OASA Primer How to choose a telescope
A guide to choosing a telescope
One of the most frequently asked questions beginners ask is, what is the best telescope to buy. The answer is always the same, start with a basic star chart and pair of binoculars and get to know the night sky first. Then look to buy a telescope. If this is not you and you feel you want a telescope then read on.
Buying a telescope can be every bit like buying a car (though not as expensive ususally!) there are all sorts of pitfalls to avoid. As a rule of thumb as there are too many to go into for this small guide it is better not to get one from the high street. Get one from a reputable Astronomy retailer. OK, but which telescope? As with any guide, we are going to describe the different types of telescope out there and let you decide. The best telescope is the one you use!
Telescopes generally fall into three categories, we’ll look at each in turn examining how they are setup and what we can expect to see and get with them. We will then discuss the pros and cons of each type. These telescopes are a) Refractors b) Reflectors c) Schmitt Cassegrain telescopes
Refractors These are generally the more common and easy to use telescopes. No doubt when you come to view you will see the same kind of tube on two types of mount. These are the altazimuth mount (left) and the equatorial mount (bottom left). The difference between them we’ll discuss later, but for now let us keep with the basic telescope tube which is the refracting telescpe.
Refracting telescopes work as described opposite, Light comes down through a lense where it is refracted down (hence the name refractor) down through to a point, we call the focal point. A eyepiece will then be inserted to magnify the image so we see the object.
Below the top diagram shows how a basic refractor works in theory. In reality cheap telescopes made with a simple convex lense like this will split light like a prism. Each resulting colour of the rainbow will then focus at different points (see middle diagram). The bottom diagram shows how this is corrected. ++
How a refractor works
Colour faultage is a massive problem with simple cheap refractors. You can see both ends of the visible spectrum here focussing at different points.. Not good With another plano convex lense fitted it helps to ensure all rays of light focus at the same common point. (ALL OASA refractors have that second lense)
Achromatic Lense
These telescopes are intrinsically smaller than reflectors (which means they tend to build reflectors bigger for slightly more money). As such these telescopes are ideal for the planets, the Moon, even some deep sky objects like M31 and M42 may also be possible. These are brighter objects so less light is needed. For fainter objects, more light is needed so a bigger entry is needed. We’ll discuss more about that later So what are the pros and cons of this kind of telescope? Pros Generally cheaper Easy to setup Good for brighter objects Light, so also portable
Cons
Smaller so only brighter objects can be easily seen If lenses are cheap, colour faultage can be a problem Can be hard to track depending on the mount used.
Telescope mounts Even the best telescope in the world is of little or no use if the mount is rubbish. You need a good steady, easy to control mount to get the best experience. So lets look at these here now. Altazimuth mount You will notice with both telescopes the actual tube you look through is pretty much the same. BUT the mount they sit on (that holds the telescope is different). The image to the left is what is called an Altazimuth mount. Good to move up down, left and right
Equatorial Mount This mount is a little more different and though a little more complicated to setup. If done correctly you will maximise your observing experience. We will now discuss both in more detail
Altazimuth Mount Here we have blown the image up to get a better look at it. You can see there is a knob on the side to allow the loosening and movement of the telescope. You then tighten it off to observe. So the idea is, use your finder to centre, look down the main tube you should see what you are looking at in the finder down the main tube. Its like a little telescope piggy backing a ride. This telescope is a gazing telescope. Point and gaze, the object will drift out of field of view
Pros Easy to use Point and gaze No major setup time needed
Cons Not readily stable Susceptible to be blown about Not easy to track Glancing telescope only. Object will move out as soon as you have it in view
Equatorial Mount
finder worm gears
align to pole
As there is slightly more to an equatorial mount than an altazimuth we’ve enlarged and labelled it for you. The finder you will see are the same on one telescope as on the other. There is a worm gear and there is a telescope which is counter weighted by a small weight which we align to the pole.
