Why I Still Use My Celestron Zoom 8-24 Eyepiece

I have owned and enjoyed my Celestron 8-24 zoom eyepiece for several years. In fact, once I got the Celestron zoom, I found that I was using my single-focal-length eyepieces less and less.

Like a zoom lens on a camera, a telescope zoom eyepiece provides a range of magnifications. This sounds great, but I need to warn you that not all zoom lenses are created equal. Some are of poor quality or have such a narrow field of view that I have found them to be undesirable.

However, I find the Celestron 8–24 mm zoom to be a worthwhile eyepiece and it works smoothly. In my opinion, it delivers image quality similar to my Plossl eyepieces. I have not found another zoom telescope that is better in the under $100 price range.

This one eyepiece, when combined with a Barlow lens that is properly matched to your telescope’s focal length, can take the place of 4-6 eyepieces and allow you to do things that single-focal-length eyepieces can’t do.

My Testing

I have tested the Celestron zoom in a wide variety of scopes, including two 80 mm f/5 refractors, a 125 mm f/15 Maksutov-Cassegrain, a 203 mm f/5.9 Newtonian, a 250 mm f/4.7 Newtonian, and a 203 mm Schmidt-Cassegrain. I have found the image to be good and comparable to my Plossl eyepieces in these scopes.

In scopes with focal ratios of f/6 or lower, I do see some aberration around the outer edges, but not enough to concern me. I would have to go to a much more expensive zoom eyepiece to eliminate these edge effects in these scopes.

This is also my favorite eyepiece for outreach events where I have people lined up to take a look through my telescope. I find it easier to share the view with others when I am using the Celestron zoom. I hand the telescope over at low magnification so the target stays in view longer. They zoom back to whatever magnification works best for them.

Also, kids love to see the zoom effect. I put the Moon in the eyepiece at low power and let them look. Then I show them how to turn the barrel on the Celestron zoom so they can zoom in. Always gets a WOW!

Magnification Range & How I Make Use Of It

Rather than buying the typical 24 mm, 14 mm, 10 mm, and 8 mm eyepieces to provide a selection of magnifications, this one eyepiece provides the whole range. And unlike single-focal-length eyepieces, it provides all the magnifications in between those sizes. So, if you would really get your best view with an 11.5 mm eyepiece, you have it. Wish you had a 9 mm eyepiece? You have it. You just have to rotate the barrel of the Celestron zoom eyepiece and watch for the best image.

The barrel is marked with focal lengths that are there for your reference. The only time I use these is when I am making notes for an observing report. I like to record that I observed Saturn at 8.4 mm or the moon at 13 mm.

From a practical point of view, unless you are making notes in an observation report, you will likely be unaware of what magnification you are using. And why should you care?  It is the image you care about, not the focal length of the eyepiece you are using.

When I am using single-focal-length eyepieces, I am very aware of what focal length I am using because I have to choose where to go next. If I have the 12 mm in the scope and want to try more magnification, I look for the 10 mm. With the Celestron zoom, I just turn the barrel. The eyepiece almost seems to disappear, and I can concentrate on the rings of Saturn or the stars in a globular cluster.

The Celestron is not perfectly parfocal through the range, but it is very close. When two eyepieces are said to be parfocal, that means that you can move from one to the other without having to refocus the telescope. I find I make very small focus adjustments as I move from one setting to another. But I would tend to do this using single-focal-length eyepieces anyway due to the change in magnification, even when they are parfocal. I will always want to be sure I have the best focus for this magnification.

A Note on the 1.25″ Eyepiece Barrel Diameter

Modern telescopes typically accept 1.25” eyepieces, so this zoom will fit them all. Some telescopes accept 2” eyepieces but they might also have an adapter for accepting 1.25” eyepieces, so you are all set if you have one of these.

Note that there are older designs that use the 0.965″ eyepiece size. This is obsolete now but you might have one of these scopes. The Celestron zoom will not fit into a 0.965” focuser or diagonal. However, there are inexpensive adapters that let the older scopes use 1.25” eyepieces. Just note that, in some cases, you may have trouble bringing any 1.25” eyepiece into focus. But it might be worth a try.

The Increasing AFOV with Decreasing Focal Ratio

AFOV stands for the apparent field of view. This eyepiece specification from the manufacturer gives us a number to calculate how much sky we will see when using the eyepiece with any particular telescope.

Celestron Zoom provides a 40-degree AFOV at the 24 mm setting and as we move up, it increases gradually to reach a 60-degree AFOV at the 8 mm setting.

Let me compare this AFOV to the other types of eyepieces that typically come with new telescopes. Kellner and Modified Achromat eyepieces, which are usually included in the lower-cost packages, provide about a 40-degree AFOV. Plossls, which are typically provided with better packages, provide about a 50-degree AFOV.

If we use the Plossl as the standard for eyepiece AFOV, the Celestron zoom runs from a little narrower to a little wider.

The benefit of having the AFOV increase as we get to the shorter focal length is that, as you go up in magnification, the true field of view naturally gets narrower no matter what eyepiece you use. With the widening of the AFOV on the Celestron zoom, you get a wider AFOV at a higher 8 mm setting where you need it. Wider than an 8-mm Plossl eyepiece, for example.

