When you begin astronomy, it’s easy to become overwhelmed with the feeling that you need lots of equipment, and it doesn’t come cheap!
Most new telescopes, for example, come with one or two mediocre-quality eyepieces. You’ve probably already been advised that you ought to replace them with eyepieces of better quality for more rewarding astronomy.
The natural next question to ask is: what eyepieces should I have?
In this article, we’re going to answer that question in a way that helps you understand how to choose the right set of telescope eyepieces to have in your collection.
What is The Ideal Eyepiece Collection?
The three eyepieces that every astronomer needs are a short, medium, and long focal length option giving a good magnification range for your telescope. This trio is perfect for viewing all of the different kinds of space objects. Ideally, you should invest in a few high-quality eyepieces.
You can effectively double your range of eyepieces to six by adding a Barlow Lens to the mix.
By high quality, we mean eyepieces that have great light transmission, minimal chromatic aberration, sharp images, and (ideally) a large field of view.
If you’d like to understand the details behind this recommendation, keep reading.
What Eyepieces Do
We’ve got a full article explaining how eyepieces work (the opens in a new tab so you won’t lose this page while you go and read it), but it’s worth producing a quick summary here before we start discussing which ones you need.
It’s easiest to think of your eyepiece (ep) as a complex magnifying glass. Its only role in your astronomy setup is to magnify the image collected by your telescope and deliver that magnified view to your eyes.
Your telescope’s optics determine how much detail is captured in the original image; the eyepiece you select magnifies it.
A good eyepiece has to reproduce the image it’s magnifying as faithfully as possible. To achieve that, a good eyepiece:
- Won’t split the whites in the image into the colors of the rainbow (known as chromatic aberration)
- Allows as much light as possible to pass through the eyepiece to our retina (known as light transmission) so that the image isn’t dimmed, and
- Makes the view to be easy for us to take in, with the right size exit pupil and eye relief
The exit pupil is the size of the image leaving the eyepiece. We need it to be no bigger than our pupil, otherwise, we can’t see all of it. For an adult, that’s about 5mm to 7mm in diameter. The higher our magnification, the smaller the exit pupil.
Eye relief measures how far away from the eyepiece your eye needs to be to see the image properly. Read the full article linked above for details, but you’ll need longer eye relief if you have to wear glasses when observing, e.g. to correct astigmatism.
Traditionally, shorter focal length eyepieces come with shorter eye relief, i.e. the higher the magnification, the closer your eye will need to be to the eyepiece.
Now, this is all useful to know when thinking about the quality of your eyepieces, but the more important considerations when deciding which eyepieces you should own are magnification and field of view. Let’s briefly consider each of these.
Working Out Eyepiece Magnification
We said earlier that the eyepiece’s job in your astronomy setup is to magnify the image your telescope captures.
The amount that an eyepiece magnifies your telescope’s image by is determined by both the focal length of your telescope and the focal length of the eyepiece. Specifically, we divide the first measurement by the second to get the magnification.
For example, if your telescope has a 700mm focal length and you use a 10mm eyepiece, the resulting magnification is 70x (i.e. 700÷10).
We know that different night sky objects can stand different levels of magnification, so this has an influence on which eyepieces should be in your collection.
Eyepiece Field of View
The field of view measurement tells us how much of the sky we can see through the eyepiece. A larger field of view means that more of the sky is visible.
We work out the actual field of view by taking the apparent field of view of the eyepiece, say 50°, and dividing it by the magnification. If, in your telescope, the 50° eyepiece gives 40x magnification, then your true field of view will be 50÷40=1.25°.
Since the field of view is dependent on magnification, it is also dependent on the focal length of our telescope because it is the focal length that determines magnification.
All of this tells us that, when compiling an eyepiece collection, we need to consider the field of view because different objects look better with a larger or smaller field of view.
That’s enough background to explain why you’ll need certain eyepieces available. However, click the link if you want to read more about fields of view.
The Ideal Telescope Eyepiece Collection
At its core, we’re looking for at least three different focal length eyepieces.
One is for astronomy at low magnification, i.e. a longer focal length eyepiece. We’ll use this ep for our initial search for an object or enjoying wider fields of view.
The second eyepiece will have a mid-range focal length and tend to be your go-to option for most astronomy. This will have slightly higher magnification and a narrower field of view. Think of this ep as the workhorse of your collection. Consequently, it should have the highest spend/quality attached to it.
Finally, the third eyepiece should have a short focal length and deliver high magnification. As a result, it’ll have the narrowest field of view. You’ll use this one when seeing is good and/or detail is important. Think lunar and planetary details, splitting double stars, etc.
Over and above these three core eyepieces, consider adding a 2x Barlow Lens into the mix. This effectively doubles the number of eyepieces you have because it combines with the core three to double each of their magnifications.
In the next section, we’ll look at the main considerations you should have as you choose the right eyepieces for you.