The idea with the polar alignment is to get the telescope roughly aligned to the pole. So we need to get the polar axis in line to the northern celestrial pole as described below
Once the telescope is aligned as below (and you may need to tilt the mount further back than it is in the picture for higher latitudes) then you should only really need to move that worm gear we spoke of earlier to keep the object reasonably well in field of view. There are motors you can get as well to help you do this but that is beyond the scope of this guide. We’ll move on to discuss reflecting telescopes now but the principle of the mount is the same. By the way the purpose of the weight is to balance the telescope. Please see manufacturers guide on how to do that
So you may need to loosen the nut on the side to get the axis pointing to the pole
Reflecting Telescopes Reflecting telescopes work on the principle of light entering the telescope tube, reflecting off the main large mirror onto a secondary mirror, then up into the the eyepiece tube where the light is brought to focus. We then insert an eyepiece to magnify the light. The diagram to the right pretty well explains how one of these work The common name for one of these is also the Newtonian Telescope, named after Sir Issac Newton who first designed one. Asides the tube there is not a lot of difference between this kind of setup or the refractor.
How a reflecting telescope works
Due to the fact that it is cheaper (and easier) to make larger mirrors than the same sized object lense for a refractor you can see why reflecting telescopes are intrinsically bigger which makes them popular amongmore serious amateur astronomers. As you can tell from the diagram if either mirror is knocked out of alignment the image will be of poorer quality. We say the telescope is out of “collimation�. This is discussed later in this guide
Reflecting telescopes then have a number of advantages in that they are intrinsically bigger than refracting telescopes which means we can see fainter objects with them. So all said and done, what are the pros and cons?
Pros Intrinsically bigger than refractors See fainter objects with them Can do more involved astronomy as well
Cons Collimation* Re silvering More fragile Mirrors hard to clean if dirty
Collimation We cannot talk about reflecting telescopes without discussing this. For such a telescope to work properly, the mirrors need to be aligned. As in the diagram previously. If it isn’t then the object will not be seen properly, or if it is, it will not be possible to properly magnify the object to see detail. When this becomes an issue we will need either a collimation eyepiece or a laser collimator to help correct the badly aligned optics. It is an easy problem to remedy and should not mean sending the telescope back to the shop. But keep in mind if you’re buying one of these then get a collimator. Oherwise its like having a guitar with no tuner! On a plus side some telescopes are locked off so you should never need one. BUT i recommend getting one anyway especially if the secondary mirror is knocked somehow.
Eyepieces and other accessories When you purchase a telescope, it is most likely going to come supplied with a small range of eyepieces and a barlow lense. In some cases lunar and solar eyepiece filters will also be provided. I am sure i don’t need to say here you should never look at the Sun directly with a telescope as it can blind you. I would certainly by the same token NEVER trust a solar filter that came with a telescope unless i was totally sure. If they crack under the heat. Again it will blind you! Lunar filters are there to help dim the lunar light. These are perfectly safe to oberve the MOON (NOT THE SUN!!!) with.
Back to eyepieces as i said the telescope may well have arrived with a selection of eyepieces. They are more than likely going to be Plossl eyepieces. Possibly with the metal 1.25 barrel which makes them useable with other telescopes with an eyepiece barrel of that size. Also there will be some numbers on the side. Lets take a look at what these are
So the above is a typcial eyepiece which might come with your telescope. We see it has a metal barrel which is typically 1.25 inches in diameter. But we also notice it says 25mm? 25 mm is the focal length of that eyepiece (it will never say what the power is as that can vary from telescope to telescope). However if the focal length of your telescope is known it is possible to calculate what the magnification is. Focal length (telescope Magnification= ___________________ Focal Length eyepiece
So for a typical telscope with a focal length of 700mm what would the magnication be for our 25mm eyepiece?
Magnification = 700mm/ 25mm 28 x OK, but i have some 6mm and 12.5mm eyepieces as well? 700/6 = 116 x 700/12.5 = 56 x Do you see a trend emerging? Yes, the higher the focal length of the eyepiece the lower the power. And vice versa
The Barlow lense will have the effect of doubling this!
