How To Choose An Eyepiece



Is to give beginners a better perspective of what's out there. Hopefully you'll gain some tips from this. I'll help you choose some eyepieces and I'll help you choose the correct magnifications.


 Remember that no telescope, regardless of its design will usually ever reveal any more detail than about 50x per inch unless you are resolving tight double star. This means that for every inch the telescope's mirror or objective lens has, you will use no more than 50x for each one of those inches. For example a 4" is capable of about 200x because 50 x 4 = 200x. Don't take these numbers too literally though. Some observers use higher magnifications sometimes, even on planets. There are other ways to calculate this, but this is just one particular way. Another rule of thumb is that most telescopes, regardless of how big they are, usually do not exceed 400x on planets because of the unsteady atmospheric conditions most observers live under. It's like trying to read the face of a penny under a stream of water.


Take the focal length of any telescope. Pretend you have a 4" refractor with a 1000mm focal length. If we use a 6mm eyepiece, we would divide 1000 by 6. This would provide about 167x which is nice for viewing Mars or resolving Saturn's rings. To resolve Saturn's rings, you only need about 30x, but to see them more clearly, higher magnifications are needed.


Let's pretend you have a 4" telescope. We now know that the maximum magnification will be about 200x. Let's pretend the focal length of the telescope is 1200mm. Take 1200mm and divide it by 200x. This equals six, which means you need a 6mm eyepiece. If the seeing conditions are not too steady, we may have to use less magnification so then take 1200mm and divide it by 150x instead. This would equal eight, which means you will need an 8mm eyepiece. Remember not to take these numbers too literally, they are simply good ball park figures.


Are you confused about which eyepiece to buy for observing planets? Here's a list of some good candidates to consider and how you should choose them. The choices should be made depending on the kind of telescope you own and how it's mounted, whether it's refractor or a reflector. For example, take the popular dobsonian telescopes. Most are wonderful for collecting light but usually don't incorporate tracking motors, therefore you will have to keep moving the telescope to keep planets centered and this becomes especially hard at higher magnification. Since most planetary eyepieces are simpler designs with smaller fields of view, this gives you less time to watch the object drift across the field. It also means that these simpler eyepieces are not as well corrected at the edge of field for coma or distortion at the edges, making the view even less impressive.

For this reason, a more highly edge corrected eyepiece like a Nagler wide field is a good choice. Since its apparent field of view is about 80 degrees, planets take longer to drift across the entire field. Not only that, but they are corrected better at the edge of field than simpler plossl and ortho designs. If this is the case, Nagler's T6 series eyepieces are a great choice. I tested the 5mm and 7mm out on a number of nights and was very impressed with their image quality. Prices are usually around $280 each. They may be pricey for some, but they are really worth it.

TELEVUE - NAGLER TYPE 6 - 5MM 7MM 9MMTeleVue Nagler Type 6

Another good candidate is Pentax's XW series. These are also wonderful eyepieces which have about a 70 degree apparent fields of view. These range at about $330, but are highly recommended. I tested the 5mm and 7mm models and was very impressed with their optical quality too. Another candidate is Televue's Radian series. These eyepiece are also well corrected at the edge with apparent fields of view around 60 degrees. They are less costly than the Pentax and Nagler series at about $240. You can also consider Meade's 5000 series. They sport 80 degree fields of view and are less costly than the Nagler's.

Pentax XW Series

TeleVue Radian Series

A telescope which has tracking grants one the opportunity to use simpler and less costly eyepieces because it's easier to keep the planets centered at all times. There are a vast amount of choices, depending on what you want to spend. I tested numerous models out. Meade's series 5000 plossls sport 60 degree apparent fields of view are a good choice to consider. Another good choice are the University Optics orthos which only cost around $60 each. University also has an HD version which stands for High Definition. These cost around $80. Both of these lines sport a 45 degree apparent field of view which is just fine for planets.
Meade Series 5000 Plossls

One of my personal favorites are plossls from Televue. These eyepieces sport a 50 degree apparent field of view and are a four element design and will easily compete with the most exotic eyepieces from Clave, Brandon, TMB or Zeiss in terms of contrast and absolute crispness with virtually no light scatter and cost around $95. They come in 8mm, 11mm, 15 all the way up to 40mm's but the shorter focal lengths are the ones to look at for planets.

Pentax now has a new series called the XO. At about $250, they are worth the cost. They come in only two focal lengths. 2.5mm and 5mm which are great fro fast optical systems like refractors with short focal lengths. I tested both eyepieces out and was stunned at the clarity. Both sport about a 45 degree apparent field of view. Another fantastic contender is TMB's Super Monocentric. This eyepiece is now hard to find, but I managed to acquire a 5mm and was very impressed with its clarity. The draw back is that the field of view is very narrow. At about 35 degrees, some may not be too thrilled, but they are for dedicated purists. Prices for these are around $200.

PENTAX - XO 2.5MMPentax XO 2.5mm

PENTAX - XO 5MMPentax XO 5mm

If you're on a really tight budget and you can only afford to spend $50 or less, Celestron has the 10mm SMA which can be barlowed for higher magnifications. Always consider a barlow. The longer the focal length of the eyepiece, the more eye relief there is. Eye relief is simply this. It's the maximum distance you eye can be in order to see the entire field of view in the eyepiece. A barlow can be used to double the magnification of any eyepiece.