I am reviewing this component with a little trepidation because of the age old problem of amateur astronomy where low end department telescopes are touted as having ridiculously high magnification. Chief among the methods used by sleezy marketers to get these numbers is the Barlow lens, a device to multiply the focal length of the telescope by a magnification factor, usually 2X, which doubles the magnification for a given eyepiece. So many of these are used to produce blurry brown images of planets which convince budding amateur astronomers there must not be much to see that they have become a sore subject for me. However, they do have good uses, and I will do my best to outline them here in this review of Celestron's superb quality Ultima Barlow lens. More general information about telescopes is in my article on Picking a Telescope.

Background

Barlow lenses are an often misunderstood optical accessory for telescopes since they do increase magnification, but it comes at the expense of image brightness. In general, the only objects to look at in the night sky where high magnification is a must are in the form of planets, a few planetary nebulas (formed by stars falling apart at the end of their lives) and splitting closely spaced stars.

The main complication is the telescope aperture has a fixed resolution capability and a fixed amount of light grasp. As a result, there is a certain space where it will be possible to produce a usable image with any given telescope. Now, since the eyepieces and other accessories each only work in their own fixed range, the telescope maker can bias the telescope towards one area of the brightness versus magnification regime. So, for example, a telescope with a very long flat light cone will have a comparitively long focal length to diameter ratio. Since a telescope does not have an iris like a camera lens, the f number is just the focal length divided by the diameter. So, a long focal length telescope will be somewhere in the f/12 to f/15 range, and is always operating towards its highest magnification potential and a barlow lens is least useful in this type of telescope. The reason is the telescope has a maximum useful magnification where its image brightness equals the unaided human eye's brighness. So, for example, in a 6" f/10 scope like the NexStar 6SE, the limiting magnification is 354X. Now, since it has a 1500mm focal length, a 6mm eyepiece with a 2X barlow gives a magnification of 2X1500/6=500X, and what you will see will look dim and a bit fuzzy, though a planet like Saturn will absolutely fill the field of view with the soft focus image. And, using a 4mm eyepiece in the same configuration will yield 750X, and an even lower quality image. These same eyepiece combinations, if used in an f/15 telescope will yield 50% more magnification; even further in the dim-blurry regime.

What is going on here is the barlow has the effect of doubling the telescope's existing focal length. As a result, it has the effect of doubling the focal ratio. So, for example, a fairly fast f/6 refractor, like the excellent AT66ED will turn into an f/12 instrument, so a longer focal length eyepiece will get to the telescope's maximum effective magnification than would otherwise be the case. But since this is a doubling factor, a telescope with a modestly higher focal ratio, like the f/7.5 C80ED is pushed all the way to f/15 by a 2X barlow.

So, since it is easy to over-run on magnification with higher power eyepieces, it is fairly clear the real value of a Barlow lens is it lets you get to the higher magnification regime with lower magnification eyepieces, such as the bright and sharp NexStar 40mm Plossl, which effectively becomes a 20mm eyepiece, but with amazing amounts of eye relief. In this mode, the longer focal length eyepieces with nice open lenses and large eyeglass friendly eye relief are able to support getting in to images of planets and stars. And another nice benefit is a well-chosen barlow lens is sort of like an eyepiece kit doubler, so, for example, you could carry 12.5mm, 16mm, 20mm, 25mm, 32mm, and 40mm size eyepieces, or just the 25mm, 32mm, and 40mm eyepieces and a barlow and have the same set of magnifications available.

The only remaining complication is every element added to an optical train is one more chance to degrade the image. As the old addage about a chain being only as strong as its weakest link, the optical quality of an image is ruled by the lowest quality element in the system. So, there is a strong motivation for a barlow lens to be a good one. The main items to fear here are image distortion (barrel or fisheye defects) and false color, where objects away from the center of the image have a blue side and a red side. A low qualiity barlow can cause both of these, while a high quality apochromatic one simply does not appear to be there (yeah- it's hard to get praise when doing a good job means no one can tell you showed up).

Description and Usage

The Celestron Ultima Barlow is an obvious releative of the other Ultima series accessories with a rubberized grip around the barrel and a stainless steel insertion sleeve. The build quality actually looks very similar to the Celestron Reducer Correcter, which is the device to do the opposite of what a Barlow does and take the telescope to lower magnification and higher brightness. Unlike the reducer/corrector, the Ultima Barlow simply slides into 1 1/4 inch size focuser sleeves, including diagonals, and the eyepiece drops in on top of it.

The eyepiece holder in the top of this Barlow is just a simple set screw, which is somewhat disappointing since a compression collar would make for better alignment and preserve eyepiece barrels from getting scarred by installation. With that said, this is the most compact Barlow size available, so it is extremely useful for cases where little space is available, such as in a fork-mounted telescope like the CPC-800 when trying to view objects near the pole.

Images through the Ultima Barlow are, in fact, color free, and it comes remarkably close to being transparent after it is installed. The color correction means it can be used in-series with a camera to get a full-color image, and so it really is an ideal pairing with the reducer corrector as a pair of elements to move a telescope to high magnification or low magnification at will.

Conclusion

The Celestron Ulitima Barlow is well made enough to make using a Barlow flat-out useful. In general, I tend to be a bit down on Barlow lenses since they are so often misused to get a big sounding number on department store telescopes. But the way I use this one is to take a large eye relief lens and get some more magnification to get detail and easy viewing with eyeglasses at the same time. And in that mode, this component really delivers. And when I am travelling, I always make sure it is packed in the eyepiece kit.

Recommended: Yes