What eyepiece shall I buy?

[Robin Scagell]

This is a common question on astro forums. People who have just bought
their first telescope soon wonder if they should buy a new eyepiece to
give them better views. Then the questions are what eyepiece to buy,
and whether the eyepieces supplied with the original telescope are
good enough.

Many of the more expensive Celestron or Meade telescopes come with
only one eyepiece, usually 26 mm focal length. Most complete Sky-
Watcher telescopes are supplied with two eyepieces, typically 25 mm
and 10 mm, with a Barlow lens that doubles the power of any eyepiece
it is used with (known as 2x). That is, it halves the effective focal
length, so the 25 mm eyepiece behaves like a 12.5 mm eyepiece.

You probably know that the magnification of an eyepiece depends on the
focal length of the telescope. So if you use a 25 mm eyepiece on a
telescope of 1000 mm focal length, the magnification (or power) is
1000 divided by 25, which is 40. A 10 mm eyepiece will give a
magnification of 1000/10 = 100 on the same telescope. The crucial
thing to remember is that the magnification goes up as the focal
length of the eyepiece gets shorter. So the 10 mm eyepiece gives more
magnification than the 25 mm eyepiece.

So the first question is, will another eyepiece help? If you have only
one eyepiece with your Celestron or Meade, usually a Plössl, then you
will certainly want to get more eyepieces to increase the
magnification. The standard eyepiece with an expensive telescope from
these manufacturers is usually of good quality to start with. But
choose carefully. Most astronomers have a Barlow lens in their
collection, which typically is a 2x, though 3x are also available. So
it is not a bad idea to buy a good 2x Barlow to start with, which will
give you double the magnification instantly, and will also double the
magnification of any other eyepiece you may get in future.

But don't buy the cheapest Barlows. They will almost certainly be non-
achromatic, which means that they will give false colour. A good
Barlow should last you a lifetime of observing and will be a very
useful accessory. Expect to pay about the same price as a good digital
radio or compact camera for one (note that I give equivalent prices so
that they don't get out of date, and so that they apply in all
currencies.

Any subsequent eyepiece you buy should have a focal length which is
not close to half the focal length of any other in your collection, or
you will be wasting the use of the Barlow. For example, if you have a
25 mm eyepiece and 2x Barlow, don't get a new 12 mm eyepiece as it
will have almost exactly the same power as the 25 mm with 2x Barlow.
But a 9 mm or 10 mm eyepiece would be a good idea, as it would give
you a good range of different magnifications.

Incidentally, you can buy more powerful Barlows, such as 5x, but these
are generally intended for imaging purposes where you need a very long
focal length, and are not particularly helpful for giving a good range
of magnifications.

If you have one of the less expensive Sky-Watcher telescopes, such as
the popular 130M reflector, the eyepieces supplied to start with are
not bad, but they are not as good as those on more expensive
instruments. The Barlows are usually non-achromatic, so a decent
Barlow would again be a good investment, as well as a better quality
higher-power eyepiece such as a 7 mm. You may find that the existing
eyepiece are good enough for low power views used by themselves
anyway. In some cases, the instrument may not bear a much higher
magnification than that supplied anyway.

If you have a small telescope -- meaning one of less than about 150 mm
(6-inch) aperture, don't expect miracles by buying a much higher power
eyepiece. Consider what happens when you increase the magnification on
any telescope. Usually:

* The light from planets and other extended objects is spread over a
larger area so the view gets dimmer

* The field of view generally gets smaller so the object that was near
the centre may now not be in the field of view at all

* The focus position generally changes, and if the object is dim it
might be a faint blur that you can't see at all

* On an undriven telescope, the movement of object through the sky is
increased so it drifts out of the field of view more quickly

* Any vibration of the telescope when you touch it to focus or when
the wind blows is much more noticeable

* Any optical defects of the telescope will be magnified

* The 'seeing', or blurring caused by Earth's atmosphere, will be
magnifed.

As a result, that lovely view of Saturn or the Ring Nebula may now be
replaced by zilch, or a nasty blob. You need a lot of practice to use
high magnifications, and if the seeing is bad or you have a telescope
which is not up to it, the results may be very disappointing. You may
well get a better view using a lower magnification anyway.

So don't rush to buy an expensive new eyepiece for your cheap
telescope, and don't overstretch the capabilities of the instrument or
your local conditions. And don't just blame the quality of the
eyepiece if the results are not what you hope for.

Robin Scagell
16 December 2008

[suthers]

This seems extremely useful advice, so I am setting it up as a
standalone page on the forum too so that it may be easily found.

Paul