BBC Focus Magazine

[M Paul Lloyd]

Making the right choice.

The mistake many people make when choosing a telescope is to assume that magnification is the most important factor, but it is actually the amount of light that is conveyed to our eye from the distant objects that is of paramount importance. The average human eye has a light receptive area, or objective, in the region of only 2.5 cm which unaided covers some 67 degrees of sky and anything that improves on this is obviously going to be a distinct advantage. So the greater the light gathering capacity that an instrument can offer the better it will be for astronomical observations.

As a rough guide, increasing the overall diameter of the objective by 50% will increase its total area by a factor of 2.5 . This means that a pair of 10x50 binoculars (two eyes does not, surprisingly, make any difference to the light information our brains get to process) have an objective area of almost 62 cm covering around 7 degrees of sky whilst a 76mm (3 inch) telescope has an objective area of almost 200 cm which will resolve mere fractions of a degree and gathers some 80 times as much light as the unaided eye can see.

Another important factor to consider about magnification is its effect on the quality of image you will see, especially if you wish to observe very dim nebulae when again actual light gathering is far more important than magnification. Even when making lunar or planetary observations it is generally the case that atmospheric perturbations makes it almost impossible to observe anything clearly above 200x magnification from anything near sea level and this should be born in mind when making your choice of scope and more importantly the eyepieces. It is the focal length of the eyepiece that determines the magnification ‘power’ of the scope or more correctly the degrees of sky in view.

The magnification ‘value’ is derived from dividing the overall focal length (the distance from the objective or main lens to the eyepiece) by the focal length of the eyepiece in use and the maximum ‘power’ should not really exceed twice the diameter of the objective in millimetres. So for a 100mm diameter objective scope with a 1,000mm focal length fitted with a 10mm eyepiece you will get 100 x magnification and with a 5mm eyepiece you will be at the usable limit of 200 x magnification. For most astronomical purposes this magnification in degrees is converted to minutes of arc with 60 arc minutes to the degree and 60 arc seconds to the minute. So if you know an objects size in arc minutes then you can fit an appropriate eyepiece so that a given area of sky is known to be observable. For example the moon is only around some 30 arc minutes in diameter, whilst the considerably dimmer Andromeda galaxy also known as M31 is actually around 2-3 degrees across. To calculate the field of view you need to divide the power of the scope into the apparent field of view of the eyepiece being used.

For example, if you were using a telescope with a 70 mm objective that had a focal length of 350mm fitted with a 7mm eyepiece, this would be 350/7=50x magnification. Then divide this into the apparent field of the 50 degree eyepiece 50/50 = 1, so the field of view, in this case, would equal 1 degree. A 90mm scope with a 1,250mm focal length fitted with the same 7mm eyepiece would be 1,250/7=178 divided into 50 would be 0.64 degrees or 38.46 arc minutes, or a little over the size of the full moon.

These days any half decent 'entry level' off the shelf scopes usually come with at least two eyepieces of around 10 and 25mm focal length but something in the region of 35 or even 40mm might prove useful giving 25 x magnification with the 100mm objective whilst maximising the light gathering ability of the scope. Remember it is light gathering that counts not magnification.

Back in the 1960’s and 70’s ‘proper’ amateur astronomical telescopes were not such good value for money nor so easy to come by as they are now. The most usual advice being ‘spend the time waiting for something to become available saving up for the next largest scope to the one you can afford, and then save some more for the stuff to go with it’. It was generally the case that if you intended to take the subject of astronomy at all seriously then some very serious saving was going to be called for.

My interest in astronomy started when, as a child, I was extremely fortunate to find myself living in an area of extremely clear air and very dark nights with no light pollution to worry about. On a good night you could see the milky way arching across the sky in all its glory and even with the naked eye such pleasures as the Andromeda galaxy and Orion Nebulae looked quite spectacular. Not having much money of my own to spend I started with a pair of binoculars borrowed from my dad, and I have to say that a pair of 10x50’s is still an essential part of my current viewing equipment. They are extremely portable and give a good wide field view of the sky, and provided the sky is dark enough they can be used to pick out the faintest of nebulae that the unaided eye can only barely detect. One thing to bear in mind is that we have been rather spoilt by modern images from the likes of the Hubble space telescope which often show deep sky objects in bright false colours, many people are a little disappointed with what they actually see through a scope for the first time, as it will have little or no discernible colour value and will almost certainly appear somewhat dimmer than a computer enhanced image. Many modern day amateurs having invested the cost of a medium sized family car in a scope, observatory, digital imager and computer equipment to produce images that rival the best observatories of 40 years ago, but the owner of a modest ‘beginners’ scope will have to be content with something a little less spectacular.
Nevertheless, there is still something very special about observing the stars first hand and to know that the tiny photons of light that enter your eye, via your scope, have travelled so very far across empty space, and you are the only person ever to see those particular photons.

A lot of local clubs, societies and internet communities dedicated to astronomy have emerged in recent years but If you do not fancy the commitment involved in joining any of these then the BBC Sky at Night newsletter by Patrick Moore is a great low cost, and to the point, way of keeping up with what you can see over a three month period.

