What Is A Zoom Eyepiece?
Zoom eyepieces are a single eyepiece that you can adjust to alter its focal length which changes magnification. Put the eyepiece into your telescope and by simply turning the barrel of the zoom eyepiece you’ll zoom into or out from whatever you’re looking at.
They tend to occupy the middle ground of focal length/magnification, so you may still need a very high and low magnification eyepieces (short and long focal length, respectively), but a zoom could handle everything in between for you.
On the face of it, they are a fantastic tool, but nothing is perfect. If you’re considering adding a zoom eyepiece (EP) to your astronomy kit, carry on reading to understand the pros and cons of owning and using one.
Advantages Of Zoom Eyepieces
There are a number of advantages to owning an eyepiece that you can alter the magnification of. Some are more obvious than others but what follows is our comprehensive list.
One Eyepiece Doing The Job Of Many
The two common focal length ranges covered by zoom EPs are 7mm to 21mm and 8mm to 24mm. You can select any focal length between these minima and maxima, which gives a lot of variety of magnification.
In a regular collection of fixed eyepieces, we might find an 8mm, 12mm, 18mm, and 24 mm. In this example, a zoom eyepiece can replace four stand-alone EPs and offers magnification in between each of those sizes which you wouldn’t have without it.
This is the most obvious advantage to owning an adjustable EP: you have one eyepiece doing the job of four.
Coupled with a Barlow lens this number is doubled, so now you have two eyepieces doing the job of five individual ones (four standard and a Barlow).
Cheaper Than Owning Separate Eyepieces
Hand in hand with our first advantage goes one of price. It is almost always going to be the case that one zoom eyepiece cost less than four individual fixed focal length eyepieces.
For example, see the current price of an 8mm-24mm Celestron zoom eyepiece:
- Zoom from low to high power in an instant with this versatile eyepiece!
- Compatible with any telescope that accepts 1.25” eyepieces
- This fully multi-coated Premium eyepiece zooms to any focal length between 8 mm and 24 mm - pick the best magnification for your subject
While it certainly isn’t the best zoom EP available, you can see how it will be cheaper to buy this that four separate eyepieces.
No Changing Of Eyepieces
With a zoom eyepiece there are no swapping between eyepieces to change magnification. Let’s say you have a planet in view at a low magnification. Normally, to increase magnification, you would have to switch out for another eyepiece.
Not with a zoom though! With a zoom eyepiece, there is no need to break away from your observing to change the focal length. Instead, simply rotating the collar will increase magnification, letting you see more detail on the planet’s surface.
Depending on the model you have, you may still need to adjust your telescope’s focus, but if you go for a parfocal model, like this one from Orion, you won’t have to.
The internal lenses of a parfocal zoom eyepiece move inside the barrel as focal length is adjusted to keep focus. Ranges of fixed EPs can also be parfocal, like the Luminos range from Celestron.
Match Magnification To Seeing Conditions
Having flexibility of focal length means you can make small adjustments to reflect the current seeing conditions. Rather than having to swap between your standard eyepieces, small adjustments of the barrel on your zoom may be all that is needed to present a decent image.
Put You More In Touch With Space
This last advantage comes through a combination of others we’ve already looked at.
Being able to seamlessly move from one magnification to another, without breaking off to change eyepieces, and being able to make the most of poor seeing conditions means longer spells of time at the eyepiece.
Not having to break away from your view gives you the feeling that your telescope is no longer there and, instead, you are looking out into space through a window.
As you move from low to high magnification, for example, you can see double stars split in real time, planetary surface features ping into existence that you couldn’t see a moment ago, and you’ll tease out new details that might be missed when swapping from one eyepiece to another.
In summary, you can always find the best magnification for the job you’re doing and you can always find the sweet spot for the conditions. Your astronomy could be much more rewarding with a zoom eyepiece than it is with individual eyepieces.
But, if that’s the case, why isn’t everybody using a zoomable eyepiece in their telescope? The simple answer is that there are also disadvantages to using a zoom, and we’ll cover those next.
Disadvantages Of Zoom Eyepieces
The disadvantages of a zoom eyepiece are all linked to compromise. The fact is, design compromises have to be made to wrap multiple focal lengths into a zoom’s barrel.
