By Damien Jacobs, March 2012
Astronomy is one of the oldest of the Natural Sciences, if not the oldest and one which can be taken up by practically anyone with very little knowledge or equipment from the outset. It’s a science, in which amateur astronomers can provide results of genuine interest and further the understandings of astronomy at large. Like all hobbies and past-times it has a start place and everyone has to start somewhere.
The object of this, the first of the Wex Guides to Astronomy, is to provide start point for people considering their first telescope, or for those whom already own one and wish to consolidate and expand their knowledge.
The guides, as you read them, will build and provide a good grounding in the basics of astronomy, the equipment and its use, without being overly technical and attempting to limit its readership to postgraduates or mathematical masterminds and done so with a nod and a wink, and a gentle play of the hair towards humour. After all, astronomy isn’t all about intangible concepts and unapproachable societies; first and foremost it should be fun, awe-inspiring and the start of something better and bigger!
Hopefully, by the end of these guides you’ll have a better understanding and knowledge to buy your first telescope with confidence, or if you already own one, a greater understanding of how it works.
So roll up, be inspired, and get excited, as Astronomy is the new Rock and Roll! – It even has the ex-keyboard player of D:ream as its modern-day poster boy!
The Optical Assembly
So where do we start?
The “business end” of course, the meat and veg of the telescope, namely the Optical Tube Assembly.
Now all optical telescopes operate on the principle of gathering as much light as possible and bringing that light into a common focus point whereby it can then be magnified by an eyepiece. This can be done in one of several ways depending on the tube’s optical design.
There are three designs in use pretty much exclusively nowadays and they are; Refractors, which use a lens to bring the light into common focus, Reflectors, which use mirrors and Catadioptrics, which use a combination of a lens and mirrors.
All three designs use what is called a primary lens or mirror, of a set diameter to bring the light into the all-important ‘Common Focus Point’. This diameter becomes what is known as the telescopes aperture.
Now the first rule of thumb in buying a telescope is to remember the larger the aperture the more light the telescope will gather and therefore the better the resolution and contrast of the resulting image.
– So as with most things in life, bigger is better, unless that is you’re talking about mortgages?
Now let’s have a closer look at each of the optical designs.
A refractor uses the simplest method to gather light. It has a lens at the front of its optical tube where the light enters and is refracted to a focus point at the rear, where it can then be magnified.
Refractors can have one individual or several glass elements to make up its main primary lens, with benefits increasing as the more elements are used. By using more than one element the lens can more accurately bring the different wavelengths of light in to focus, thereby reducing colour (chromatic) aberration.
Because refractors can use several glass elements each of which have to be precisely ground and polished, they can be very expensive size for size, compared to reflectors. Refractors are though, size for size, considered the best at resolving the moon and planets due to the fact they have no central obstruction reducing the amount of light entering the tube. Also, as they use a fixed lens there’s no collimation or dust issues within the optical tube assembly.
Reflectors use a concaved mirror as its primary with the mirror diameter referring to its aperture.
The mirror is located at the back of the tube assembly with the light entering from an open front end. The light is then reflected from the mirror back up the tube to another “secondary” mirror set at 90 degrees, which reflects the light out through an opening and to the eyepiece.
The disadvantages of reflectors are that as they are an open tube system, dust on the primary mirror can become an issue, although to be fair, a layer of dust will only stop a minute fraction of a degree of light, so that unless the dust extremely bad, leave it (dusting with an incorrect cloth or cleaning material can do more damage and reduce reflectivity more than the dust itself).
Also due to their open tube assembly, differences in the air temperatures within the tube and the air outside, can cause convection cells, and reduce the “seeing”.
On the plus side and of most importance, large mirrors are less expensive to manufacture than complex lenses, so reflectors are far cheaper than refractors or Catadioptrics size for size, (Remember the bigger is better lesson?). Also, with reflectors no light is lost during the ‘gathering’ process (it reflects it all) as opposed to refractors, which unless a very expensive lens design is used, some light will be scattered, and to a degree lost.
Now these clever fellers use a combination of a lens and mirrors, with the mirror being the primary. Light enters the front of the telescope through a lens called a Correcting Plate, it then reflects off the mirror at the rear back up the tube to a smaller mirror fixed to the middle of the corrector plate, back down the tube again and though a small hole at the centre of the primary mirror to the eyepiece. Now the benefit of Catadioptrics, having folded the light twice within its tube assembly, is that they’re the most compact and portable of the three designs. They are also considered a very good all-rounder for viewing most celestial objects, and have no internal dust issues. Their main downside being cost. They generally come out as the most expensive design, size for size what with all their ground glass and mirrors.
OK, we’ve had a look at the Optical Tube Assemblies, their design and their respective advantages and disadvantages, so what goes with them? What comes with them to turn them from a collection of inanimate objects into a “Window to the Universe”?
Telescopes are generally brought as a kit containing everything you need to get you going and are how most people will buy their first telescope. They will consist of a tripod, mount, the Optical Tube Assembly, and a host of other exciting parts all packaged up in small anonymous boxes.
So let’s have a look at these in a little more detail and familiarise ourselves with them.
Looking through even a low powered eyepiece and unless you’re looking for the moon, the chance of you finding what you’re looking for is slight to put it mildly so a finderscope is employed to aid you in this task.
