Eyepieces

What eyepieces should I buy for my scope?
By: JC Willette

This is a common question heard by many astronomer from beginners. Another question asked is about Barlow lenses, should I buy the 3X or the 2X? In this article I will try to answer these two questions and give support as to why I answered that way.

First I will tell you that not all eyepieces will work with all telescopes. One reason right off is that the diameter of the barrel of the eyepieces come in 3 common sizes. You can purchase eyepieces with barrels of .965 inch, 1.25 inch and a 2 inch models.

The magnification of a telescope is the ratio of its focal length to the focal length of the eyepiece in use.

Thus a telescope with a 1000 mm focal length, used with an eyepiece of 25 mm focal length, has a magnification of 1000 / 25, or 40. It makes things look 40 times bigger, or 40 times closer. Change the eyepiece to 4 mm focal length, and the same telescope now has magnification of 1000 / 4, or 250.

The apparent field of view is the angle that your eyeball must travel from one side of the view to the other side. This will vary with each eyepiece. The actual field of view is the angular width of the patch of sky you are looking at, before it is magnified.

Eyepieces come in many different designs, and they all have names. Each name refers to some property of the eyepiece. Prices vary on each type. The more expensive ones aren’t always the ones that work the best with your scope. Plossl, Nagler, Ramsden, Kellner, are some of the names of eyepieces.

Eye relief is that distance between the final glass surface of the eyepiece and the lens of your eye when looking through it.

Barlow lens are used with eyepieces to change their magnification. Barlow lenses come in two sizes, a 2X and 3X. A Barlow lens is a device which multiplies the telescope's overall focal length.

General Observations

I must admit that hands-on experience is much better than text book reading. My advice is to take your telescope to “Star Parties” and ask around if you can test other observer’s eyepieces on your scope.

Not all Barlow lenses work with all eyepieces. This is another factor to take into consideration when choosing one.

A few good eyepieces will out perform several bad ones

It is desirable to start with a couple of eyepieces then buy a Barlow lens and finally fill in the rest of the magnifications with the final eyepieces.

I suggest building a chart like the one in the article. After completion and filling in the correct numbers, shade the magnifications that are close to 300 (or twice your primary lens). I have chosen pink for the close ones. Note that the 175 is colored pink because it is more than twice the 70mm lens. Anything above the 300 magnification should be ignored. I filled them in with orange in the chart. I then filled in the rest with a blue as these will be the ones we will need to look at for selection. Now we will take a look at the more common eyepieces and try to make some selections that will give us the most ranges in magnification.

Let’s take a look at the ETX70. You will note that a 9mm and a 25mm are standard equipment that comes with the telescope. These are very good selections in this case because if you buy a 2x Barlow you will have the following magnifications with your selections: 14X, 28X, 39X and 78X. I own this scope and have the 2X Barlow, so I can verify this from personal experience. I also added the 6mm eyepiece and use it without the Barlow with very good results.

I am now stepping up to the ETX125 and have been mulling over which eyepieces I should purchase in addition to the ones I already have. According to my research, I see that the 6mm will probably be of little use. This leaves me with the fact that I must purchase more eyepieces for the new telescope. As you can see the ETX125 comes with a 26mm only. Since I have the 2X Barlow this gives me 73X and 146X. This scope will go to 300X so I plan on getting the 15mm to give me the following magnifications: 73X, 127X, 146X, 253X and possibly 317X (with the 6mm).

I also strongly recommend getting par-focal eyepieces. These are eyepieces that when exchanged on your telescope there is very little adjustment of the focusing knob to bring them into focus. Eyepieces that are not par-focal will require lots of adjustment to the focusing knob to focus on an object. This can become a big hassle and take away of your pleasure in viewing the universe.

Ok, now we have our eyepieces and Barlow lens and we are off to view the universe, so we think! In order to obtain the best possible viewing, we must take into consideration of a few elements.

Tripod - I will not emphasize this enough, a solid tripod is almost mandatory to good observing. Remember we are out in the night unprotected from the elements including the wind. Many of night viewing has been destroyed due to the wind shaking the telescope.

Clouds - This is a given. If it is cloudy you can bet your viewing will be hampered. Cirrus clouds are the high wispy clouds and will be transparent. If they are light enough, they will not bother your observing to the point of leaving. Stratus clouds will cover a large area in sheets. These clouds are usually a menace and cause problems in a pleasant night of viewing. Cumulus clouds are puffy and tend to have gaps between them. They are a menace also, but with a little patience they will pass and you will be able to observe your object.

Transparency - This is just what it means. How transparent it is from the end of your telescope to the object you are looking at. This is a calculation of how much water vapor is in the air. If you are planing on observing deep sky objects like galaxies and nebulae, you will need excellent transparency. Here in the desert we seem to have an abundant nights of good transparency.

Seeing - This is the ability to observe fine details in the planets, galaxies and nebulae at high magnification. At poor seeing times, the objects will appear to be floating in swirling water. Periodically your object will “pop” into view and you will see some fine details. If you continue to observe the object, more and more of the details will be revealed.

Darkness - I classify this as how close does the night sky get to the color of pure black. Many things will depend on this. Sky-glow from local towns, the reflection from the moon in the sky or even the reflection from the snow or light sand you are standing on. The more you can get away from populated areas, the better and the darker the night sky becomes. The darker the sky, the more you will see in your telescope.

My advice to anyone who is interested in buying a telescope or adding eyepieces to an already existing telescope, is to go out and look through different telescopes and inquire what eyepieces that person has. Each telescope is different and each astronomer has their own favorites. Just inquire, gather all the information you need to make your decision and then press on.

Until later - keep your eye on the sky!