I would say that before even contemplating what telescope you want to buy you first get to know your way around the nights sky, learn the main constellations, the path of the moon and planets and how the changing seasons, coupled with the Earth's rotation, affects your view. Learn how to distinguish a night of ‘good viewing’, even a clear sky may not produce the best results and weather forecasts are notoriously inaccurate from a local pint of view. There is no real substitute for stepping out of the back door and having a proper look for yourself and being prepared to take advantage of some opportunistic viewing. To this end the actual size of your intended scope is worth considering and how long it might take to set up.

It is also worth considering some useful accessories you might need such as a planisphere, or astrolabe as I believe they used to be known, which is a simple mechanical calculator that allows you to get an idea of what is going to be in the sky at any given time of the night over the course of a year. A red light torch, usually obtainable from camping shops, will allow you to find your way around without seriously affecting your night vision and in addition to warm clothing (lots of thin layers works best with decent gloves and footwear) a CD player can also help to wile away the time waiting for your scope to cool and the ideal conditions to present themselves. In my experience the best viewing is obtained in the small hours around 2 am after the night air has settled down and the scope has cooled, then with any luck, in the peace and quite of a dark night when everyone has gone to bed and turned off those infernal ‘insecurity’ lamps, you will have a view of the universe that can take your breath away.

Most importantly try to decide exactly how much you are likely to use a telescope, and how seriously you intend to take the hobby, after all it will be an expensive luxury item if you only use it occasional

An understanding of what advantages, if any, a telescope may offer is useful, many people go and spend a fortune on exceptional equipment that turns out to be wholly impractical because it is just too big to carry from the house to viewing place and they could have saved themselves a lot of money if they had only taken the time to consider the alternatives. There is an old saying, ‘A small scope, outside, gathers far more light than a big scope that is indoors’.

The Different Types of Telescope.

Ok so we have three basic types.
The Refractor, the Reflector and the Catadioptric. Each have their respective strengths and weaknesses which can have quite profound consequences for the type of viewing you might be thinking of doing, so making the right choice is really quite important.

The Refractor.
is what most people have in mind when thinking of a telescope, a long metal tube with glass lenses at each end. This type tends to give a dimmer but sharper image than the Reflector with the added problem of colour distortion through the prismatic properties of lenses. However In modern refracting telescopes the lenses are no longer made from single pieces of glass but are compound units made up of elements sandwiched together to help counteract the chromatic aberrations,that manifest themselves as rainbow edges around objects. The objective lens of this type is really quite difficult to manufacture and this is why refractors are generally the most expensive type of telescope for a given objective diameter or ‘aperture’. However, a good modern refractor can produce very sharp images which are not only corrected but posses a high contrast, which means they are most suited for observing fine lunar and planetary detail, or say for separating closely paired double stars. One of the greatest advantages of refractors is that they are solidly built and unlikely to need collimating (re-aligning) after being transported. They also have the advantage when it comes to maintenance as a good Refractor will, if stored correctly, last almost indefinably with minimal attention.

The Reflector.
As its name suggests this type of telescope uses a large parabolic mirror as its objective to focus light onto a secondary mirror that then reflects that light at right angles and into a focusing eyepiece. Mirrors are much easier to make than lenses and thus the Reflector is generally the least expensive form of telescope for a given objective diameter but it is also worth noting that a Reflector of the open tube Newtonian design will need to have an Objective that is larger than the equivalent Refractor, as a rule a 100mm Refractor equates to a 125mm Reflector. Possibly the most important feature of the Newtonian Reflector is that they are generally quite compact and light in weight for a given aperture especially if installed on a Dobsonian mount, a sort of pivoting box arrangement, which greatly simplifies the setting up process.
Personally I tend to think that for general amateur viewing the open tube Reflector requires a little too much maintenance to be practical but if you are planning on some really deep sky observation of very dim objects then it is going to be an all but essential purchase. However there is a third possibility that should be considered with care.

The Catadioptric
Ideally a telescope should deliver an image to the observer that is both free from distortion or aberrations and which is not only compact but light and easily maintained. You could say that the ideal Telescope would incorporate the image sharpness and ease of maintenance of a Refractor with the light gathering and economy of a Reflector.
This was for many years a lot to ask until that is the Maksutov-Cassegrain system was developed back in the 1960’s for installation in spy satellites. By combining both lenses and mirrors it was found that a long focal length could be built into a physically short sealed tube, which delivered both the sharpness of image of the Refractor with some of the light gathering qualities and compact design of a Reflector of equivalent aperture.
A feature of this design is that they have an optical window that seals the end of the tube, which would otherwise be open in a conventional Newtonian Reflector and so the internal mirrors are better protected from dust and damp.
This is not to say that the Catadioptric is the answer to everyone’s needs, from a sheer light gathering capacity they can never compete with the open tube Newtonian Reflector and by their very nature they require a far more complicated and heavy mounting.
However it is fair to say that a 100mm Catadioptric is going to be significantly shorter and lighter than a 100mm Refractor, which will result in a much smaller mounting making it easier to set-up and use in an amateur setting.

Mountings.
Generally speaking the most common form of mounting incorporates some form of tripod but the most important feature of any set-up is the actual mounting that attaches the Telescope to that Tripod. For example a lot of bottom end (cheap) telescopes feature the Alt-azimuth mounting which is really not suitable for astronomical observations. What you need is an Equatorial mount that is designed to allow the Telescope to follow the path of the stars and planets as they cross the nights sky due to the rotation of the Earth.
More to follow.
M Paul Lloyd 2006

This is well worth a look. http://www.bbcfocusmagazine.com/blog/vi ... -telescope