Your telescope is as good as the eyepiece you use with it, but you don’t need to spend a lot of money to get decent views. Most of the beginner to medium-range telescopes come with a basic eyepiece which lacks a lot of key features.
As it is said – An eyepiece is a key to upgrading your telescope and using its full potential. In this detailed analysis, we will find out – what is the main difference between a cheap and expensive eyepiece and how big of a difference does upgrading your eyepiece makes?
The default eyepieces which comes with your telescope have a very simple design. They generally come with a two or three element lens which has limited optical capabilities.
Qualities of a good eyepiece
The prime duty of an eyepiece is to show the image with as much clarity as possible. Here are the four standards to measure your eyepiece quality.
Sharpness – Keeping every object-on an image in focus without any visible distortion is what we call sharpness. The chances of an image being out of focus increases as we move away from the center of the lens. You can check for Sharpness by optically inspecting the image of a star and looking for any loss in image quality as it moves from the center to the edge.
Brightness – When the light goes through a lens some of it reflects back while others pass through, which results in a dimmer image. Most of the high-end optical lens comes with a special coating which reduces reflection and makes the image brighter.
Well, brightness may not affect the already bright objects like the moon but it has a noticeable effect on dim stars.
Contrast – Contrast is the difference in luminance between the brightest and darkest areas of an image. A good eyepiece which does not scatter light has good contrast.
The light can scatter when the edges of an eyepiece are not properly sealed off. The shiny internal casing can also help to amplify the scattered light by reflecting it. That is why most high-end eyepieces come with matte black internal casing.
Using a low-quality glass and bad arrangement and calibration can also result in scattering and dispersion of light.
Comfort – Properties like a longer eye relief (distance from the lens within which the full viewing angle can be obtained) along with soft eye rest which makes the viewing experience more comfortable. A wider apparent field of view is also desirable.
Difference between Cheap and Expensive eyepiece
A good eyepiece has a number of design features that help with producing a clearer image. Some of these features are lacking or totally absent from cheaper eyepieces and that is what makes all the difference.
Here is the list of design features that you are likely to find in a good eyepiece…
Quality of lens
Very cheap telescopes can sometimes come with a plastic lens. The plastic lens is very cost-effective and easier to make but has many disadvantages and should always be avoided.
Nowadays most of the lens are made of glass but as you go from the cheap eyepiece to expensive the quality of glass also increases. A good eyepiece has a really high-quality lens with no structural deformity and perfect shape. These lenses do not suffer from optical aberrations.
Matte black paint
Scattering can lower the quality of the final image and is also responsible for strange artifacts which appear when looking at a bright star called ghosting. It happens due to stray light reflecting off the internal walls of the eyepiece,
A very simple way to reduce this is to paint the internal walls and the edge of the eyepiece with matte black paint. A more sophisticated eyepiece can also have the edge of the lens painted black to block stray light from interfering.
Baffles
Baffles are the raised rings inside the barrel which reduce reflections and captures stray rays.
Metal Casing
Cheaper eyepiece often comes with plastic casing instead of metal. The downside to plastic is – the matte paint has a hard time sticking to the plastic, over time it may chip off. An eyepiece with metal casing lasts a long time and gives better protection against accidental drops.
Elements
An entry level H (Huygens), R or SR (Ramsden) type eyepiece use two lenses. They have an apparent viewing angle of around 35º∼40º and usually used with Short focal length and long f-ratio telescopes. These eyepieces are some of the oldest designs and have limited optical capabilities.
A K, Ke or RKE (Kellner) or MA (modified achromat) is what you will likely find in the budget to medium range telescope. They use three element lens and have a slightly wider apparent viewing angle of ∼45° which gives you decent viewing experience.
If you really want to step up your game than the 4 elements Plossl eyepiece can be a great starting point. It can reach ∼50° field of view with around 20mm eye relief. They are not expensive and can work with any kind of telescope. They sometimes come as a kit lens in medium to expensive telescopes.
