Beginners’ Guide to Telescopes

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 The eye-pieces are very important.  They control the magnification you will see and can make a big difference to the quality of the image.  Eye-pieces are labelled with a size in millimetres which is their focal length.  All you need to remember is that the smaller the eye-piece focal length then the higher the magnification eg a 4mm eye-piece will magnify 5 times more than a 20mm.  Also, the more magnification then the smaller the field of view (and the harder it is to find and keep a target in view) so you will often begin an observing session with a longer (lower power) eye-piece like a 20mm or 25mm then switch to a higher power (eg 10mm or 4mm) for more magnification.  One tends to switch eye-pieces a lot when moving from target to target during a session.  Eye-pieces slot in and out of a holder on the scope near the focussing knob.  A starter telescope will include one, two, or three eye-pieces, and you can buy additional eye-pieces in a wide variety of focal lengths, qualities, and prices.

 

On a refractor or Mak the eye-piece holder is usually part of a piece called a ‘diagonal’ or star-diagonal which contains a mirror or prism to turn the light path through a right-angle so the eye-piece is held at a comfortable viewing angle for you.   Starter refractors and Maks usually have a diagonal included.

 

The eye-piece in use determines the magnification you will see.  To calculate the actual magnification divide the focal length of the main telescope (eg 700mm) by the focal length of the eye-piece – so a 25mm eye-piece will magnify x28 in a 700mm scope, and a 10mm eye-piece will magnify x70.   Adverts for reputable scopes will seldom mention magnification but always tell you the focal lengths (or focal ratio) of the main scope and eye-pieces included.  You can safely ignore ads for cheap telescopes that talk for example about ‘675 times magnification’.  In fact there is a practical limit to the maximum magnification that a telescope can deliver which you can take as 2 times the aperture in millimetres.  So a 90mm aperture would have a practical limit of 180x magnification.

 

 

Finally, by way of background, to have fun remember some other essentials –

Dress properly for the conditions.  It can be very chilly after dark and star-gazing is no fun if you’re freezing.  In cold weather a hat is essential.

Never look directly at the Sun or via binoculars or a telescope without a proper solar filter!

Have something to help find your way around the sky – a star chart from a book or magazine (eg Astronomy Now) or a planisphere.  You can download some excellent free sky chart software from www.stellarium.org.   If you don’t have some sort of sky chart you’ll soon run out of things to look at.

People differ in their eye-sight.  Some have much better sight and will be able to see things that poor moles like me can only imagine!

The condition of the atmosphere and local lighting makes a big difference to what can be seen through a scope.   In towns and cities or near golf ranges etc watch out for light pollution..  You’ll see a lot more from a dark site compared to London where you can’t even see the Milky Way.

 

End of background!   Sorry about all that pre-amble but it will give you a better understanding of what you’re heading for. 

Now, what can you get for your money —

 

Eye-pieces

Star diagonal

Overview p3 / 4

Overview