Buying a Telescope by Steve Southern (revised November 2013)

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If you’re new to astronomy and you’re thinking of buying your first telescope then this article should help you

Orion SkyQuest XT6 Dobsonian TelescopeShiny new telescopes do look very desirable and despite a levelling of price in recent years a decent beginners scope is still around £150.
A telescope does not always have to be the first thing to consider purchasing when you get into this great hobby. Experienced amateur astronomers often recommend starting with 10×50 binoculars if you’ve a fixed budget of under £100. Binoculars are easy to use and transport. If you eventually decide astronomy is not for you then binoculars are always useful to have and your money wouldn’t have been wasted.
You should also consider purchasing a decent book to guide you around the night sky and a plastic planisphere to help you locate the constellations. If you’d like more techbased aids then having a decent planetarium program for your computer or tablet is very useful. I have tried a few and have stuck with Sky Safari (Mac and mobile devices only) but other options such a Stellarium and Red Shift are worthwhile too. There are plenty of apps for your tablet or mobile.

So you’re going to purchase your first telescope and you want to know how to pick the right one for you. Be careful astronomy is addictive!

Philip's Planisphere (35 degrees South)Generally don’t buy a telescope from anyone advertising huge magnifications and has glorious colour nebula pictures displayed on the telescope boxes. If you do visit department stores or TV shopping channels for telescopes then make sure you know what you’re doing! Remember, when it comes to telescopes, size does matter and it’s more important to understand the light gathering qualities of the optical tube assembly (OTA) than any alleged magnification. You need the right advice from a specialist astronomy dealer such as Telescope House, Astronomia, Opticstar, Widescreen and Green-witch. Or go along to a local astronomy society observing session and see telescopes galore for yourself. Liverpool AS society members are very approachable and will be happy to help with advice.

I have bought several telescopes since the mid-90’s. My first was an Orion Optics Newtonian 8” reflector followed by the early Meade ETX90, then the goto ETX90. I sold all those and have now ended up with a Celestron SCT 9.25” and a Borg 77mm Achromatic refractor. I’m very happy with my choices. More than just mirrors and lenses – there are 3 major elements to deciding on a suitable telescope;

  • OTA – the optical tube assembly including finderscope. Size and type
  • The mount – dobsonian, GEM or altaz, motorised, Goto computer or not
  • The tripod or pillar – portable or permanent

More about those within the article and you can’t discuss telescopes without mentioning eyepieces so see within the article for eyepieces too.

Celestron CGEM DX 1400 HD telescopeThe best telescope for you is the one you’ll make most use of. Do your research before you purchase, not after! Some good websites later in this article will help you further. Everything in this article is my personal opinion so comes with a warning that you should research yourself and draw your own conclusions!

Most decent branded telescopes will give you many years of enjoyment and they will give you good images of planets and deep sky so don’t get too obsessed with the type of tube etc, it ends up mostly being personal choice with a bias to your favourite astronomical objects! Whatever you decide to buy, remember that astronomy is not an ‘instant‘ hobby and requires developing skills such as basic understanding the stars and constellations, learning how to actually observe through a telescope and learning how to make best use of your new telescope – including that goto computer control if your telescope has one!

The OTA – Optical Tube Assembly types

Telescopes come in many varieties, sizes and shapes so plenty to think about. Let’s start with the 3 main types of optical tube assembly;

  • The Reflector (also known as Newtonian),
  • The Refractor
  • The Cassegrain or SCT

All very different to each other and with different characteristics. The bigger the primary mirror or objective lens, the more light gathered by the OTA. This means fainter objects will be brighter and can be magnified more. It’s not all about magnification, more about that later.

Newtonian Reflector

Diagram showing how a Newtonian reflector telescope works

Diagram showing how a Newtonian reflector telescope works

This kind of telescope tube was invented by Isaac Newton, hence the name Newtonian. The tube of a Reflector (above) has a primary concave mirror at one end and a secondary flat mirror at the other. The eyepiece is located towards the top of the OTA. This kind of OTA gives very good value vs mirror diameter so typically larger primary mirrors would be affordable to you with a limited budget. The f/ratio of
these kind of OTA’s are usually known as ‘fast’ being around F3 to F5. Fast f/ratio means better light gathering abilities. So the characteristics of a Newtonian reflector are better suited for faint deep sky objects. The secondary mirror does slightly reduce the amount of light hitting the primary mirror. 5 – 6” reflectors are a good beginners scopes, not too big and would cost you around £150-£250. Reflectors are
usually easy to set up and are better for Messier and other deep sky objects. The bigger the mirror diameter the brighter the image and a larger 16” diameter reflector will cost around £1,600 to £2,000 but give you fantastic deep sky images.

