We all use them every time we point our telescopes skywards, but do you understand how telescope eyepieces work?
In this article, we explain what we eyepieces do in a telescope, how they do it and everything else you need to understand to make the right eyepiece choice for your stargazing.
What Does a Telescope Eyepiece Do?
At its most basic, telescope eyepieces have two jobs to perform to improve our stargazing:
- Bring the light collected by your telescope’s lens or mirror to a sharp image at your eye, and
- Magnify that image to reveal detail contained within it
Underpinning that simplicity is a whole world of technical details, which we’ll simplify.
Creating a Sharp Image
All lenses (and there are loads of different types – just click here for a sense of the selection on offer) operate using the same physics.
At their most basic there are two lenses: the field lens which points into the telescope, and, appropriately enough, an eye lens at the end you look through.
The field lens takes the image from the objective lens of your telescope, focuses it to the eye lens, which then moves the beams of light to a focus point where your pupil will be so you can see a great image.
There are a three challenges with this:
- Getting all the different coloured beams of light to the same point
- Having the right size exit pupil
- Decent levels of eye relief
Chromatic Aberration (or, breaking the white light)
Remember the story about how a prism splits white light into the colors of the rainbow?
Well, stars that generally look white are actually comprised all those colors.
Why is that of crucial importance for telescope eyepiece design?
Because physics says that when that white light passes through the lenses of a telescope eyepiece, the blue light will bend at a different angle from the red, which means (without correction) they will end up at different places when they hit your eye.
This phenomenon, where there is a blurring of the colors around stars when you look through your telescope, is called chromatic aberration.
In the example on the right, you can clearly see where the colors of the rainbow have separated out around the pillars, with yellow, red, purple and blue all quite distinct.
If you see anything like this when looking at objects through your telescope, you have a poor lens!
Chromatic aberration is obviously a BAD thing for astronomers!
Thankfully, there are ways of minimising this effect using modern lens configurations and coatings within telescope eyepieces to maximise our viewing pleasure.
In ‘achromats‘ two different pieces of glass are cemented together as a single lens which brings all the colours of light to focus at the same place.
Other lenses solve the problem by using ‘low dispersion glass‘ which contains elements (like fluorite) to minimise aberration.
The exit pupil measure is simply the size of the image that is formed at the eyepiece.
I’ve found there are only two things to note on exit pupils:
- The adult human eye, dark adjusted, has a pupil diameter of 5mm – 7mm (smaller as we get older) so any exit pupil larger than that wastes light
- The exit pupil at the eyepiece is calculated by dividing the aperture of the telescope by the magnification of the lens (see below for how to calculate magnification)
To take the second point, if my telescope has an aperture of 100mm and magnification is 100x, then my exit pupil will be 1mm (i.e. 100 ÷ 100)
Move the magnification down to 20x (100 ÷ 20) and the exit pupil is now 5mm.
Keep going down to 10x and the exit pupil is now 10mm, much bigger than the dark-adjusted pupil in my eye, and so light is lost.
The result of this?
Don’t reduce the magnification to increase image brightness if the exit pupil becomes bigger than the pupil in your eye.
Why is this relevant to eyepieces?
Because the size of your scope determines the magnification of your image, so a smaller aperture telescope generally only benefits from lower magnification eyepieces, whereas a larger telescope can use larger magnification and still generate a decent exit pupil size.
The final consideration for your telescope eyepiece regarding the image it creates is the eye relief it gives.
Every eyepiece is designed to focus the image just outside the glass surface you put your eye to, i.e. the image should hit a focus at your pupil.
The distance between the surface of the eyepiece and where the image is brought to a focus is called the eye relief, and is the distance marked by the number 3 in the image below.
This space is important for three reasons:
- If the eye relief is large enough, glasses wearers can leave them on
- You’re not in danger of smudging the glass or poking your eye
- You won’t move the scope by touching it with parts of your face (which is also a benefit for your face!)
As noted here, a long eye relief is also useful at star parties to save refocussing the scope for those with and without glasses.
You will have to find your own comfort zone for eye relief, either too little or too much will be a pain, but deciding what is ‘just right’ is down to personal taste.
That said, it seems that most of us find 15mm – 20mm is ideal.
Chromatic aberration, exit pupils and eye relief covered the sharp image role of an eyepiece, so now it’s time to move on to the fun bit:
Telescope Eyepiece Magnification
I said at the start that the eyepiece has two jobs:
- Make a sharp image at your eye
- Magnify that image
In this section, I’m going to look at magnification.
Eyepiece Magnification: How Much and How to Calculate it?
Let’s start with the basic question: how much magnification will my eyepiece give?