If budget is a major constraint, and you can only afford to upgrade or add one eyepiece at a time, then it makes sense to invest in the eyepiece that’s going to get the most use.
In regular astronomy, that’ll be the lower magnification one (longest focal length) but it could be that you want to spend more money on a higher magnification ep because you love exploring the moon and want to get into those details.
In the next sections, we’ll look at the different eyepiece groups to think about.
Lowest Magnification Eyepiece
There is a lower end for magnification that every telescope can handle well and below which we don’t get good results. The simple way to figure this out for your model is to take the aperture of your telescope in millimeters and divide it by seven.
For example, if your aperture is 150mm, then the minimum magnification your telescope can handle is 150÷7 ≈ 21x.
We can use this number to work backward and see what the longest focal length eyepiece we should have in our collection is.
Let’s say the scope we just mentioned – the one with a 150mm aperture – has a focal length of 750mm. We know anything less than 21x magnification is no good to us, but what eyepiece focal length does that equate to?
Well, we know that the focal length of the telescope (750mm) divided by that of the eyepiece (?mm) needs to equal no less than 21.
With a bit of algebra (sorry), we see that this converts to the eyepiece of ?mm = the focal length of the telescope (750mm) divided by the magnification (21x).
In other words, the largest eyepiece we need in this scenario is 750÷21≈36mm
In reality, you should get something shorter than this, e.g. 30mm, but this longest eyepiece will be your ‘hunter’. This is the ep you’ll use to get to the right part of the sky and begin your search for an object before increasing magnification.
Maximum Useful Magnification Eyepiece
Here, we are talking about the maximum useful magnification. This is generally accepted to be twice the aperture size in mm (or 50 times the aperture size in inches). For example, an 8” scope is approximately 200mm of aperture because 8 (inches)x50 = 400x and 200(mm)x2=400x.
Working backward again, assuming a focal length of 750mm, we find that the smallest ep we should have is a minute 2mm.
Does that feel off to you?
It should, but not because the math is wrong but because we’re all doing astronomy from under a thick layer of turbulent atmosphere.
That constantly swirling air means we rarely have conditions that allow us to use more than 200x magnification usefully. Even if we could, the high level of magnification dims the image, because it spreads the light out over such a wide area, and so only works for double stars, the moon and planets, and similarly bright objects.
Practically, any eyepiece that gets you to 200x magnification (which would be 4mm in our example) is more than enough.
When you’ve worked out the top and bottom end of the magnification chart, we need an eyepiece sitting in the sweet spot between the two.
With 30mm as our largest ep and 4mm as the smallest, we’d be looking for something in the order of 12mm-18mm in the middle (offering 42x – 63x magnification).
Add a Barlow Lens
The final consideration is a Barlow lens. This works together with your eyepieces to add extra magnification, the most common option being a 2x Barlow.
For example, if you have an 18mm ep giving 42x magnification, you can use it with a 2x Barlow to provide 84x magnification.
They make the image darker than just an eyepiece on its own, but they’re an affordable way of effectively doubling the number of eyepieces in your collection, without actually doubling them.
Range of Magnifications for Different Focal Lengths
In the table below, we’ve looked at a selection of telescope focal lengths across the top and of eyepieces down the side. In the main body of the table, you’ll see the magnifications that these produce, so you can work out the sorts of eyepiece sizes you’ll need for your telescope.
Ideal Eyepiece Collections
Whatever your viewing aspirations, it’s much better to spread your budget across three or four higher-quality eyepieces that cover the range of magnifications we all need, than spread it across several not-so-great eyepieces.
When we thought about what we’d be looking at (above) it became clear that we only need eyepieces that deliver great results at low, medium, and high magnification. And, if we can afford to have a wide field of view at the same time, that’s even better.
If you follow that recommendation, these are the kinds of objects you should expect to use each eyepiece size for:
- 4mm = Best for moon and planets with a short focal length telescope
- 10mm = Best for moon and planets with a medium focal length telescope
- 10mm-20mm = Best for DSOs and full moon – planets become dots
- 20mm-30mm = Best for DSOs
- 30mm+ = Best for large DSOs
Are Zoom Eyepieces a Good Alternative to Separates?
An alternative to owning a collection of separate eyepieces is to buy a good quality zoom eyepiece.
These have their pros – such as being cheaper than separates and easier to use than changing in and out a different ep every time you want a magnification change – and their cons – such as lower optical quality and smaller fields of view.
If you’re interested in discovering more about zoom eyepieces, click this link which will open in a new tab so that you don’t lose this page.
It’s easy to get sucked into buying lots of different sized eyepieces to cover all eventualities, but this will either be very expensive, or you’ll end up with lower quality, and it isn’t necessary either!
Instead, use a few minutes to understand the ideal sized eyepieces for your particular telescope (using the table and information above) and then – one at a time if you need to – invest as much as you can afford in three good quality, wide field of view eyepieces, plus a Barlow.
With these six different magnification options in your collection, you’ll be marveling at everything from lunar craters to close double stars.