The Refractor

Diagram showing how a refractor telescope works

Diagram showing how a refractor telescope works

In the diagram, the light enters the tube from the left and is refracted by the objective lens to a focal point. It is then usually reflected and focussed to the eyepiece. The eyepiece being at the end of the OTA.

Lower priced refractors can suffer from poor optics so avoid plastic optics completely. Optical lenses can vary enormously in quality. So avoid those £50 ‘toy’ refractor telescopes.

The high quality ones will cost between £800 and £1500 for a small 70-80mm diameter objective. These are referred to as APO (Apochromatic) refractors. A 90mm Takahashi will set you back about £2,000 and that’s just for the OTA! I have seen refractors that cost £25,000!

However do not despair as there are lower priced alternatives that also produce very good images starting from about £150 for 70-80mm objective lens. If you’ve a bigger budget then there are mid-range refractors that offer excellent optics for half the price of an APO. At around £450 for 70-80mm you’ll have a fine refractor with excellent images.

Decent refractors give sharp clear images as they do not have a secondary mirror in the light path so they are really good for planets and brighter objects. In simple terms the more you pay the more advanced the primary lens will be and the better the image.

Small refractors of 80mm and less are very portable and give good image views. F/ratio tend to give a wide field of view, the image can be magnified up depending on lens quality and seeing conditions. Smaller refractors are good wide field instruments but can also give lovely planetary views.

Cassegrain, Catadioptrics and SCT’s

Diagram showing how a Cassegrain telescope works

Diagram showing how a Cassegrain telescope works

This type of OTA is known as a Catadioptrics or Cassegrain or SCT and are amongst the most popular type of telescopes owing to their performance and relative portability. Light enters from the left and is reflected off the primary mirror to a secondary. This then aims the light through a central tube and a hole in the primary mirror to the eyepiece. The light path is “compressed”. The eyepiece is at the back of
the OTA. This type of design gives longer focal lengths and higher f/ratios of f10 which means higher magnification. These type of OTA’s will have narrower field of views, give excellent images of planets and will give large magnification – but then I’m biased as these are my favourite scopes!

SCT’s are very good for star clusters, planets, Messier and deep sky in general but with a higher f/ratio they have a narrower field of view meaning wider field objects may not fit and a focal reducer might be required at some point. In my opinion Cassegrains are the best all round telescopes and the nearest you’ll get to an “all purpose” scope – but they’re a little more expensive.

Meade and Celestron are the main manufacturers of SCT style OTA’s and there are other manufacturers too. This type of telescope has a corrector plate at the front of the tube assembly and this is the difference between a Schmitt Cassegrain and a Maksutov Cassegrain. The Maksutov has a concave shaped corrector plate to minimise the size of the central mirror.

Schmidt-Cassegrain Optical Tube Assembly (OTA)

Schmidt-Cassegrain Optical Tube Assembly (OTA)

Maksutov-Cassegrain Optical Tube Assembly (OTA)

Maksutov-Cassegrain Optical Tube Assembly (OTA)

Maksutov Cassegrain design tube on the right and the Schmitt Cassegrain on the left

Click here for more information on Catadioptrics
http://en.wikipedia.org/wiki/Catadioptric

SCT’s are portable by design and also very adaptable for permanent observatory use with the larger OTA’s. Meade and Celestron make 8” to 16” models usually mounted on an U-shaped altaz mount called forked mount. 8” aperture models are nice and portable and fit in a car quite easily. Up to 10” is still portable but anything over that starts to get a bit big. I like the size of my 9.25” diameter OTA as a reasonable size to
use and transport.

Smaller models of Maksutov type from 90mm are obviously extremely portable. One of the best images I ever had of Jupiter was through my old Meade 90 ETX so whilst the image of the fainter objects won’t be as bright as a larger scope, don’t rule out 90mm apertures. Indeed, I had taken it to Spain on holiday several times and the views of the Andromeda galaxy and Whirlpool galaxy were superb.

