Astronomers Guide To Eyepieces

08/24/2014 Facebook Twitter LinkedIn Google+ Astronomy Help Articles


Eyepieces Buying Guide

I am going to talk about Plossls, mostly, as they are the best value for money. If you get a branded Plossl, you will seldom get a piece of junk. You can expect reasonable sharpness across most of the field in all but the fastest scopes. Plossls also have a field of view of 50 – 52º, which is quite reasonable. I am also going to suggest a set of three or four eyepieces, and no Barlow,except in the case of a fast scope.

You should have a high power, a medium-high and/or medium-low power eyepiece, and a low power eyepiece. The eyepieces that came with your scope probably fill the medium-high and low power slot. If they are satisfactory, keep them for now. If they are marked ‘H’ or ‘SR’ don’t even think about keeping them! If they are marked with a ‘K’, they are Kellners, which are generally acceptable eyepieces, but a little limited on field of view, being about 45º, usually.

Find out the focal ratio of your scope. It should be printed on a plate on the scope, usually near the focuser, and be represented by a number like f/5 or f/8. F/6 or lower is a fast scope, and f/7 or higher is an intermediate to slow scope. Scopes with focal ratios of f/8 or higher are generally more forgiving of lower-quality eyepieces, while fast scopes tend to reward lower-quality eyepieces with fuzzy stars anywhere from 1/3 to 1/2 way from the edge to the centre.

If you can’t find the focal ratio, but you know the aperture and focal length, the focal ratio is (focal length/aperture).

Take your focal ratio, and multiply it by 3/4. So, if you have an f/8 scope, the result is 6. If you have an f/10 scope, the result is 7.5. This result is the length in millimetres of your high power eyepiece. It will give about 2/3 of the theoretical maximum power of your scope. This is the actual maximum if you do not always enjoy perfect seeing and transparency. If you have a 100mm scope, this eyepiece will give 133x.

IF YOU HAVE A FAST SCOPE, say, f/5, this formula will suggest a 3.75mm or 4mm eyepiece. Looking through a Plossl at this length is a miserable experience. If this is the case, I would suggest you buy an eyepiece with a length equal to 1½ times your focal ratio, and buy a 2x Barlow lens in the same price range as your eps. These purchases give you your high power and medium-high power magnifications, so skip the next paragraph.

Now multiply your focal ratio by 1¼. For our f/8 scope, the result is 10, and for an f/10 scope, the result is 12.5. This is the length of your medium-high power eyepiece. For our 100mm scope, it gives a magnification of 80. Eyepieces in these lengths are not hard to find, and you can go up or down a millimetre if your dealer doesn’t stock them.

Multiply your focal ratio by 2, now. By now, you can do the math yourself! In our 100mm scope, this gives a magnification of 50. This is your medium-low power eyepiece, and your low power eyepiece is given by multiplying your focal ratio by 3, and you get a magnification of 33 in your 100mm scope. IF YOU HAVE A FAST SCOPE, you want an eyepiece of 3 to 4 times your focal ratio, or 15 to 20 mm for an f/5 scope as your medium-low power eyepiece, and about 5 times your focal ratio for your low power eyepiece.

An  eyepiece of 5 times your focal ratio also gives you an ‘exit pupil’ of 5mm. This is the longest eyepiece you want to use if you are older, as this exit pupil is approximately equal to an older (45+) person’s maximum pupillary dilation. You can’t use more light than that. If you are younger, you could go up to 7 times your focal ratio, or an exit pupil of 7mm.

To summarize, for an f/8 scope, we suggest a kit consisting of 6, 10, 16 and 24mm. For an f/10 scope, 7.5, 12.5, 20 and 30mm. For an f/5 scope, 2x Barlow, 8, 18, and 25mm.

If your budget allows for only three eyepieces, drop one of the medium power eyepieces. If you are a lunar/planetary observer, then we would suggest dropping the medium-low eyepiece, and if you are a DSO observer, the medium-high eyepiece. In the latter case, we could suggest dropping the high power, but let’s face it, there will always be times  you want to get a good look at Saturn, or a good planetary nebula, so keep the high power.


Dobsonians tend to be large, fast scopes. If your Dob is 6″ or less, you can safely follow the guidelines for the scopes listed above, as the highest magnification this will give you is 200.

At about 200x, it gets hard to follow things with a Dob. Some people can do it, and your ability to follow objects will improve with time, but 200x is a good start. You will want to have an eyepiece kit between 200x, and a 5mm (or 7mm if you are a youngster) exit pupil. Suppose you have a 10″, f/5 Dob. You will have a focal length of 1250mm, and will get 200x with a 6.25mm eyepiece. In practical terms, a 6.5 to 7.5mm eyepiece will be what you will find available. To get a 5mm exit pupil out of a 250mm mirror, you will need an eyepiece that gives you  50x. This means a 25mm eyepiece. To get a 7mm exit pupil out of the same mirror means a magnification of 36, and a 35mm eyepiece.

Having decided on your low and high power, it is fairly easy to pick two more eyepiece focal lengths that will fill in the gap. If your spread is 6mm to 25mm, try 10mm and 16mm as your intermediate lengths. If the spread is 6mm to 35mm, then use 12mm and 20mm as your intermediate eyepieces.

So, for an 8″ f/5 Dob, you would be getting something like a 5mm, 10, 16 and 25mm.

These guidelines will give you a useful set of eyepieces without breaking the bank. You can buy one eyepiece a month until you have your set, and use the eyepieces you have until your set is complete.If you can afford slightly better eyepieces, then buy those, with the length guidelines still in mind. If you have a fast scope, ask specifically if the eyepiece you are considering is appropriate for a fast scope. Some less expensive wide-angle eyepieces perform well only in a f/8 or slower scope, and you don’t want to buy a set of these with a fast scope


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