterintuitively, the shorter the focal length of an eyepiece the more it will magnify the image you are looking at.
Magnification = Telescope Focal Length / Eyepiece Focal Length
For example, if your telescope has a focal length of 1000mm and you use a (reasonably wide) 25mm eyepiece then you have a magnification of 40x. Swap out that 25mm eyepiece for a much shorter 5mm on the same telescope and you then have a magnification of 200x.
Because of this you will probably find you want a selection of eyepieces to give you a range of magnifications with which to observe. Longer eyepieces tend to give a wider field of view, more suited for larger or dimmer objects, whilst shorter eyepieces tend to give a narrower view, which is better for smaller, brighter objects.
|2mm - 10mm||High Magnication - Ideal for viewing the moon and planets.|
|10mm - 20mm||Medium Magnication - Can be used for brighter deep space objects and can also be used for the moon and wider views of planets (ie. Jupiter and it's moons).|
|20mm - 40mm||Low Magnication - Good for larger and feinter deep space objects such as nebulae. Also good for large groups and clusters of stars. Can also be used for navigating round the sky.|
Field of view refers to the amount of sky you can see when you look through the eyepiece. Generally being able to see more sky at a given magnification is better, but you will have to trade this off against quality and price. As mentioned above, shorter eyepieces tend to have a narrower field of view to longer ones. However, there can be large variations in the field of view between the different types of eyepiece. This is especially important for longer eyepieces as they will allow you to see the whole of larger objects. Seeing more of the surrounding sky also makes it easier to navigate the sky and locate the objects you are looking for as well as meaning that objects stay in view for longer without moving the scope.
Brightness and contrast are linked but not the same. Contrast is the difference between the brightest and darkest parts of the image you see, and although brightness is important without contrast you will not be able to pick out the object you are looking for. Some designs of eyepiece deal better with stray light being reflected inside and this gives better contrast as the sky will appear darker.
Quality can mean many things, but here we are generalising about the quality of the image you see. There are many aspects to this, and it really goes beyond the scope of a beginners article, but things such as the image being in focus across the whole field of view, a lack of reflections or ghost images and a lack of artifacts or false colours (known as chromatic aberation) are all things to aim for. Again, some designs are better or worse than others but the quality of the optics used to build the eyepiece will also affect this.
This is the distance your eye needs to be from the lens of the eyepiece to see the image properly. Longer eye relief means your eye can be further from the lens, which can be particularly useful if you wear glasses. On the other hand, the further your eye is from the eyepiece the more ambient light is likely to sneak in. Some eyepieces have rubber lips round the edge to help with this.
Now we have talked about some of the things you'll want to think about when choosing eyepieces let's have a look at some of the main designs. Many of these have been around for some time and some are better for particular types of telescope or observing. More expensive eyepieces often have more lenses in, but this is not always better as more lenses means more light can be sent the wrong way or lost.
These two designs use two lenses and were designed in the late 17th and 18th century respectively. They are fundamentally obsolete and if you come across them you shouldn't consider purchasing them anymore. Hopefully even the eyepieces that came with your telescope are not of this design.
These eyepieces have three lenses, and use a fancy lens called a achromatic doublet to eliminate some false colour problems. Often eyepieces using this design have the letter K in their name. They are not a terrible design but are quite outdated and are not likely to be an upgrade from eyepieces you already have. If you do want to buy some and are on a limited budget they tend to cost under £20/$30.
As suggested by the name these are a modified version of the Kellner/Achromat design. An improvement over standard Achromat eyepieces, and possibly what you got with your telescope. Again, these are not bad eyepieces but if you are buying an eyepiece other designs are likely to perform better for little extra outlay. Again they should cost under £20/$30.
Designed in Austria in 1860 these contain four lenses. These are very popular and quality can vary, but they are probably the best choice if you are looking for an upgrade from your starter eyepieces. They tend to have lower eye relief than other designs, especially at higher magnifications but offer better contrast and a large field of view at a more reasonable price.
Due to their popularity there are many variations out there and some are better than others. Make sure you check out reviews by other astronomers to ensure you get the best value and quality you can. Prices vary from similar to Kellners all the way up to the £100/$200 region, depending on size and quality.
Another four element design, these offer better image quality than Plossls for the same build quality, but a much narrower field of view. Though they have gone out of fashion recently they are good for looking at smaller and closer targets such as the moon and planets, and you may be able to find them at a bargain price due to their reduced popularity.
There are a number of other designs. Some, such as the Nagler, are very high end and expensive. Others such as the Erfle are less exclusive but still tend to cost more than many beginners would want to spend. The resources we list at the end of the page have more information on the many other designs we don't cover here as you may want to consider them in the future.
A Barlow Lens is an adaptor you can attach to other eyepieces to alter their magnification, usually by a factor of 2x or 3x. This means that a 20mm eyepiece will behave similarly to a 10mm eyepiece when used with the Barlow. There are side effects and compromises in using them, but for a beginner they can double the range of magnifications you have available for the cost of a single eyepiece.
Recommending specific eyepieces is difficult as so much of it will depend on your needs and telescope. Telescopes with a longer focal ratio (the relationship between the focal length and the aperture) will tend to handle lower quality eyepieces better than a telescope with a shorter focal ratio, and planetery observing will need different choices to observing deep space objects.
As a rough guide, if you want a reasonably priced upgrade from the lenses you have then consider Plossl eyepieces. As for the focal length, for a first choice you probably do not want to duplicate a size you already have. We would suggest considering a good quality 32mm Plossl, which will be an excellent wide angle eyepiece and will probably give a much clearer and wider view than the 25mm or 20mm eyepiece that came with your telescope.
It is also worth picking up a reasonable Barlow lens to use as this will double the range of magnifications you have available.
There are two main sizes of eyepiece in use today. 1.25" and 2" are the standards. For a beginner there is not a great deal to choose between them, so check what your telescope needs. Some can take both sizes with the use of an adaptor. In short, 2" eyepieces may offer a larger field of view but are likely to cost significantly more than an equivilent 1.25" eyepiece. This wider field of view may also be wasted as some telescopes are not large enough to fully utilise the larger eyepiece.
If you have a choice and you're not sure we'd say that at this stage you should choose a 1.25" eyepiece over the 2".
Most lenses and optics used in telescopes and eyepieces will have some form of chemical coating on the glass. These have a number of purposes including protection and reducing stray reflections. Depending on the design and price of the eyepiece these will be used on some or all of the glass surfaces. Generally coated optics are better, but at this stage you can rely on the reviews and opinions of more experienced astronomers to guide you.
There are a lot of brand names out there. Some are better than others and many are manufactured by the same companies. Check out the reviews of the individual eyepieces you are considering to see how it performs. Common brands you might want to consider include Celestron, Skywatcher, Revelation, Baader and Meade.
There is an extremely good and detailed guide here by Robin Wilkey. He goes into much more detail about the design of eyepieces and recommends some particular models.
Wikipedia also has a guide to different types of eyepiece.
This page goes into depth on the maths behind choosing an eyepiece.
A more indepth guide to many of the topics discussed above.