Smaller Masutov-Cassegrain telescopes are usually fork mounted and many have motor control or goto computers – more about those later. Prices start from around £350 for a typical Celestron Nextstar 90 with goto computer, alt-az mount and decent tripod – plus usually at least 1 eyepiece. Bigger SCT’s will cost you between £1500 and over £3000 so avoid those unless you’re a real enthusiastic amateur observer!

And not forgetting finders

Skywatcher FinderscopesFinders can be overlooked but a good finderscope is essential. You use your finderscope to aim the telescope. Today there are other options such as red dot pointers and ‘telrad’ types. All are fine and
the smaller scopes can benefit from red dot pointers. Avoid laser pointers, we’ve banned them from our observing sites for safety reasons. But a traditional straight through finderscope is the best
option if available. And don’t forget to align it once you’ve bought your telescope! For more info follow the urls below.
http://www.dummies.com/how-to/content/aligning-your-telescope-and-finderscope-for-for-st.html
http://www.firstlightoptics.com/finders/telrad-finder-astronomy.html
http://astronomia.co.uk/index.php/accessories/finderscopes/sky-watcher-red-dot-finder.html

So what does focal length and f/ratio actually mean?

Diagram showing focal length

As with camera lenses, OTA’s have focal length and f/ratio characteristics. The focal length is simply the length from the primary mirror or lens to the point were the light is focussed into a point. f in the diagram A is the diameter of the primary mirror or lens ie the aperture. f/ratio is focal length divided by the aperture in mm. ie f/A
So my Celestron has a focal length of 2350mm and an aperture of the primary mirror 235mm making it an f/10 scope (f/A = 2350/235). Simple!

More significant is why it’s important. Lower f/ratio’s have “faster” light gathering systems. So f4 is faster (faster meaning it gathers more light) than slower f10 and an f10 telescope will have a smaller field of view than an f4 so for the same eyepiece an f/10 will produce a larger image. Aperture is the other major component of light gathering abilities – bigger the aperture, the better the light gathering abilities of the OTA. Try this link for more information on telescope design, focal length and f/ratio
http://www.chuckhawks.com/telescope_focal_length.htm

Understanding the focal length and f/ratio helps you make decisions about what the scope will be best for and what kind to buy. Faster f/ratio’s tend themselves to fainter deep sky objects whereas slower f/ratio’s tend themselves to planets and brighter deep sky objects such as globular clusters. When purchasing or using eyepieces in your telescope knowing the focal length of your OTA system allows you to calculate the magnification a particular eyepiece will give you. For instance my Celestron SCT has a focal length of 2350m so a 12.5mm eyepeice (fe in the diagram) will produce x188 times magnification. Putting the same eyepiece in my refractor with a focal length of 500mm produces x40 magnification. Magnification is OTA focal length/eyepiece focal length (2350/12.5=188mag, 500/12.5=40mag). So focal length
is important, equally so is aperture for light gathering abilities, f/ratio describes the relationship between the two elements.

Eyepieces

Eyepieces design, types and manufacturers is a complete article in itself. When you’re starting off in astronomy the main consideration is to have at least 2 eyepieces of different focal lengths so you can alter the magnification of the image. A x2 Barlow lens would be useful too, this magnifies x2 any eyepiece inserted into it. Typical sizes to start off with are 25mm and 12mm. A good rule of thumb is no more than x50 magnification for every 1 inch (25mm) of aperture. There are many different eyepiece types and manufacturers, stick with the telescopes’ brand to start with and you should be fine. When you’re more experienced then look at the higher quality manufacturers such as Televue (www.televue.com) and the variety of eyepiece types.

I’m going to quote from the Scottish Highlands Astronomical Society telescope basics website……
http://www.spacegazer.com/
quote “Although eyepieces usually consist of more than one lens, they can be regarded optically as a single lens. In many books on telescopes, you will read that the eyepiece “magnifies the image which has been focussed by the objective lens”. This description, although theoretically correct, is a bit vague. It would be much better to say that the purpose of the eyepiece is to put the expanding light rays
beyond the focal point back into a bundle of parallel rays again, in order to reconstruct the image. For that purpose, the eyepiece needs to be placed exactly at the right distance from the focal point. Looking at the diagram above, you would expect this distance to be the same as the focal length(fe) of the eyepiece (E) itself, which is indeed the case.” unquote.