You can go higher in terms of price and find eyepiece with 8-9 or more elements with an enormous apparent field of view and great eye relief.
Eyepiece with few elements is not a bad thing provided they have been properly weighed. They can give a very bright and sharp view if the optics have been crafted carefully enough.
Lens Coating
Lens coating reduces reflection and ghosting (appearance of secondary “ghost” images). A high-quality eyepiece has multiple layers of coating, sometimes on every single element.
Rubber Eyecup
Rubber eyecup gives comfortable padding for your eyes to rest. It can be helpful if you wear glasses.
Shortcomings of cheap(low-quality) eyepiece
Cheaper eyepiece ofter uses optics which are plain and simple in design. They may also have various structural deformity. All these factors put together is responsible for many of the optical abnormalities like lens aberrations. Some of the most common ones are listed below with simple illustrations for better understanding.
Spherical aberration
When a lens does not focus light at a single point and the center rays meet at a different location from the rays coming from edges. It happens due to imperfections in curvature of the lens.
Chromatic aberration
Also called color fringing where the lens is unable to bring all the wavelength of color at the same focal plane. Different spectrum of colors focuses on different focal planes which result in the image having a shifted color tint.
Coma (comatic aberration)
In comatic aberration, all the rays focus on the same plane but they are stretched out and does not focus on a single point. It results in the star looking like a comet with a tail.
Lateral color
It is the combination of chromatic and comatic aberration(COMA), where the different wavelengths meet at different points on the same focal plane.
Ghosting
Ghosting is the faint appearance of the secondary image of the stars or any other bright object in the sky. These secondary “ghost” images move around as you move your telescope.
Eye Relief
It is the distance from the last lens of the eyepiece where the image forms. Ideally, you want a longer eye relief which can really help if you wear glasses. Eyepieces with basic designs have very short eye relief.
Apparent field of view
How wide of a view can you see through a telescope is what we call the apparent field of view. The eyepieces play a big role in giving a wider view. The wider view gives you a more immersive viewing experience.
The field of view is measured in angles or sometimes radian. Eyepieces can be divided into three categories on the basis of their apparent field of view.
- Standard (25° – 60°)
- Wide angle (60° – 80°)
- Super wide angle (80° – 120°)
Click here for more info on optical aberrations (opens in new tab).
Why do telescopes come with a low-quality eyepiece?
Saving costs is the main reason why telescopes come with just a basic eyepiece. Unlike the main mirror and lens – an eyepiece is one of the most replaceable (interchangeable) parts of the telescope which can be easily swapped.
Saving a few bucks on the eyepiece allows the manufacturers to use better materials for the more important components like the main mirror or the lens. Which cannot be upgraded later and you are left with what you get out of the box.
This selective compromise helps us get fairly good telescopes at a very inexpensive price.
Should you upgrade your eyepiece
There are way too many advantages of upgrading your eyepiece, it steps up the performance of the whole telescope and does not cost much in most cases. So after you had your initial look from your telescope you should upgrade them.
Which eyepieces need an upgrade
A telescope comes with two sets of eyepieces. One of them usually have a smaller focal length of around 10mm and the other one is the 20-25mm eyepiece.
Ideally, you want to upgrade both of them but if you have to choose the one you should consider upgrading the smaller 10mm eyepiece. The 20-25mm eyepiece has lower zooming power and overall they are not that demanding.
Alternatives for your default eyepieces
Upgrading your eyepieces could be tricky. You may find the eyepieces which come with your telescope are not that bad and you may not feel the need to upgrade them. In that situation, you should go with a different focal range.
For example, if your telescope comes with 10 and 25 mm eyepiece you can go with a 15-17mm eyepiece. It will give you a new range to play with and you can also get the performance of 9mm eyepiece using a Barlow.
Related topic: Reflector vs Refractor telescope
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