Eyepieces can make or break your telescope images, and I do mean it. You may find that the eyepieces that come with telescope kits, usually a 12mm and 20mm, are usually not the best ones for your scope.

On the topic of eyepieces, many terms appear, such as focal ratio or aperture and field of view. It’s important that we understand these terms and how they affect a telescope before progressing into calculations that link everything together.

Focal ratio, length, and aperture

Firstly, we have focal length, focal ratio, and aperture. Focal length is the distance light travels in a telescope. They are normally in mm, and it just shows how much light has travelled from the primary lens/mirror of the telescope to your eyepiece. Focal length is written on the telescope or in the telescope manual. If not, a google search of your scope will probably do the job 😉

Aperture is the diameter of the primary lens/mirror (the big one). This also is written on the telescope or online, and is labelled as either aperture, or objective.

From our focal length and aperture, we can calculate the focal ratio (sometimes written as f/ratio or f/stop). Focal ratio is simply just the speed of the optics, but it can be used further to find out other info such as exit pupils.

All there is to calculating our f/ratio is simply to divide focal length by aperture (or objective).

For example, if my newtonian has a focal length of 700mm and objective of 76 mm, so my focal ratio is 9.2 to 2.s.f.

What does this mean?

A lower ratio is usually better for deep sky images, because of it means fast optics, low power and bright images. This is normally f/ratios below 5. However bigger focal ratios (6+) are great for planets, the moon and very close stars (not the sun unless you are equipped for it!)

Eyepieces have focal lengths as well as scopes, and they are measured in mm. Most telescopes can accommodate most eyepieces, but once the focal length of the eyepiece is too big, the light isn’t magnified that much and hasn’t much use.

We can calculate magnification from both focal lengths, by dividing the telescope’s focal length by the eyepiece focal length.

For example a 700mm telescope and 12.5 mm eyepiece has a magnification of 56x (700/12.5) . The same telescope with a 20 mm eyepiece has a magnification of 35x (700/20) . We can deduce that the bigger length an eyepiece has, the smaller magnification we can get.

Don’t be fooled by this and buy smaller eyepieces, because your telescope can only be magnified so much until the images become hazy, blurred and bad in general. I recommend to stick to less than 6x the f/ratio. Even with my telescopes f/ratio being 9, I would never use a 54mm eyepiece unless I had a very powerful Barlow.

Barlows, Reducers, and Diagonals

Barlows and reducers are a great way to increase the variety of magnifications you have with just one or two accessories. A Barlow increases the magnification from 1.5x up to 5x, which can prove very useful for people using large length eyepieces. Using an eyepiece of 25mm on a 1000mm telescope, our magnification will be 40x, adding a 2.5x barlow our magnification is now 100x.

However the warning about magnification still applies; do not try to push your telescope beyond its magnification limits, and don’t stack barlow on barlow thinking that it will make your telescope Hubble 2.0.

Reducers do the opposite; they reduce. For small telescopes, there really isn’t any need for a reducer, especially with large length eyepieces. Reducers are ideal for big high power + magnification telescopes that are built for deep sky observation. Using a reducer here means that we can adjust the magnifications to see planets or the moon.

Our last piece of equipment is the diagonal. This is a must for refractors or catadioptrics. This piece is used before the eyepiece and all it does is reflect the light upwards so you don’t need to observe in an uncomfortable position; it’s like a periscope. The diagonal saves achy backs and knees from all kinds of discomfort.

To conclude this part of my post, I suggest investing in an eyepiece or two of varying length as a start. If you are looking to do planetary/ lunar observations it would be wise looking at eyepieces with slightly larger focal lengths, or consider a barlow for deep sky objects, to maximise magnification (to a suitable level). I also encourage those with joint or spinal issues to buy a diagonal for reducing the discomfort that may come with cassegrains or refractors.

Happy Stargazing!