Telescope Mounts

Steve Southern with his Celestron 9.25inch SCT, mounted on a Vixen GP German Equatorial Mount with a Skysensor Goto hand computer and an Orion Field Tripod

This is me with my Celestron 9.25” SCT, mounted on a Vixen GP German Equatorial Mount (GEM) with a Skysensor goto hand computer and an Orion Field Tripod. An equatorial mount is one type of mount Telescope OTA’s can be attached to.

2 basic mount types are available today Alt-Az and equatorial. Either type can have computer controlled goto’s. Alt-Az is so called as it moves in Altitude ie up or down and Azimuth ie left to right. A particular kind of Alt-Az is very popular with Amateur telescope makers, the Dobsonian mount. Alt-az mounts are also popular with SCT manufacturers such as Meade and Celestron who use fork mounts.

A Meade LX90 on an Alt-Az mount

A Meade LX90 on an Alt-Az mount

A Newtonian Optical Tube Assembly (OTA) on an equatorial mount

A Newtonian Optical Tube Assembly (OTA) on an equatorial mount

A Dobsonian Alt-Az mount from Orion Optics

A Dobsonian Alt-Az mount from Orion Optics

Which mount do I choose?

The mount is pretty much personal choice based on price, usability and how “hi-tech” you want to go. A simple Dobsonian mount with a large aperture Newtonian reflector OTA of say 10” is a very simple instrument to use and most of your spend goes on the OTA mirror. Perhaps not for children due to it’s size but good value adult size instrument. Spending your money on aperture size instead of a more expensive OTA type or hi-tech mount means you can quickly and easily set up and point your 10” Dobs up to the sky and start observing. Typically a 6” dobs will cost around £200-£300 up to about £1,200 for a 10” mirror size. Dobsonians as they are called takes you to the world of amateur telescope makers. Take a look at the TROK telescope, a 30” home made dobsonian built by some LAS society members.
http://datscope.wikispaces.com/30+inch+TRO+Telescope
Altaz mounts generally take on the form shown in the diagram below left. They are typically simple to set up and use. The Meade and Celestron U-shaped altaz fork mounts are some what different but still the same basic design. The higher tech Meade and Celestron Altaz mounts shown below right have altitude and azimuth motor control or goto computer controlled.

A simple Alt-Az mount

A simple Alt-Az mount

The 12" Meade LX200 telescope, which was installed at the Leighton Observatory on the 5th of September 1999.

Left is a diagram of a simple Altaz mount.
Right is the Liverpool Astronomical Society’s 12” Meade LX200 located at the Leyton Observatory, Pex Hill. This image clearly shows the U-shape altaz mount of this type of telescope.
Altaz mounts are generally lower priced compared to equatorial mounts but can also be hi-tech. Altaz mounts have a problem pointing the telescope at the zenith. Goto technology means the Meade and Celestron type Altaz mounts can track the celestial object under observation as the night sky moves from East to West. Of course the sky isn’t moving the telescope is compensating for the
movement of the Earth by slowly turning the Altitude and Azimuth motors by the same rate. This does produce a step type movement albeit very small steps!


 

Diagram showing Celestial Sphere with Equator and poles

Diagram showing Celestial Sphere with Equator and poles

Equatorial mounts differ in that they require more setup. In northern England one axis is tilted to 53 degrees (your location latitude) and pointed at the North Pole in the sky. It is now called a Right
Ascension RA axis. What you are doing basically is setting up the RA axis of the telescope to be parallel to the north/south pole line. As the earth rotates, the RA motor of the mount moves at the same rate
keeping the celestial object in the eyepiece view. So an equatorial mount can keep in sync with the object you’re observing by moving the RA axis. The other axis is called the Declination axis. The RA & Dec
produce a smooth movement and very suitable for astrophotography. One other advantage is the object stays in the same orientation and doesn’t rotate in the eyepiece as an Altaz mount does. Pretty key for long exposure photography! On a personal note, I believe that an equatorial mount is more accurate when it comes to tracking the sky and slewing in goto mode over an Altaz mode.


