(Note by MG, 11th May 2013: The prices quoted in this article were accurate as of December 2008 but will likely have changed by now! It’s possible some of the prices given will have dropped as new brands/models have become available in the intervening time.)
If you’re new to astronomy and you’re thinking of buying your first telescope for Christmas then this article should help you choose an appropriate instrument.
Firstly let me say don’t buy from anyone advertising huge magnifications and has glorious 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. Come along to a society observing session and see telescopes galore for yourself. Society members are very approachable and will be happy to help.
Telescopes are expensive and if you decide at some point later that astronomy is not for you then you’re stuck with it (there’s always e-bay today of course!). Ifyou’re not sure then it’s much better to spend your money on a decent pair of binoculars. 10×50 are the most popular (10 means they magnify by 10x and 50 is the diameter of the optical lens). A reasonable pair of 10×50’s will cost between £40 and £100.
You can search the night sky and spot planets, nebulae and star clusters with binoculars. They are cheaper to buy, easy to use and easy to transport. Then if youdo find astronomy is not for you, binoculars have many other uses.
Of course binoculars don’t let you observe quite like a telescope. With a telescope you can have a steady image, vary the magnification and gather a lot morelight to allow you to see much more.
I have bought several telescopes since the mid-90’s. My first was an Orion Optics Newtonian 8” reflector followed by the first 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.
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 – dobs, GEM or altaz. Goto computer or not
- The tripod or pillar – portable or permanent
The best telescope for you is the one you’ll make most use of. Do your research before you buy one, not after! Some good websites 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!
So what does focal length and f/ratio actually mean?
As with camera lenses, f/ratio is focal length divided by the aperture in mm.
So my Celestron 9.25” SCT has a focal length of 2350mm and an aperture of the primary mirror 235mm (9.25”) making it an f/10 scope.
Focal length is the distance the primary objective lens or mirror focuses light to a point. Aperture is the diameter of the objective mirror or lens. f/ratio = FL / A
More significant is why it’s important. Lower f/ratio’s have “faster” light gathering systems. So f4 is faster than f10 and an f10 telescope will have a smaller field of view than a f4. Aperture is the major 15 component of light gathering abilities – bigger the aperture, the better the light gathering ability but also read on as it also depends on OTA type.
Try this link for more information on telescope design, focal length and f/ratio http://www.astro.ufl.edu/~oliver/ast3722/lectures/Scope%20Optics/scopeoptics.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. When purchasing or using eyepieces in your telescope knowing the focal length of your OTA allows you to calculate the magnification a particular eyepiece will give you. For instance in my Celestron 9.25” SCT with a focal length of 2350mm, a 12.5mm (fe in the diagram) eyepiece will give f/fe (see diagram above) or 2350/12.5 = 188x magnification. Putting the same eyepiece in my Borg 77mm refractor f6 with a focal length of 500mmgives 500/12.5 = 40x magnification.
So focal length is important, equally so is aperture for light gathering abilities, f/ratio describes the relationship between the two elements.
Eyepieces design, types and manufacturers is a complete article in itself. I’m just going to quote from the Highlands Astronomical Society telescope basics website http://www.spacegazer.com/
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 below, you would expect this distance to be the same as the focal length(fe) of the eyepiece (E) itself, which is indeed the case.
OTA – Optical Tube Assembly
A Newtonian reflector is a good value OTA.
The tube has a primary mirror at one end and a secondary at the other.
This type gives you the best price vs aperture size value so you can consider 5” (130mm) diameter mirrors for lower prices than the other OTA types.
More info here > http://en.wikipedia.org/wiki/Reflecting_telescope
In the diagram above, light enters the tube from the left and is reflected off the primary mirror up to the secondary mirror then the image is observered in the eyepiece.
Reflectors (also called Newtonians) are useful for deep sky observing. They are usually around f4 to f6 which is a good ratio for light gathering and hence observing fainter objects. Reflectors usually give you a wider field of view than SCT’s so again, suitable for deep sky observing. Add to that the lower price per inch diameter then if deep sky is your main interest then a reflector would suit you.
5” reflectors start at about £140 from Celestron and other manufacturers. There are usually very reasonable Synta, Chinese built models that are good value for money also available. There are quite a few brand names that use the Synta manufactured tubes. Orion Optics is a local Crewe based manufacturer of Reflectors. I wouldn’t begin to pretend to be a mirror expert but the society has some very experienced amateur telescope makers that can talk about mirrors.
The refractor is what most people recognise as the “typical” telescope design.
In the diagram above, the light enters the tube from the left and is refracted by the objective lens to a point. It is then usually reflected and focussed to the eyepiece.
