Most people start out in astronomy with a telescope bundle – that’s the scope, tripod, finderscope and other accessories. It’s just the sensible way to start.
Amongst the most important contents of these kits is the eyepieces you get in the box. Generally you find two or three eyepieces covering a range of different focal lengths.
But, like when you buy a DSLR, the lenses that are supplied with the scope might not be the highest quality – they’re just there to get you going.
When I’m asked by clients what eyepieces they should buy when they’re picking up their new telescopes, I generally tell them to wait until they get a feel for the eyepieces they have at the moment. Once you know which targets you like, and you get a feel for how those targets fit into your field of view, then it’s time to decide about upgrading eyepieces.
I had a bit of a look at the kit lenses supplied in common bundles.
Sky-Watcher, saxon and Celestron bundles tend to be supplied with 25mm and 10mm eyepieces. For some bundles, such as the popular 80 or 90mm refractors sold under both brands, the lenses supplied are the 3-element black plastic “super” types, of the Kellner design Super 10 and 25. Looking through these lenses, you have a fairly narrow field of view (meaning the circle of image you see isn’t very large). You also have to hold your eye very close to the lens, especially with the higher-power ones. Chromatic aberration is also noticable, meaning stars – and especially those at the edges of the field – will shimmer blue and red, and the overall image won’t focus really sharply. There’s also an issue of field curvature, where if the centre of the image is in focus, the edge isn’t. If you were to buy one of these lenses, they retail at about $37.
Some other saxon and Sky-Watcher bundles, such as the 127mm Maksutov, come with a better-quality, metal kit lens. The saxon-branded bundles come with a pair of silver coloured Plössl eyepieces Silver plossl 10 and 25, and the Sky-Watcher bundles have recently changed to a black-coloured one with “Super Plössl” written on them SW super plossl 10 and 25. These black and silver Plössl eyepieces are around the same quality and performance. They have around 52° of apparent field of view, which is wider than the basic eyepieces, and are a 4-element design, so the flatness of the field is a little better. Field curvature is a little better, but chromatic aberration is still significant in these lenses, although I rather like the longer 25mm one. The eye relief (the distance you need to be from the eyepiece) is still quite small, especially for the 10mm one, so if you wear glasses, these will be a little uncomfortable.
When you’ve decided to upgrade your eyepieces, there is a plethora of different brands and levels of quality. I’ll describe just three 1.25″ eyepieces here. You can see them in a photo here. You’ll notice they’re all significantly larger than the kit lenses. They’re heavier too. Paul and I tested these lenses using a fairly basic refractor, and found the differences are noticable. We compared them (somewhat unfairly) against the 25mm black plastic “super” we got in a saxon refractor.
First, the Sky-Watcher LER (which stands for long eye relief) 15mm. This is a 7-element Plössl, with an apparent field of view of 58°. If you wear glasses, this lens will suit you quite well, as it has 20mm eye relief, which is the distance your eye needs to be away from the glass. These eyepieces typically retail for around $110.
The eyepiece we tested was the 15mm Sky-Watcher LER, and the increase in contrast was noticeable. The higher quality glass improved focus and cut the haze that the cheaper lens was producing. The lens has a roll-up that allows you to position your eye at the right distance, meaning you never see the “kidney bean of death”, that black blob that obscures the image if you have your eye in the wrong place.
Next, the saxon Cielo HD is 6-element eyepiece lens. At 60°, it has a slightly wider field than the Sky-Watcher LER. The feature that is important here is extra-low dispersion glass. If you are using a reflector or Cassegrain telescope, or a refractor that also uses extra-low dispersion glass lenses, the false colours associated with chromatic aberration will be heavily reduced. The focus will also be sharper. These eyepieces normally retail for around $150.
I’ve had a soft spot for the saxon Cielo for a long time. I do think it’s the best value for money in the upgrade group, and the view is better than the Sky-Watcher LER. The difference in the apparent field is not noticeable, but the ED glass produces a better focused, higher contrast image with better colour reproduction. We tested an 18mm.
The third eyepiece in the photo is a Baader Hyperion. This is an 8-element lens with a much wider field of view than the others, at 68°. This gives you a far more immersive experience at the scope, with stars coming closer to the edge of your vision. One party trick that this lens has is that you can change its focal length by adding space between the first element (which is all the way down the bottom in the 1.25″ barrel) and the second element. This doesn’t make it a zoom lens, because you need to partially disassemble the eyepiece to make the change – and I certainly don’t recommend trying this at night outside. But it does give you a bit of flexibility when buying expensive lenses. The Hyperion normally retails around $240, but at the time I wrote this it was on special – click the link to see the current price.
The eyepiece we tested was a 17mm one, and the image was the best of the three. The increase in magnification over the 25mm eyepiece was not as noticeable as the others due to the much larger field of view. Objectively, we were using higher magnification, but because the wider field was getting more of the view to our eyes, the overall effect was diminished. On the other hand, having the image extending into our peripheral vision was great. That black circle you see around the edge of the narrower view just wasn’t noticeable. I don’t know what type of glass the Baader Hyperion uses, but the contrast and colour suggests that it was ED.
So if you’ve got a telescope and you’re using the eyepieces that were supplied in the box, take your scope to a nice dark sky site and spend some time with a few of your favourite targets. You’ll soon get a feel for which targets need a little more or a little less magnification, and from this, you’ll figure out what focal lengths you will get the best out of. If you like, compare what you see with some of the field of view simulators, such as this one.
Then we can help you choose your next eyepiece.