Lower Optical Quality
Zoom eyepieces contain many lenses. Some of these move in relation to each other as the amount of magnification is adjusted. Often what this highlights, particularly in lower quality models, is internal reflections between the lenses.
The second area where sacrifices have to be made for optical quality is in relation to focal length. Unlike a fixed eyepiece, zoom eyepieces for telescopes can’t be optimised around a single focal length.
The lenses in any eyepiece are made of glass and have a fixed curvature and size, so they deliver optimum optical performance at a single focal length. Outside of that – above or below – their performance is below perfect.
In a zoomable eyepiece, the focal length changes and so the lenses inside are not always working at their optimum state.
This is just a consequence of physics and there is nothing that can be done about it, however higher-priced models will compensate through use of better quality glass and more lenses.
Whilst it is a genuine disadvantage, in reality, you will be unlikely to notice the impact in day-to-day use with a zoom of decent quality.
Smaller Apparent Fields of View
In zooms, the apparent field of view tends to be smaller than a fixed eyepiece. This is particularly the case at higher end of magnification, with the obvious example of the Baader Hyperion seem which will look at later.
This means you’ll see less of the sky through your zoom eyepiece than you would through a fixed eyepiece at the same magnification (focal length).
When deciding whether to buy a zoom eyepiece this is probably the most significant aspect to pay attention to. However, if you plan to use the eyepiece in a telescope mounted with go to tracking, the impact is mitigated as tracking keeps the desired object centred.
Costs More To Give Long Eye Relief
The final ‘con’ of adjustable eyepieces is that it’s not easy – hence, cheap – for manufacturers to provide long eye relief.
Eye relief is a measure of how far away you can have your eye from the eyepiece and see the image. Short eye relief means your eye needs to be very close to the EP, which makes for less comfortable viewing.
While, for reasons of cost, it may not be a problem in higher price models, you may find that cheaper models are not as enjoyable to use because the eye relief is just too short.
Be sure to pay attention to specifications when buying.
How Much Do Zoom Eyepieces Cost?
As with all astronomy equipment, prices of adjustable focal length eyepieces vary tremendously, as you can see in the table below.
At the cheapest end you can buy a model that will cost you much less than $100. Mid price points are in the $100-$200 range, these tend to be perfectly decent models.
At the top end, you might pay as much as $300 or more. This is the price point for the darling of the zoom eyepiece world, seemingly loved by all who use it, the Baader Hyperion Zoom Eyepiece Mark IV.
Briefly below, we take a look at why this model is so widely acclaimed amongst amateur astronomers. Good sleep
The Baader Hyperion Zoom Eyepiece Mark IV
- Hyperion Universal Zoom Mark IV: fourth generation of Baader-Zoom, 68° Widefield eyepiece with five magnifications - focal lengths 8, 12, 16, 20 and 24mm clearly defined with "smoothed-out" improved Clickstop Action (a must for effortless binoviewing)
- Includes 2" and 1¼" barrel, usable for all telescopes with 1.25" or 2" focusers, and some spotting scopes with 1 3/8" thread (Celestron / Sky-Watcher and others)
- High Quality Phantom Coating Group Multicoatings! For faintest objects, maximum contrast and total freedom from reflections
- Large adjustable rubber eyecup, smaller straight rubber eyecup and winged, foldable eyecup included, to fit over M43 photo/video-thread
- M43-Video-Photo thread for afocal and classical projection photography, permits optional adaptation of practically any camera with optional accessories
Comments as to why it’s loved include:
- Really dark coatings to remove reflections
- Click-stop functionality, it ‘clicks’ into place at specific focal lengths
- Long eye relief
- Wide apparent fields of view, from 68° at 8mm focal length to 42° at 24mm
- Glass type and polish optimized for maximum magnification
All in all, if you can afford the cost of the Baader, you may find it occupies space in your kit box as the main eyepiece for most of your viewing.
Zoom eyepieces your telescope can prove a wonderful investment.
Sure, they have their downsides in terms of specification when compared to a fixed focal length eyepiece. However, many of these issues vanish when you can afford to buy higher price model.
What can’t be argued against is that even a lower-priced model still provides fantastic value when compared to the cost of four individual fixed eyepieces.
Last update on 2020-01-17 / Affiliate links / Images from Amazon Product Advertising API