Hi there, welcome to Part five of the video series, “Telescope Basics.”
My name is David Fuller, host of the “Eyes on the Sky” weekly stargazing series
that can be found on eyes at the sky dot com.
In this video, I will explain what a barlow lens is and how it works – and what the
benefit is to you in owning one.
A barlow lens something known as a “telenegative amplifier.”
That's a fancy way of saying it increases the magnification of your existing eyepieces,
simply by placing the barlow between your telescope's
objective lens or mirror, and the eyepiece you view through.
So if your 900 millimeter focal length telescope produces 90 times magnification with a 10
millimeter eyepieces, a 2 times barlow can produce 180 times magnification.
Why not just get another, shorter focal length eyepiece?
Well, a barlow effectively doubles your eyepiece collection, for starters.
Let's say you have the following three eyepieces: 25, 16 and 10.
If you were to get the same doubling of magnification, you would now have to purchase 12, 8 and 5
millimeter eyepieces.
But with ONE barlow lens, you can effectively get that same magnification with the 25, 16
and 10.
In other words, it's a lot less expensive way to increase your
eyepiece collection.
There is a caveat here though, that is both good and bad.
Using a barlow increases the eye relief of a given eyepiece.
For shorter focal length eyepieces, like 10, 12, or 16, this can be quite beneficial,
as it is like using a 5, 6 or 8 millimeter eyepiece with greater eye relief.
The problem comes with longer focal length eyepieces; this increase in eyepiece
is even greater with longer focal length eyepieces.
Since a 32 millimeter eypiece, for example, may have
an eye relief of 16mm – a normally comfortable distance – a barlow may now cause it to
have 32 millimeters of eye relief.
This can actualy make it difficult to see the entire field of view, because your
eye is so far away from the lens.
One thing to remember is that this effect is greater with shortie barlows
than with longer ones.
Either way however, this is a good thing for shorter focal length eyepieces where eye relief
tends to be short, and barlows are typically of best use anyway.
So what kind to get?
There are short and long versions, achromatic types, and various factors of magnification,
like 1.8, 2 and 3 times, just to name a few.
Partly this will depend on your scope: Short ones are usually required for reflectors,
as they tend to work better in the shorter focusers.
Refractors, on the other hand, are often the best fit for the
longer ones, but this is not a hard and fast rule.
And by the way, to get more magnification place the barlow AHEAD
of the star diagonal when using them with refractors, to get the maximum effect.
Whatever you do decide, make sure to choose one that does not create the same
magnifications for eyepieces you already own.
For example, getting a 2 times barlow if you already own 20, 10 and 5 millimeter eyepieces
won't help you much, as the 20 and 10 eyepieces when used with
a 2 times barlow would be identical to the 10 and 5.
In that case, a 3 time barlow might make more sense.
Conversely, you may not need to purchase eyepieces for
magnifications you can achieve with an already purchased barlow, so owning one early can
help with making future eyepiece decisions.
So what is most important to look for?
I'd say fully-multi-coated lenses.
By adding another set of lenses into the optical system, there is potential light loss.
By using a higher-quality barlow with minimal reflections, you minimize stray and reflected
light, maximize light throughput, and also -very likely - get a better
quality piece of equipment.
Eyepieces may come and go, but many amateur astronomers hold on to quality barlows for
a long time.
Well worth it to spend a few extra dollars and get a quality one.
So that is a quick look at barlow; in the sixth and final video in this series, I will
cover various finderscope options.
And for planets you may want to use high magnification to see, check out this week's
“Eyes on the Sky” videos at eyes on the sky dot com.
Thanks for watching; keep your eyes on the sky and your outdoor
lights aimed down, so we can all see, what's up.