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Learn How A Barlow Lens Works, And See If You Need One

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How The Telescope Objective Works



Lens Diagram

Above is a simple telescope diagram of how the objective lens of a telescope works. Stay with me, I'm getting to the issue of what a Barlow lens does. The telescope objective is represented by a simple convex lens. In truth, modern refractors usually have two lenses that make up the objective, and they may be convex (curved out on both sides) or plano-convex (bulged out on one side, flat on the other). The multiple lens setup is used to compensate for chromatic limitations of a simple single lens. The reflector type telescopes use a concave mirror as an objective instead of a lens.

The purpose of the objective is to take incoming light from a distant source and bring it to a focus. In the diagram, light from a desired target enters from the left, and is bent to a focus on the right. An eyepiece placed at the focus will create an image for the observer's eye.

In the case presented, the focal length (FL) of the lens (L) is the distance from the lens to the convergence point. Since telescope magnification results from the ratio of the objective focal length to the eyepiece focal length, it follows that the longer the focal length of the telescope objective, the more magnification any given eyepiece will provide.

Magnification = Objective FL / Eyepiece FL.


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In astronomy, there are a couple of upper limits to magnification that plague observers. One, that magnification of over 50 times per inch diameter of the objective tends to produce fuzzy and low contrast images. Not only does magnification make an image larger, it spreads the light from an image over a larger area, making it dimmer. So a 3 inch diameter telescope, as an example, tends to start running out of gas (image-wise) at 150 times magnification, or thereabouts.

Atmospheric Scintillation

The other aggravation, illustrated at left, is that the scintillation of the atmosphere, which varies on atmospheric conditions and observer altitude. It tends to limit the average backyard astronomer to between 200 times and 300 times magnification, whatever the telescope, with most nights allowing even less magnification. You can find more information about the effect of atmospherics on viewing at Seeing and Transparency.

But -- if you happen to have a relatively short telescope, one only 20 to 40 inches in focal length, your problem isn't likely too much magnification or atmospheric turbulence, but not being able to magnify enough to properly view high resolution targets. For instance, I have a 30 inch long Newtonian telescope. It's super for observing wide star fields, but even with a relatively short focal length eyepiece like 10mm, it only magnifies 75 times. To get to 150 times or more, my telescope needs a boost.

The Barlow Lens To The Rescue

Barlow lens diagram

One easy way to get that boost is to use a Barlow lens. The image above cycles through a series of images to illustrate what a Barlow lens does when placed in the path of the converging rays of the objective. The initial image shows the same simple lens diagram shown before.

Positioning your computer mouse over the image causes an animation to cycle at about one frame every 3 seconds. The legend below explains each image in the sequence.

The animation sequence is as follows:

  • 1) Basic objective and focal point

  • 2) Barlow insertion

  • 3) Barlow extends focal point

  • 4) Projected focal distance to virtual objective

  • 5) Effective focal length of optical system

  • With a Barlow lens between the eyepiece and the telescope objective, one computes magnification by dividing the effective focal length by the eyepiece focal length. Since the effective focal length is much longer than the objective's inherent focal length, the magnification of any given eyepiece will be much greater.

    That, in a nutshell (or animated gif) is how a Barlow lens increases magnification for any given eyepiece. Seems magical, no?

    Barlow Lens Pitfalls

    Edmund Barlow Lens

    Above is an image of my old (circa 1964), trusty, Edmund Scientific Company Barlow lens. It is a simple device, a tube that's narrow enough at one end to go into a telescope focuser, and large enough at the other to accommodate an eyepiece. Within is a negative lens (actually an achromatic negative lens) that decreases the convergence angle of light from the objective.

    It seems too good to be true that one can just put such a simple device into play, and get a much longer telescope for the bargain. There must be pitfalls.

    There can be. To get the longer focal length, you must insert another piece of glass into your system. That gives more opportunity for additional distortions or optical errors, and some (though small) inevitable loss of light. One notorious problem that poorly made Barlow lenses can cause is glare.

    The inexpensive Barlow lenses that come with department store telescopes can actually cause enough of these problems to render them nearly useless, especially glare. One might reasonably have had such expectations from the trusty old Barlow shown here. It only cost me about $8.00 back in the 60's. For a measly $8.00, what could I expect?

    As it turns out, the only anomaly I ever discovered with my old Edmund Barlow was the glare problem. I didn't even notice that until I used it to take some photographs of the moon, using my 35mm camera in place of the eyepiece. The exposure times were proper, but all of the images were ruined by glare.

