Page 39 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

FINDING YOUR WAY This Schmidt-

Cassegrain has
AROUND THE SKY an 8x50 fi nder

Most telescopes come with a second scope:
a lower magnifi cation fi nder

One of the most important parts magnifi cation. Only the
of a telescope is its fi nder. This cheapest of scopes comes
little gadget with its little bolts is with no fi nder or too small a There are many
the key to whether you will enjoy fi nder to be useful – be warned, fi nders to choose
observing or not. Finders give some are as small as a miserable from, such as this
you a wide fi eld view at a low 10mm. For the others, it’s just a laser pointer
magnifi cation, which you use to matter of setting it up accurately.
line up the telescope accurately You can always change a
on the object in space that you fi nder if it’s letting you down or
wish to view. A good fi nder will you’re a gadget fi end – there
have an objective lens above are some brilliant laser fi nders
25mm in diameter, with 5x or 6x around nowadays.

mirrors and lenses to do all that telescope focusing refl ecting, folded light-path optics mean what
stuff. There are several designs, all with great would have been a rather long telescope ends up
names – there’s the Schmidt-Cassegrain, the really quite neat and compact.
Maksutov-Cassegrain and the slightly rarer
and more specialised Ritchey-Chrétien. SCHMIDT VS MAKSUTOV
I say slightly more specialised, when I The Schmidt- and Maksutov-Cassegrain designs PROS & CONS
should say very specialised: Ritchey-Chrétien differ in two main areas. Firstly, the lens that lets
telescopes are found in professional observatories the light in is thicker and does a slightly different Pros
such as the Keck Telescopes on Hawaii, the Very job in the Maksutov, which makes it a heavier scope • Light and portable
• Great all-purpose
Large Telescope in Chile and the than the Schmidt. Secondly, because the Schmidt
telescopes
Hubble Space Telescope. It’s probably lens is lighter and does less of a job in bending the • Cheaper than a refra-
best to discount this design until you light, it needs a corrector plate along the light path tor of the same aperture
either a) own a volcano or b) have some to sort out the fi nal image. • Good for
means of putting your telescope into In other words, the Maksutov is a simpler design. astrophotography
orbit and controlling it remotely. Maksutovs do give very good images, but due to • Durable
So we’re going for a Schmidt- or a their heavier lenses they take longer to adjust on
Cons
Maksutov-Cassegrain, then. But why cold nights if you’re taking them out from inside
• More expensive than
should this be any better than a refractor – and their fi eld of view isn’t as large as you’ll get a refl ector of the
or a refl ector? Well, if you’re not into big with a Schmidt. So there’s really no substitute for same aperture
telescopes then the Cassegrains are just the trying out each design and seeing which one best • Secondary mirror
instrument for you. Their lens-bending, mirror- suits your observing needs. causes a slight loss
of light compared to
a refractor
• Will need collimating
Primary from time to time
mirror • Images not generally
Starlight as bright as other
Eyepiece Meniscus telescope types due
lens to the number of
optical elements
Coated spot

Starlight

MAKSUTOV-CASSEGRAIN
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