How The Telescope Objective Works
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.
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
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
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
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
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
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
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
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
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
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
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 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
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
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
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
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.