By comparison, I have seen zooms that run from 30 degrees AFOV to around 45 degrees AFOV, which I consider too narrow. Why have such a restricted view when Celestron’s version is available at such an affordable price?

Would an Eye Relief of 15mm to 18mm Ideal for You?

The 15 to 18 mm eye relief specification for the Celestron zoom is important for anyone. And this becomes even more important if you wear glasses.

Eye relief is the distance you must place your eyeball from the top lens in order to see the full field of view. I find anything more than 10 mm to be comfortable, with 13 or more to be preferable. However, if you wear glasses, you can’t get your eye that close to the lens. Many, but not all, glasses-wearers will find 15 mm to be sufficient eye relief. Some may need more.

If you are a glasses wearer, you are going to have to determine if the 15 to 18 mm range provided by the Celestron zoom is adequate for you.

I wear progressive lens glasses and have tested them with my glasses on. I find it sufficient, but I typically remove my glasses when observing. The primary value of the glass I wear in real life is to bring images into focus. But have a focuser on the telescope to do just that. If you have other eye issues, like astigmatism, you may have to keep your glasses on.

Eyepiece Weight and Balance Issues

At 8 ounces, the Celestron zoom is larger than the typical Kellner, Modified Achromat, or Plossl eyepiece. These normally range from about 3 ounces to about 5 ounces.

On very small scopes, the weight of the Celestron zoom may cause a slight imbalance of the optical tube. I have not found this to be a problem with my 80-mm scopes. I have larger 1.25” eyepieces that weigh more than the Celestron zoom that I have used in these scopes.

You learn to adjust the balance of your scope to account for heavier eyepieces, but this is just something to be aware of. I don’t think it will be a problem for you, but the eyepiece is larger than the typical starter eyepiece.

Illustration of Magnification Range

Let’s take a quick look at how we measure magnification provided by a telescope eyepiece. The formula is simple:

Focal length telescope / focal length eyepiece = magnification or power

To illustrate, let’s look at two typical eyepiece focal lengths that are included with beginner telescope packages: 25 mm and 10 mm. If you have a telescope with a 1000 mm focal length, the 25 mm will provide 40X magnification, and the 10 mm will provide 100X magnification.

The Celestron zoom will, by comparison, provide 41.67X to 125X. But the extra benefit is that it provides all the magnifications in between. Normally, you would buy additional eyepieces to fill in that gap, but the zoom covers it all.

If we match the Celestron zoom to a 2X Barlow in our example scope, we get double the magnification, ie. 83.3X to 250X. In many cases, that will cover the full range and capability of the scope.

However, if this is a large aperture scope, say over 200 mm, then we might match the zoom to a 3X Barlow for 125X to 375X. We achieve this range with one eyepiece and one Barlow.

Just to be complete in the analogy, I would still recommend a low-power wide-view eyepiece in the 28 to 40 mm range for targets that work best at very low powers or that require a wider field of view.

Naturally, you can also use single-focal-length eyepieces in conjunction with the zoom. Some people like to use the zoom to find the best magnification and then switch to a single-focal-length eyepiece that has a wider AFOV. Use it in whatever way works best for you.

Other Advantages of Using a Zoom Eyepiece

• Within the magnification range, you eliminate eyepiece swapping.

With a zoom, the eyepiece seems to disappear, as you can just zoom between lower and higher magnifications.

If you like to split double stars, you typically move from a lower-power to a higher-power eyepiece, swapping eyepieces to see if you actually separated the double stars. However, with the Celestron zoom, you can watch the single star resolve into two or more stars as you turn the barrel. There can be no doubt that you split it.

• With the Celestron zoom, one filter serves over a wide range of magnifications; there is no screwing and unscrewing to try other eyepieces for other magnifications.

If you use filters, you typically have one filter of a particular type. If you want to try that filter at various magnifications, you typically remove it from one eyepiece and screw it onto another eyepiece to try that filter at a different magnification. This is a slow and cumbersome process, which is avoided in the case of zoom eyepieces.

• The Celestron zoom lets you move smoothly between small magnification changes, so you know you are always working at the optimum magnification for this target.

When “seeing” is poor and the atmosphere is turbulent, this provides significant advantages over single-focal-length eyepieces that do not allow small magnification changes.

• I do not engage in astrophotography, but it is worth pointing out that the rubber eyeguard is removable. Underneath are threads that work with camera adapters, so you can use the zoom with your camera.

• My eyepiece case has been greatly simplified.

I have over 20 eyepieces, but 90% of the time I use one or two low-power wide-view eyepieces, then move to a zoom eyepiece for my midrange magnifications. If I need high magnification, I drop the zoom into a Barlow that is appropriate to the focal length of that scope.

Summary

The Celestron 8-24 zoom is a top performer in its price range. I recommend it to newbies who are on a budget and to those who are looking to fill out their magnification range.

There are more expensive, more capable zoom eyepieces on the market. I have a second zoom that costs over 4 times as much as the Celestron zoom. But the Celestron zoom stays in my eyepiece case and gets its share of sky time.