 

Diagram showing a correctly set up equatorial mounted telescope with the RA-axis parallel to the Celestial North/South pole line of the Celestial sphere

Left is a diagram showing how a correctly set up equatorial mounted telescope has the RA axis parallel to the north/south line.

A Vixen GP equatorial mount

Right is a typical equatorial mount, a Vixen GP mount to be precise

Click here for a good basic guide to aligning an equatorial mount
http://arnholm.org/astro/polar_alignment/


A simple equatorial telescope mount, with manual and slow-motion controls

Above is a typical equatorial mount – manual with slow motion. After a few attempts and practice, setting up your scope will be a simple task. Do it during the daylight before attempting an evening’s observation.

So to summarise telescope mounts, out of the 2 types the Alt-az type tends to be simpler and less hi-tech such as Dobsonian mount, meaning the cost of the telescope goes on the mirror size. The Meade and Celestron U-shape forked mount however is an ideal match to their SCT telescopes making for a portable hi-tech instrument from 90mm up to 16” with well designed motors and/or GOTO computer
control. Equatorial mounts need a bit more setup, the RA axis must be pointed north at the pole star. The RA axis also needs to be parallel to the north/south pole line so a latitude adjustment is also needed. Very accurate alignment will keep the object in the eyepiece for as long time if the mount tracks and is used for astrophotography. They can also be simple manual movement or manual slow motion control. German Equatorial Mounts like the Vixen or other similar mounts can be easily motorised or computer control added – very often as a later purchase. The equatorial mount however takes a little more getting used to than an Alt-az type.

Motorised or goto

More to think about with mounts is whether you want to manually move the telescope, use a motor to move & track or go for a full computerised goto mount.

Again I’ll quote my own experiences, my Celestron has an equatorial Vixen GP mount with a goto computer control that drives a motor on each axis of RA and Dec (Right Ascension and Declination). So takes a bit of time to set up with polar alignment of the RA axis and then setting up the computer control by aligning it to 3 bright stars. Once that is completed then I can simply key in “Jupiter” or M31 and away it goes! Whereas my small refractor has an Alt-az design mount with manual slow motion movement so this is simple and fast to setup and easy to use (modern day term being a grab and go scope!!).

Tripods and Pillars

Just as important as the mount or the Optical Tube Assembly is the tripod or pillar. A good solid tripod goes a long way to eliminating vibration. In my photo earlier, the Orion Optics field tripod is a good solid setup – and it’s portable. A suitable tripod is a compromise between portability and a solid base to mount your telescope. Pillars these days tend to be permanent fixtures in observatories. Orion Optics used to make and supply a pillar with their Newtonian telescopes but these days they seem to include tripods. Tripods are more convenient for transporting. Always check whether a tripod ‘flexes’ and vibrates as this is a good sign of the build quality.

A typical telescope pillar

A typical telescope pillar

A typical Celestron Newtonian system from David Hinds with a typical tripod

A typical Celestron Newtonian system from David Hinds with a typical tripod

 

In summary…

So now you’re armed with all the explanations and jargon and are probably even more confused! Here’s my advice for a beginners telescope.

  1. Try to contact your local society for practical advice or an observing visit
  2. Go to a recommended telescope dealer
  3. Decide on what you want to observe
  4. Decide on your budget and this does reflect on your final decision.
    Recommend you don’t buy a telescope for less than £130. Don’t forget to ask for a good deal with extra things thrown in.
  5. Go for a 5-8” reflector and a simple mount to start with, maybe an equatorial motorised or goto. Priced between £150 and £300 (more if goto included)
  6. Or go for a small Cassegrain type on an Alt-az mount with computer control if your budget stretches to £350-£450
  7. Be patient with your new toy and give yourself time to learn the new skills required
  8. Popular brands are Skywatcher, Meade, Celestron and Orion

Here’s some good websites to help you further:

Cloudynights –great review site
http://www.cloudynights.com/

Celestron UK
http://www.dhinds.co.uk/

Meade UK
http://www.meade.uk.com

Sky and Telescope magazine-what to know
http://media.skyandtelescope.com/documents/AboutScopes.pdf

Good telescope dealers……
http://www.astronomia.co.uk
http://www.telescopehouse.com
http://www.green-witch.com
https://www.widescreen-centre.co.uk
http://www.opticstar.com