    A Peek Inside My Old Barlow Lens

    Edmund Barlow Lens

    Above is shown what's behind the curtain so to speak. The inside of my Edmund Barlow is a simple but clever design that consists of two slip rings with an achromatic negative lens cell between them. It is designed in this simple way so that the user can slide the lens to any position within the tube.

    When slid to the eyepiece end (the big end of the tube), the Barlow gives an effective focal length of double the original telescope focal length. When placed at the opposite end of the tube, it gives an effective focal length of triple the original. Modern Barlow lens come in fixed settings, either a 2x or 3x magnification increase.

    With my old Edmund Barlow, I had to get rid of the glare to be able to use it for photography. Now that initial photos revealed the glare, I even noticed it when viewing bright objects. I was sure the glare was reducing contrast, and in so doing hiding details I should be able to see.

    I found that I could easily solve the problem with a simple aperture stop made from a piece of black poster board. The stop is just big enough to fit in the tube, and has about a 3/8 inch diameter hole in it. The illustration shows the stop rotated so that you can see the hole, and in practice it's placed between the lens and the slip ring on the eyepiece side of the tube. It's amazing how well it works, and it's a solution you may want to try on any glare-prone Barlow you may have.

    Lunar Appenine Mountains With ETX 90
    Lunar Apennines Through ETX 90 and Edmund Barlow

    As proof of the stop effectiveness, examine the image above and other images on my ETX 90 Astrophotos web page. Those images were all taken using my ETX 90, a simple web cam conversion, and the old Edmund Barlow lens. The clarity of these images suggest that it's hard to argue that the old Barlow isn't performing rather well with the aperture stop.

    How The Barlow Lens Is Used

    Edmund Barlow Lens In Use

    At left you see how simple it is to use a Barlow. In this case, I just removed the eyepiece from my telescope focuser, inserted the Barlow tube, then placed the eyepiece in the end of the Barlow tube. Now the light operated on by the eyepiece appears to have been produced by a much longer focal length telescope.

    By pre-positioning the slip rings and lens within the Barlow I can control the effective focal length of my telescope to between 2 to 3 times the original focal length. I use it a lot with my short focus 6 inch reflector. With only a 30 inch inherent focal length, getting to high enough power to appreciate views of the planets and the moon is not possible with just my eyepieces. But with the Barlow lens, my 10mm focal length eyepiece can provide 75x (alone), or 150x to 225x with the Barlow lens. Enough to provide some wonderful views.

    A Modern Barlow Lens

    There have been a number of breakthroughs in telescopes from my days as a youngster, and many of those have increased the niche for the Barlow lens. In my days of youth, the typical Newtonian telescope had a focal ratio of f/8 or larger (f ratio = focal length the objective diameter). Now it is common to be able to purchase reflector telescopes at focal ratios of f/5 or even lower. My own Newtonian is f/5, making it great for taking in grand star views.

    Similar changes have occurred in the refractor world. The old classic refractor telescopes were generally f/15 or greater, making them very long. The length was necessary to curb chromatic aberrations, a nemesis of refractors. But breakthroughs in lens manufacture have resulted in some low dispersion types of glass that greatly reduce the chromatic problem. This allows refractors to also be made in low focal ratios, making available some short and very portable refractor telescopes.

    But take any short focus telescope and try to produce high magnification. What's a natural solution? Ah, you've been paying attention: It's the Barlow Lens. But no one who's spent a premium to get a quality short telescope wants to spoil the views with a cheap and cheesy Barlow lens. Fortunately, there are now some modern Barlow lenses made to great quality that give that extra magnification without introducing any anomalies -- even the glare.

    The image at left shows a Televue Barlow lens, which I have. It's not just some slip rings and a small lens, but a quality set of lenses that accomplish the mission without introducing errors. And it's a well baffled system, screening out any annoying glare as was prevalent in my old Edmund Barlow of 50 years ago (though my simple aperture stop greatly reduced that problem).

    So if you have one of those convenient short telescopes of Newtonian or refractor design, you can either buy some very extravagant and expensive eyepieces (which basically have a Barlow built in), or continue to successfully use your current collection of eyepieces and get a single, quality Barlow lens, like the Televue shown. If that Barlow is a bit expensive for you (I had to save awhile before I went for it), consider the more affordable yet perfectly functional Celestron Omni 2X Barlow Lens . Not everyone likes Barlow lenses, but I've used them for, well -- since the 60's, and wouldn't be without one.