This is quite easy to calculate. You need the focal length of your telescope and the focal length of your eyepiece. Divide the first by the second to get magnification.
As in this example, if your telescope’s focal length is 700mm and your eyepiece’s is 25mm, then you have a magnification of 700mm/25mm = 28 (or 28x).
Change the eyepiece to one with a focal length of 7.5mm, and now the magnification is 700mm/7.5mm = 93.3x.
Give me more magnification!
It’s easy to assume that to get the best details of that nebula, or pick out a certain faint star in that globular cluster, we need more eyepiece magnification.
Use an eyepiece with a focal length of 2mm and I’ll get a magnification of 350x – awesome!!
But (and sorry if this bursts your bubble like it did mine) that kind of misses the point of what that eyepiece lens is actually doing.
Think about eyepiece magnification a slightly different way:
The image you are magnifying is ‘fixed’, and that fixed image is created by the aperture of your telescope and the quality of the mirror or lens in that aperture.
So, all the eyepiece lens does is zoom into that fixed image, it does NOT increase the detail of brightness of the image itself.
If you’re struggling to get the distinction (I’ll admit, it took me a while – the articles I read on the subject are quite ‘techy’), think about a digital photograph.
The image is fixed, but you can zoom in (or magnify) it to get more detail but, eventually, the magnification becomes pointless as the image just starts to blur or pixelate… there is no more detail to be had no matter how much we magnify.
Just like your telescope eyepiece, when you are magnifying a fixed image there comes a point where more and more magnification will just degrade what you are looking at, to the point of it being a useless dull smudge.
How Much Magnification can my Telescope Take?
Thinking about the right level of magnification for your particular telescope is a much more sensible approach.
How much your scope can handle is determined by the quality of the fixed image it can capture.
To begin with, think about it from the digital photograph perspective again: if you have more pixels (think 20megapixel SLR camera versus 8megapixel iPhone camera) you can magnify the image more and still keep detail that’s worth looking at.
The equivalent to megapixels in a telescope is its light gathering capability.
And light gathering depends on aperture – or how big the end is that you’re pointing to the heavens – the bigger your aperture is, the more light it collects.
More light means a more detailed ‘fixed’ image (think more megapixels) which you can magnify more and still deliver a useful image to your eye.
A helpful rule of thumb for maximum magnification seems to be to limit yourself to double the aperture of your telescope in millimetres.
For example, a 6″ aperture telescope is roughly 150mm. Double that and you have a useful maximum magnification of 300x.
A 250mm telescope (10″) can give you as much as 500x magnification at the top end and there will still be something worth seeing.
Sky & Telescope have a really simple magnification calculator that you can use for your telescope and eyepieces.
The simplest rule in the end, is: the bigger the telescope, the more magnification you can use.
Other Things to Think About with Magnification
Image quality has to be your first concern with magnification, but it is not the only one:
Keep in mind that the more magnified the image, the smaller the area of sky you are seeing at the eyepiece.
This means there are two other factors that you need to take into consideration before ramping up the magnification:
- Stability: If you (like me) don’t have a brilliantly stable mount – like this bad boy – then you will experience some wobbling of your image. The more magnified the image, the more pronounced this will become… to the point of causing cussing!
- Earth’s rotation: Everything we look at appears to move across the night sky. When we use just our eyes, this movement is glacial and we don’t notice it minute to minute. Use a low magnification in the scope and it is noticeable – we have to adjust the scope every so often. But… use a high magnification and the image will seem to scoot across the eyepiece like a roadrunner. Unless you have motorised tracking, this will stop you using über magnification with your scope.
There are other minor considerations, but aperture, stability and tracking cover off the essentials for us beginners.
There is though, one final thing we need to consider before moving on from magnification to look at the different types of lens, and that is field of view.
Field of View
As the name suggests, the field of view describes how big the circle of night sky is that you will see through your eyepiece. There’s a dedicated article all about it here.
This size is measured in degrees. To give a frame of reference, the full moon occupies a circle of about 0.5°.
Just because we (apparently) don’t like to keep things too simple in astronomy, there are two measures of the field of view: apparent and true.
The apparent field of view is the limit of what the lens would show you if there were no magnification involved (I know that sounds odd, but it will make sense in a moment).
Most eyepieces will have an apparent FoV of 40° to 50°, but they can be purchased up with viewing circles of up to a massive 100°.
The true field of view is the measurement that matters.
True FoV is calculated by dividing the apparent FoV (this is where it makes sense) by the magnification (which, as we saw earlier, is worked out with the focal length of your particular telescope).
To simplify, we worked out above that a telescope with a 700mm focal length using a telescop eyepiece with a focal length of 25mm gives a magnification of 28x.
