Beginners’ Guide to Telescopes

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More on Image Quality

APERTURE SIZE — is the diameter of the main mirror (called the primary) or lens (called the objective).  The brightness & contrast of the image you see is determined in large measure by the aperture size, the bigger the better for image quality.  Go for the largest aperture size you can, subject to the other considerations here.

You will find the aperture size of a telescope listed in millimetres (mm) or inches (in).   Manufacturers often offer the same model in a range of different aperture sizes.  Light-collecting power goes up according to the square of the aperture size, so a 102mm objective or primary has about double the light-collecting power of a 70mm (roughly 7,850 sq. mm v. 3,850)

Watch out for restricting ring baffles called ‘stops’ deliberately put inside some cheap refractors to disguise false colour fringes.  These reduce the effective aperture making a 70mm scope into a 50mm for example.

You can get more aperture for your money with a REFLECTOR than with a refractor (refractor lenses have two optical surfaces to make and so are broadly more expensive than reflector mirrors which only have one optical surface).  Dobsonians (Dobs) are liked because they are a cost-effective way of offering a big reflector on a simple mounting.

Patrick Moore’s advice is to go for at least a 70mm / 3-in refractor or 150mm / 6-in reflector if you can afford it.

The PRACTICAL LIMIT OF MAGNIFICATION is also determined by the aperture.  The practical limit of magnification for any particular scope can be taken as 2 x per millimetre of aperture (50 x per inch),  so a 70mm scope will have a practical limit of about 140 x magnification.  If you try to use higher powers of magnification than this, the image will be dim and washed-out, unless the scope has very high-quality optics (much more expensive).   Even the practical limit may not be obtainable with less expensive scopes or when the conditions (seeing) are poor.

The ACTUAL MAGNIFICATION observed is determined by the EYEPIECE in use and the FOCAL LENGTH of the scope. The focal-length is the distance taken by the optics to bring an image to focus, and is usually listed in millimetres.  In practice the physical size (length) of a refractor or Newtonian reflector will be close to its focal length, but the tube of a Mak (catadioptric) or ‘compact’ or ‘short’ Newtonian (really a kind of catadioptric) could be just half its focal length — much less prone to vibration and more portable.   Actual magnification is calculated like this:

Times Mag. = [focal-length of the main lens or mirror] divided by [focal-length of the eyepiece]

So a scope of main focal-length 1000mm used with a 20mm eyepiece will give 50 x magnification.  An eyepiece with shorter focal-length will produce higher magnification so a 10mm eyepiece will deliver twice the magnification of a 20mm. 

EYE-PIECES are easily interchangeable during use, and can make a big difference to image quality.  Typical starter telescopes will be supplied including two or three different size eye-pieces.   You can easily swap eyepieces in use, but for all normal purposes the focal-length of the main scope is fixed.  Look at the number and quality of eyepieces supplied with the scope.  The focal length of each eye-piece is stamped on it — typically 20mm, 15mm, or 10mm with a starter scope.

Focal length also determines the FOCAL RATIO or f/NUMBER of a scope (photographers call this the ’speed’ of a camera) — a measure of its ability to deliver bright wide fields of view or higher powers of magnification.   The f/number is calculated like this:

f/number  = [focal length of the scope] divided by [aperture size of objective or primary]

so a scope with focal length 700mm and aperture 70mm will be f/10 (700 / 70).  Scopes with small f/numbers (eg f/5) can deliver bright wide fields of view but will not support higher powers of magnification so well.   Scopes with higher f/numbers (eg f/11) can support generally higher powers of magnification (subject to the practical limit) but with a smaller field of view.  Higher magnification may be important, for example, to show detail on planetary targets.   Regardless of f/number, the practical limit of magnification for any scope is as described above.

Consider the QUALITY OF THE OPTICS.  Higher quality lenses, mirrors, and eyepieces will deliver better images, at a price.   The optical quality of the kind of budget starter scopes discussed here (costing about £100 to £250) is usually acceptable for a beginner, and gives value for money.  You may want to upgrade as you gain experience.  Cheaper refractors suffer to some degree from ‘chromatic aberration’ which causes false colour fringes to be seen around the image.  An ‘achromatic’ lens has been built to partly minimise this, but an ‘ED’ (Extra-low dispertion glass) achromatic will be much better, and an ‘apochromatic’ or APO will not show noticeable false colour fringes (but at much higher price).

Look for A FIRM MOUNT with the minimum of vibration and shaking.  When the scope is touched to move it, focus it, or by accident, the tube will shake and the image will jump and blur which can be very annoying.   Make sure the mounting is firm and stable with minimum vibration when the scope is touched.  Short tubes as on catadioptrics will generally be less prone to shaking than longer refractor or Newtonian tubes.

OBSERVING CONDITIONS significantly affect what you can see.  In towns and cities there’ll be significant light pollution and you shouldn’t expect to see faint ‘deep sky’ objects.  You may well want to take your scope out into the countryside where it is darker (see ‘portability’).  The state of the atmosphere (called the ‘seeing’) changes over time, and will also affect image quality.

Your eye-sight will also, of course, affect what you can see.   There is a big difference between what images can be seen by different people.

More on IMAGE QUALITY.  What factors influence the quality of the image you’ll be able to see?