(Click the banner above if you do not see a navigation menu to the left)
To properly select an eyepiece you should first be armed with the focal length of your telescope. For the ETX90 this would be 1250mm and for an 8" SCT this would be roughly 2000mm. If you are unsure of the focal length of your scope it is typically printed on the ring around the corrector or objective lens. Since the ETX90 comes with a 26mm eyepiece we will use that as an example. The formula for figuring the "power" or magnification (in X) is to divide the telescope focal length by the eyepiece focal length (both measured in millimeters).
Now let's complicate things just slightly. If you own a barlow (our example will be a 2X barlow) you would just double the resulting magnification. A stock ETX90 with a 26mm eyepiece and a 2X barlow would give 96X magnification.
There are other things you should know about a Barlow. First, buy one, even before you buy other eyepieces. A Barlow essentially doubles your eyepiece collection by doubling the available magnifications of all of your existing eyepieces. For example, if you use a barlow with a 26mm eyepiece it will be the same as if you were using a 13mm eyepiece. So you wouldn't want to go and buy a 13mm eyepiece or you would be duplicating powers. Of course if money is no object it is nice to have a complete set of eyepieces regardless. Eye relief is another factor, this is the distance your eye will have to be to the eyepiece to view the image. Another advantage of a barlow is that it can double the power of an eyepiece while still maintaining the eye relief of the original eyepiece. This can be an important factor as when you start getting down below 10mm eye relief can be very short. If you MUST wear glasses while viewing eye relief is certainly a consideration. One final note on barlows, if you insert one before the diagonal (not possible with the ETX) a 2X barlow will yield 3X!
As mentioned in the last paragraph eye relief gets shorter as the magnification goes up. At the same time the field of view gets narrower. So to sum it up using our ST Series examples a 40mm eyepiece will give you a nice wide field of view, you will have plenty of eye relief, but the magnification will be low. By contrast the 4mm would give you a very narrow field of view with very short eye relief; but magnification will be very high. If you wanted to look at a large extended object such as M31 you would want to use the 40mm. But if you wanted maximum magnification (on a very clear night) to see the finest detail possible on the moon you would want to use the 4 or 6mm eyepiece.
My honest recommendations for a starter set of eyepieces are as follows, I am assuming you already have a stock 26mm. First buy that barlow, it is not only like getting a 13mm eyepiece it also will double the magnification of every eyepiece you buy in the future.
For your first actual eyepiece I would highly recommend the 40mm. That way with a barlow you will have the equivalents of 13mm, 20mm, 26mm, and 40mm. Quite a nice spread of magnifications for such a small investment!
Many people new to the hobby will buy the maximum magnification as their first eyepiece, this can be a big mistake. High power eyepieces are good only during the very best of seeing conditions and even then the image is darker, harder to focus, and less crisp than a more reasonable power eyepiece. You will be surprised at how good things look in a wide field eyepiece, globulars are suddenly worth looking at! The 40mm also is a great first choice as the wide field is a definite advantage for locating objects, even with a computerized scope! Once located and centered you can up the power.
Next?? I would recommend the 15mm. This will give you a good mid/high power eyepiece for planetary viewing as well as (with barlow) 7.5mm high power eyepiece! So by buying a barlow and only 2 eyepieces you now have a selection of 7.5mm, 13mm, 15mm, 20mm, 26mm, and 40mm! From there on out it becomes more subjective depending on what you like to look at.
After owning the above recommendations for a short while you will probably know where you would like to go next without any help from me. One rule of thumb you should remember is that maximum usable magnification is typically at 40-60 times the telescope aperture in inches. For an ETX90 (3.5 x 60 = 210X) this may actually be conservative as it does well at even higher powers if the object is bright and atmospheric conditions are excellent. Nevertheless it is a good basic rule of thumb for most telescopes under most conditions. So, there are quite a few factors to consider and even a simple choice can get very complicated. I have simplified the factors here as much as possible to try to make it understandable. To ease your choice even more I have compiled a table below of all ST Series eyepieces and the magnifications they will provide in an ETX90 with and without a 2X Barlow.
| Eyepiece | Telescope | Magnification | Mag. w/barlow |
| 40 mm | ETX90 | 31X | 62X |
| 25 mm | ETX90 | 50X | 100X |
| 15 mm | ETX90 | 83X | 166X |
| 9 mm | ETX90 | 139X | 278X |
| 6 mm | ETX90 | 208X | 416X* |
| 4 mm | ETX90 | 312X* | 624X* |