If that same eyepiece has an apparent field of view of 50°, then the true field of view is 50° ÷ 28 = 1.78°, which would fit the full moon in about 3 times, side-by-side.
If we had a 50x magnification with a 50° apparent field of view, our true field drops to 1°, and so on.
The diagram below (which is from this rather technical site) illustrates really well the impact of magnification on the apparent field of view.
With that, it’s time to move on and answer the ‘so what?’ question… which eyepieces do I have and what do I need?
Telescope Eyepiece Types
There are many different types of eyepieces, and this helpful page covers them in more useful detail than I can do here.
What I will do is look at the eyepieces on a scale from simple to complex.
These consist the Huygens and Ramsden types.
Cheap to make because they only have two lenses in them (so often packaged with low-end telescopes) they both suffer from aberration defects (see above).
These are not go-to lenses, obviously.
Moving up the scale, these eyepieces have additional lenses (and so, cost) to reduce the achromatic aberration effects.
A Kellner lens steps up from the Ramsden design because it includes an achromatic lens (discussed above).
Cheaper versions still tend to suffer from ghosting though, which is where fainter versions of the main image can be seen alongside it.
The ‘ghosts’ are caused by reflections within the eyepiece between the different lenses. Manufacturers reduce or practically eliminate them using special coatings on the glass lenses themselves.
Move along the eyepiece quality scale another notch and we come to the Orthoscopic eyepiece.
These are four-lens eyepieces and are thought to be best for observing the moon and planets.
Generally speaking, higher magnification works for the moon and planets too, because they are brighter objects to train your telescope on.
Orthoscopic eyepieces do have a restricted field of view, in the region of 35° to 50°, but deliver good eye relief.
Plössl eyepieces are the most well known (it is certainly the most fun to say!) and they sit at the end of this list with their four-, or even five-lens structure.
The Plössl is so desirable because it offers great standards of colour and eye relief. Like the Orthoscopic, the field of view tends to be limited to 50°.
The smaller mm lenses are relatively cheap to buy, but the recommended 15mm to 30mm start to become more pricey.
See this example of a 32mm Plössl for sale on Amazon at the time of writing.
Wide Field Eyepieces
To get above the 50° field of view, you’ll need to invest (and it can be a significant investment) in one of these types of eyepiece.
The cheapest of the wide field eyepieces is the Erfle.
The downside is they tend to suffer ‘ghosts’ at edges of the image, so they don’t work well for looking at the planets, but they do give over 60° of view.
The König eyepiece is similar to the Erfle but with a shorter focal length and so a greater magnification.
This brings us to the ‘daddy’ of all lenses:
The Nagler is a relative newcomer, only becoming available in the early ’80s.
It is a seven-lens behemoth, and all that glass means it can weigh well over a pound; it’s no good attaching one of these to a puny telescope on a poor mount.
Delivering a massive field of view in excess of 80° and suitable for very ‘fast’ (low focal length) telescopes, the Nagler gives great performance.
An improvement on the original Nagler is the Ethos which was developed in 2007 and offers a massive 100° to 110° field of view.
Of course, this kind of greatness comes at a price:
You will not believe how much this Nagler costs!
As this Wikipedia article says, as Nagler comes in at the price of a small telescope and “hence, these eyepieces are regarded by many amateur astronomers as a luxury.”
Let’s wrap this up… have I learned enough to solve my original dilemma?
Answering the Original Question
This article could still go on for a long time, as the amount of detail on eyepieces is staggering (see these telescope eyepiece specification tables as an indication of the complexity you can take on) but that feels like enough detail for this post.
The last piece of business is to answer the question I had posed myself at the start: what were those markings on the lenses I’d found with my telescope?
Well, now I know: H12.5mm is a Huygen lens with a 12.5mm focal length and the SR4mm is a Symmetric Ramsden which offers more magnification than my 2-lens Huygen.
If you read the next quote from here:
“Unfortunately some entry level telescopes come with eyepiece designs that are not as good. These typically include the Huygens (marked with an “H”) and Symmetric Ramsden (typically marked “SR”). These are common designs in entry level telescopes as they are inexpensive to make (however their performance is often not so good compared to the better Plossl design).”
You’ll see that I also learned that I have bottom-of-the-pile eyepieces and it probably means I need to invest, but…
…at least decent eyepieces are going to be cheaper than a new telescope!
Example of chromatic abberation Credit: Stan Zurek
Diagram of achromatic doublet Credit: DrBob
Diagram of exit pupil Credit: Tamasflex
Field of View Image Credit: Bruce MacEvoy
All eyepiece designs Credit: Tamasflex