Lower priced refractors can suffer from poor optics and avoid plastic optics completely. Optical lenses can vary enormously in quality. The better ones usually from companies like Televue will cost between £700 and £1500 for 70-80mm diameter objectives. These are referred to as APO refractors.
Lower quality optics have a problem focussing all the various colours to the same focus point. This results in slight distortions of the image. Some bright objects will have a violet tint around the edge of them. This varies depending on the quality of the lenses. My Borg 77mm has a very slight violet tint around the edge when observing the Moon. Invisible most of the time but slightly apparent with the bright the Lunar image.
More here http://en.wikipedia.org/wiki/Chromatic_aberration
Refractors will give you sharp clear images. If you haven’t £1,500 spare to buy a Televue APO then fear not. Synta refractors such as Evostar, Startravel and Starlight are fine scopes from around £129.The Synta made Chinese refractors are very good value for money and a good price for beginners.
Otherwise stick to the well known brands like Televue, Williams Optics, Hutech-Borg, Vixen, Celestron and Meade for higher quality more expensive models. My Borg 77mm Achromatic is a compromise between price and APO quality.
Small refractors are very portable and give good image views.
F/ratio tend to be low so give a wide field of view, the image can be magnified up depending on lens quality and seeing conditions. Theory is that a good 3-4” refractor will be as good as an 6-8” reflector as they have no secondary mirror obstruction.
Refractors can also be larger, higher f/ratio design too and these types have very good quality images but can be quite long.
More information here http://www.astronomics.com/main/category.asp/catalog_name/Astronomics/category_name/Why%20buy%20a%20refractor?/Page/1
Schmidt-Cassegrains & Maksutov-Cassegrains are the third OTA type that are popular with today’s amateur astronomers.
These are known as Catadioptrics 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 hole in the primary mirror to the eyepiece. The light path is “compressed”. This type of design gives longer focal lengths and higher f/ratios of f10 which means higher magnification.
SCT’s will have narrow field of views, give excellent images of planets and will give large magnification – but then I’m biased as these are my favourite scopes! Not good for wide deep sky objects but good for star clusters, planets, Messier objects http://en.wikipedia.org/wiki/Messier_object and deep sky in general.
In my opinion Catadioptrics 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’s with Orion Optics also now including a
Maksutov type in it’s catalogue. Synta branded tubes are also found.
These type of telescopes have 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 shaped corrector plate.
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. 8” aperture models are nice and portable and fit in a car quite easily. Up to 10” is still very portable but anything over that starts to get a bit big. I like the size of my 9.25” 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 smaller ones will give dimmer views of the fainter objects, don’t rule out 90mm apertures. Indeed, I had taken it to Spain on holiday several times and the views of the Andromeda galaxy were superb.
And not forgetting finders.
Finders can be overlooked but a good finder scope is essential. You use you finderscope to aim the telescope. Today there are other options such as red dot pointers and even laser pointers. All are fine and the smaller scopes can benefit from red dot pointers or laser pointers. But a traditional straight finder is the best option if available. Personally I’m hopeless at aiming telescopes so went for a computer goto making the finder only necessary at alignment time. My small scope has a laser pointer attached to it. That way I have a chance of actually observing rather than spending ages trying to find objects!!
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 Amatuer telescope makers, the dobsonian mount. Alt-az mounts are also popular with SCT manufacturers such as Meade and Celestron.
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 OTA of say 10” is a very simple instrument to use. 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 £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-shapedaltaz 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.
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 slow turning the altitude and Azimuth motors by the same rate. This does produce a step type movement albeit very small steps!
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. It is now called 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 objectin the eyepiece view. So an equatorial mount can keep in sync with the object you’re observing by moving the RA axis. This is 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.
Click here for a good basic guide to aligning an equatorial mount http://www.adur-astronomical.com/simplepolar/simple_polar_alignment.htm
So to summarise telescope mounts, out of the 2 types the Altaz type tends to be simpler and less hi-tech like Dobsonians.
The Meade and Celestron U-shape however is an ideal match to their SCT telescopes making for a portable hi-tech instrument from 90mm up to 16”.
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. Very accurate alignment will keep the object in the eyepiece for as long time if the mount tracks and is used for astrophotography.
Below is a typical equatorial mount – manual with slow motion.
Tripods and Pillars
The third element of a telescope system is the pillar or tripod. 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.
Right – a typical Celestron Newtonian system from David Hinds with a typical tripod >>
So do you now have a better understanding of what kind of telescope you’re going to purchase? Come along to Pex Hill any Wednesday evening and see for yourself. Don’t forget to read the Astronomy magazines for reports and adverts.
Here’s some good websites to help you further:
- Cloudynights –great review site
- Celestron UK
- Meade UK
- Orion Optics
- Scopes n’ Skies
- Telescope Warehouse
- Stockport binocular and telescope centre
- SCS Astro