Eyepieces: A primer

In my opinion, an eyepiece is about a 40% contribution to what you see - but it shouldn’t be that much of your budget.

The chief factor is power, or magnification — this is just the scope’s focal length (1200mm is common) divided by the eyepiece’s focal length. We will discuss magnification in more detail later. There is a tradeoff as you increase power: The view gains contrast/detail, but it dims. Likewise, you observe a smaller and smaller field in the sky. For planets, you likely want the highest you can manage. 200x is challenging but workable for beginners. For nebulae, 50x to 120x is often a sweet spot. Clusters/rich fields of stars, you want the lowest you can manage - often around 35-40x.

Secondary factors include eye relief and “apparent” field of view. Eye relief is simply a measure of how far from the eye-lens you need to put your pupil. This is a concern for glasses-users, and too little can make things challenging. Apparent field of view (or AFOV), is a measure in arc-size of the image circle. 40 can feel narrow, and 100 can feel like a porthole into space. A tertiary factor is “correction.” Telescopes with a very low focal ratio (<4) start to have a lot of problems that beginners generally shouldn’t worry about. Telescopes f/8 and "slower" can get away with very cheap eyepieces without running into problems.

Correction and AFOV are by far the most expensive factors (escalating prices into the hundreds).

For new astronomers, the recommendation is often one each for low power and high power - for planets, and large deep sky objects. Correction and huge AFOV aren’t important considerations at this stage: A $20-30 eyepiece works just fine without obvious issues - a 6mm (200x) and 25mm (48x) are delightful for a beginner telescope with 900 to 1200mm focal length.

Pictured: From left to right: 2x Barlow 2", 4mm Lacerta UWAN, 7mm Lacerta UWAN, 11mm ES, 16mm UWAN, 30mm UFF, 32mm Plossl for scale. All have an AFOV of 82 degrees, except for the 30mm (70 degrees) and the plossl, (52 degrees).

A go-to recommendation for beginners is a 58 degree "Planetary" or 65 degree "goldline" (SVBONY, Orion, Agena etc) in the size that gets you as close to 150-200x as possible. Pair with a single low power eyepiece (a 25 to 32mm plossl, a 23mm aspheric etc.)

FAQ/ do I need a barlow/ other eyepieces?

Barlows: Experienced hobbyists often like at least a small collection of eyepieces. This is mainly focused on nicely framing any given target - enough magnification to make it clear, not so much that you lose some. But most Hadley users are likely only looking at the moon, planets and a few large/bright deep sky objects (similar in scale to the moon) - without targets filling in the sizes between "tiny planet" and "moon" - there's no want or need of different power steps, at least in my opinion.

I only recommend extra eyepieces to users who have enough experience to answer this with confidence: "What exact niche will the new eyepiece serve? And why would it do that much better than my other eyepieces?" EX: I grabbed a 38mm Q70 for my large dobsonian, to serve as a largest possible "true field" and see huge extended structures. EX: I am in the market for a quality 4mm, to serve as a high power eyepiece in certain extremely short telescopes, as well as to push my biggest telescope beyond 400x magnification, without the added glass of a barlow.

Find some nebulae/deep sky stuff that your high power dims too much, but is too small in your low power? That's a good reason.