A brief guide to optical instruments for beginners

by Jason Woolley

1. Binoculars

Don’t ignore the humble binoculars – they’re often better than the cheaper types of telescopes. 8×50 or 10x50s are probably the most popular. Don’t be tempted into getting too large a pair as they become heavy and unwieldy.

Very wide field. The ultimate ‘grab and go’. Relatively inexpensive – I think I paid about £150 for a good pair but my favourite wide field ones came from a charity shop for £10!

What you can see with them:
Great for rich star fields, a couple of the larger galaxies (Andromeda), some nebulas (Orion Nebula). The Moon, Jupiter and it’s moons, all of the other planets – don’t expect to see any detail on the planets though.

2. Refractors

These are the type of scope most people think of when you mention telescopes. Simultaneously they can be the cheapest type (often on sale in supermarkets – best avoided) and the most expensive.

The type we’re talking about here are the good quality short focal length refractors with an objective diameter of between 70 and 90mm 

Wide field – don’t underestimate how difficult it is to find objects in  the night sky. As the focal length increases so does the magnification and the field of view decreases. Refractors with a short focal length (often called ‘rich field telescopes’) are great for  wide fields of view and so it’s quite easy to track down the fainter objects. Another huge advantage with them is that they are relatively light and portable this is especially important when you have few opportunities to observe (due to commitments or more often the weather).

It’s true that the best instrument you have is the one you use most often – I have a set up like this – it’s the smallest and importantly most used.

What you can see with them:
Great on the Moon, The brighter nebulas and galaxies. If you’re into photography they’re perfect for taking pictures of the night sky – the better quality instruments are also pretty good on the planets (see note below) and you will be able to increase the magnification (via the eyepiece) to see details on some of the planets. The rings of Saturn and its largest Moon Titan for example are very obvious.

‘Chromatic aberration’ Refractors have to be ‘colour corrected’ as cheaper models create rather alarming violet halos round the brighter objets. Generally the better the colour correction and larger the diameter of the objective the more you will pay for the instrument. look out for the terms ED in the description generally this will have good correction. Apochromatic or ‘Apo’ refractors are the best but you’ll pay a high price for them.

Cost: £150 for the less expensive types (with a mount!) to £600 (tube only) for a good quality ED short focus refractor.

Places to buy: Have a look at companies like ‘William Optics’ at First Light Optics for the higher range instruments.

‘Sky Watcher’ have some good price refractors.

3. Reflectors

Where Refractors use lenses, Reflectors use mirrors. The most common type of reflectors are called Newtonian telescopes. You may come across various different types particularly ‘Dobsonians’ which refer to a very economical type of Newtonian telescope on a very simple mount that can be set up in minutes.

The fainter the object you want to look at the more light you need to collect and this is where using a mirror comes into its own. Very good Newtonian telescopes are common place at 6, 8 and 10” for a few hundred pounds, a comparable colour corrected refractor can cost tens of thousands!

What you see with them:
The larger instruments are are often referred to as ‘Light buckets’ and they are perfect if ‘Faint Fuzzies’ i.e. Galaxies and Nebulas are what you’re into. It’s a common mistake that magnification is the most important aspect of telescope – in fact it’s probably aperture. An example would be the Andromeda Galaxy an ‘Island universe’ of about 1 trillion stars and the closest galaxy to our own – It’s actually appears in the sky about 6 times the size of the full Moon! It’s just very faint.

They take a bit more looking after than other types of scope in particular the optics need to be well aligned (collimated) and though fairly easy to do it needs doing regularly. As the mirrors are open to the elements these will also need re-coating from time to time.

An important point: More important still than aperture is the quality of the sky – It doesn’t matter how large the telescope is – if your sky is heavily light polluted you’re not going to see Galaxies easily – it’s probably best to stick to the Moon and planets – unless you’re willing to travel.

4. Cassegrain Telescopes

These are usually compact telescopes with a combination of lenses and mirrors. They are usually characterised by having quite short bodies but as the optics are ‘folded’ they have relatively long focal lengths.

What you can see with them: The smaller instruments (about 5”) are great for the Moon and planets – instruments this size often have focal lengths of around 2 meters and so you will get a good image size of say Jupiter (even with a low power eyepiece) the banding should be obvious on the planets surface and with favourable conditions you should be able to see the shadow of the moons crossing and even the ‘Great red spot’ (actually quite difficult to see!). Saturn’s rings are mesmerising!