Star diagonal This is seen in the pictures, but what are they? What are thet used for? A star diagonal is only ever seen with telescopes such as refracting telescopes. As you can imagine if an object is way up in the sky, the eyepiece end is going to be virtually close to the floor. To mitigate the need to bend down the star diagonal allows for the light to be reflected from a prism (or mirror in cheaper versions) up to the eyepiece. Eyepiece inserted here Star diagnonal here (some contain a prism, cheaper ones contain a mirror)
Dobsonian telescopes Invented by the late John Dobson, this telescope incorpates the principle of the altazimuth we saw earlier with a massive light bucket of a reflector. These are plonk and go telescopes which combine their usually high aperture with simplicity and ease of use. They are excellent family telescopes and though large are generally quite portable. There is also not a lot out of range of the larger ones. They are not easy to attach motors to for long term tracking so not ideal for astrohotography or serious amateur work. That said one was used for hunting supernovae
So we’ve seen the two basic types of telescope an amateur might use. So which is the best type? Well that as the reader is the decision you must make. We’ve seen the refracting telescope both with an equatorial mount and simple but cumbersom altazimuth mount. We’ve also looked briefly as to what such an instrument can see. We have also seen conversely what a reflecting telescope is and does. The final decision is down to How confident you feel with something more advanced How knowledgable you are about the equipment What you want from the telescope. Don’t forget, the best telescope is the one you use!
Schmidt Cassegraine Telescope These telescopes are like a hybrid of refractors and reflectors in that that have the front lense like a refractor has (but its called a corrector plate). The main mirror does the same as the reflector does in this case. Take a look at the diagram. below
So as you can see the eyepiece is placed at the back end where the mirror is. See our friend the star diagonal is there too. These telescopes are often used by serious amateur astronomers but are a favourite with beginners as they are goto’s (a compurer controlled telescope that goes to target as desired by the user)
Final Jargon Buster Aperture This term is met and used often and used to describe the diameter of the objective lense of the telescope or the diameter of the main mirror of a telescope Eyepiece Used to magnify the image once its been focussed to a point by the telescope. The smaller the mm on the side the higher the magnification.
So what can we expect to see with these different telescopes? 50- 70mm refractor is ideal for Moon Can certainly see many of the fine features of the Moon well worth sketching Planets, Venus: just possible to make out the different phases Mars, can see the darker areas when it’s closer to us Jupiter, easily make out the belts and the Moon Rings of Saturn and some Deep Sky Objects. Great Orion Nebular, M42 Globular Cluster M13 Andromeda Spiral, M31 6 - 8 inch reflector Still good for Moon, Planets but more detail will start to be visible on the planets especially Jupiter and Mars Also more fainter deep sky objects will be visible, especially as you creep up to 8 inches 10 inch upwards Finer detail in the planets becomes visible and we start to resolve galactic structure in some of the closer galaxies like M31,
So what’s next? What we’ve tried to do is to offer an overview of some of the main telescopes and mounts used in Astronomy. We’ve looked at the pros and cons So what you want from the telescope will depend on what type you get. Remember though it is far better to buy a smaller easier to use telescope than a combersome instrument that needs loads of setup time and thus is never used. “The best telescope, is the one you use” The Online Astronomy Society Academy also offers a range of telescopes representing many of the types described in this small guide We also offer courses to aid people in how to use them (which in many cases are free with the telescope!)
Some objects to try for with a small telescope
Grosely over exposed image of Jupiter with its moons
Nice close up of the Moon, most decent small telescopes will show the Moon like this
Another shot of Jupiter with its Moons, these details have been recorded with a webcam
Another close up
Part of Jupiters family
M42, the great Orion nebula is always a delight in small telescopes. We see it in black and white though but you will recognise some of the structure
This is M51, the whirlpool spiral in the Plough. A ten inch would show structure. Its more fuzzy in smaller telescopes I have seen it in a 7x50- binoculars as well!
The first time i ever saw the lunar craters was at half phase, amazing view in any telescope
The Moon always presents something new.
M13, globular Cluster in Hercules, nice binocular object
Jupiter, imaged with the authors 8 inch telescope
Meet the Author and his beauty!
We hope you have found this guide useful, in it i have tried to answer many of the more common questions i get. I have also tried to make this self explaintory. Please support the Online Astronomy Society Academy by purchasing courses and equipment from us. It is not just about making money, though yes we have bills to pay, but its also about helping in the community. We hope this has aided you in making your decision of what telescope to get. There is one book we do recommend, Turn Left at Orion by Guy Consigmaglio who also sketches how many of the objects we’ve discussed in this guide and many more so people can see how they look through a small telescope.
www.onlineastronomycourses.co.uk We offer a variety of astronomy courses by distance learning
GCSE Astronomy Maths Astrophysics and Space Science A range of courses for the amateur astronomer Also offering a range of telescopes and other equipment to help the beginner get started