The larger instruments of around 8” are as close as you get to an ‘all rounder’ and are good for nebulas and galaxies too.

One particular advantage is cost. The instruments are greatly favoured by the large american companies and so you can get fantastic packages with computerised ‘GOTO’ mounts and even photographic packages with automatic guiding included at a relatively low price.

The smaller instruments (about 5”) are highly portable and are evan available with GOTO mounts at a very low cost.

Personal note:
This was the first telescope I had and on the first night saw Galaxies, planets, Globular star clusters etc. etc. The computer just points from one object to the next and the controller includes tours like ‘tonights best’. A bit of snobbery surrounds these things and some will say there’s nothing like knowing the night sky – in some ways they’re right but if you’re short of time and don’t want to spend hours hopping from one star to another to find a ‘faint fuzzy’ it’s a great advantage just to have to press a few buttons and automatically go straight there. 

I sold my 8” LX90 (pictured) a few years ago – and have always regretted it.

Due to the long focal length the field of view can be quite small and so low power eyepieces are necessary for wide views – these can be quite expensive. 

The LX90 and scopes of this type are heavily dependent on batteries or a power pack.

Take a look at: Meade Instruments

Unfortunately the telescope is only 1/3 of the story – It’s often said that a telescope is only as good as its mount and this is true – there is no substitute for a good solid mount.

There are two main types:

ALT-AZ Mount. 
These are the most simple type which move the telescope through an x,y type movement. Most are unpowered and are most suitable for lower power Refractors. They are fantastically easy to set up and often require no more than dropping the tripod on the porch before you start viewing. 

There are versions that with varying degrees of complexity can track the sky. Quite inexpensive versions can be bought which as the Earth turns will counteract the movement. This is essential when using high power instruments with long focal lengths (i.e. scopes for planets). The smaller cassegrain instruments are often sold quite inexpensively with this type of mount.

If you want something a bit more sophisticated there are also quite a few GO-TO Alt-Az mounts available too. One important point with these is that if you do run out of batteries they pretty much useless.

Equatorial mounts:
These are rather elegant pieces of equipment which have an axis which is oriented parallel to the Earth’s axis – only one control is required to counteract the rotation and this can easily be motorised. This is a great advantage when looking through the scope as even under low power it’s quite surprising  how quickly an object and move and be lost out of view. 

Again GO-TO versions are available – these tend to be more expensive than the ‘Alt-Az’ types but if you run out of batteries they can be manually controlled.

Many of the above optical tubes are supplied with either of the two main types of mount.

The Eyepiece is really half the optical system and is just as important as the telescope.

There are a bewildering amount of types and you can spend anything from £20 to £700 on them. Really to some degree you get what you pay for … but below is a brief summary of some of the more common types:

Orthoscopic: Good image quality but narrow field of view – Ideal for planets. Long eye relief so good for people who wear glasses.

Plössl: Good field of view. Good quality and relatively inexpensive eyepieces as they are the mainstay of some of the larger corporations. The eye relief can be quite short and can become uncomfortable at higher powers especially if you wear glasses. 

Nagler: Ultra wide field of view – often described as giving a ‘space walk’ experience. the main disadvantage is that they are hugely expensive.

If you’re feeling flush, visit  the Televue website: www.televue.com 
These are some of the best eyepieces you can buy … but there are plenty of less expensive ones that will give very satisfying views.

There are many other suppliers in the UK – such as:

Some good makes:
Televue www.televue.com
Sky watcher www.skywatcher.com
Orion Optics (Crewe) www.orionoptics.co.uk
Meade www.meade.com
Celestron www.celestron.com

Just a few basics: 
The magnification of the optical system is equivalent to the focal length of the telescope divided by the focal length of the eyepiece so for a 500mm refractor with a 10mm eyepiece the magnification would be 50x – Don’t be tempted by excessive magnification. 

It follows that the shorter the focal length of the eyepiece the higher the magnification.

Generally the lower power eyepieces with wide field of views are the more expensive but often give the more satisfying views 

Also a wealth of information can be found by visiting the various societies there are a dozen or so in the North West.

Good luck!