They all operate on the same principle regardless of their design which can be of a straight, 90 degree, or new-fangled ‘red-dot finder’ which projects a small red dot on to a clear window within the finder body.
In use, you first “Sight” the object in question through the finderscope, (safe in the knowledge that you have previously aligned the finder to the optical tube assembly), and once centred in the finders cross-hairs, you’ll move to your eyepiece and lo, the object in question should be in the middle of the field of view.
As mentioned a finderscope requires alignment to the OTA which is a simple enough task and best done in daytime.
First locate and centre an object (not the Sun!) in a low powered eyepiece which is at least 500 meters away. Then simply direct the finderscope to the same object using the method for that particular design of finder, be it moving the cross-hairs, a red-dot, or the finder itself.
This isn’t an accessory as such but a vital part of a telescopes design so warrants a few lines to describe it. In short, for refractors and reflectors it moves the diagonal or eyepiece holder along the focal plane so focus can be achieved for a particular individual, eyepiece or camera adaptor. Design is usually of a rack and pinion type, although some “draw-tube” designs can still be found in older telescopes. What to look for, is a smooth, slick action, and if possible with a dual-speed focuser for ultra-fine focusing.
With catadioptrics, the design is simpler where the primary mirror is generally moved back and forth via a small knob to change the distance of the focal point along the focal plane, and hence achieve focus.
In to the focuser will sit either, an eyepiece, eyepiece holder or diagonal of some flavour.
A diagonal (if required) is situated between the eyepiece or eyepiece holder and the focuser and generally only used on refractors and catadioptric telescopes. Its function is mainly as an aid in viewing comfort by reflecting the light coming out from the telescope, therefore allowing a more suitable position for the observer when viewing for long periods. Diagonals are also available to “erect” an otherwise inverted image making the telescope suitable for terrestrial observations.
The Eyepiece holder:
Normally an integral part of a Diagonal or focuser, but can sometimes be a separate accessory supplied with a kit if required. In essence, it does what it says it does. It securely locates and holds the eyepiece in the required correct position by a means of 2 or 3 tightening screws around a short barrel in which the eyepiece would sit.
A vital element of the telescope and effectively useless without them! They provide the all-important magnification of the image at the focal point and come in a variety of designs and “strengths” of magnification. An eyepiece is generally referred by its focal length, which divided by the focal length of the telescope will give a figure to the resulting magnification. An example being a 25mm eyepiece being used on a telescope with a focal length of 1000m would therefore give a 1000/25= 40x magnification. The second part of an eyepiece name will be its optical design, be it a Plossl, Wide-Angle or Long Eye-Relief amongst many others, whereby differing designs giving different characteristics for specific viewing applications. In general most kit lenses are the extremely good performing Super Plossl.
Tube-rings and Mounting bar:
These simply allow for the attachment of the OTA to the mount and are generally unexciting in themselves, but like all grand-scheme-of-things, are an essential part of the “whole”.
Tube rings fit around the OTA, to which is then bolted the Mounting Bar, which itself fits to a matching slot in the mount from where it can be fixed, (much like a quick release plate on a photographic tripod). Mounting bars can also be directly fixed to the OTA on smaller, lighter tube assemblies.
Mount and tripod:
Tripods generally get a lot of bad press, usually as the transportation method of choice for invading Martians, but in astronomy, they are vital. The mount and tripod can be thought of as the firm foundations of a telescope. The tripod provides the support, height adjustment and weight-carrying role, with the mount providing the ‘securing’ ability and to move and adjust the telescopes orientation.
If you’re buying a telescope as a kit then the manufacturers will match it up with a suitable mount and tripod and you may think no more on the matter, unless of course you’re looking to upgrade, or putting together your own kit, in which case a little more consideration will be required prior to buying.
An easy way to think about the importance of the tripod, is to consider that the object your viewing has been magnified many times. As such any vibration or movement of the telescope will cause the object to dance around in the field of view until it settles about 30 annoying seconds later. So to mitigate against this a mount and tripod combination must be sought which is designed to support at minimum, the weight of your telescope set up. Unfortunately mount and tripods are not cheap and there may be a temptation to go for a less expensive option, but this may come at a cost to your hair as you increasingly pull it out waiting for that “dancing star” to settle again.
There are two flavours of mount used pretty much exclusively nowadays, and they are the AZ, or Altazimuth, and the EQ, or Equatorial. Both come rated as to their weight supporting abilities from 1 to 6 with 1 being the smallest, least expensive and lightest load carrier, with 6 being the opposite.
The Altazimuth mount, is generally used on smaller, beginner scopes, (or scopes which can also be used terrestrially). It has two axis of movement, horizontal and vertical enabling it to move in altitude and azimuth.
Equatorial mounts have a Right Ascension and Declination axis and requires an initial set up for each viewing, by which they can then follow an object using movement in just one axis. The key advantages being for astrophotographers who require tracking in one axis only and for all observers, the ability to locate celestial objects using their right-ascension and declination coordinates.
(There will be a more detailed look at equatorial mounts in the second part of the Wex guides to astronomy).
So, this brings us to the end guide. Hopefully you now have a slightly better understanding of your potential purchase, or the many bits laid out in front of you on your lounge floor as you assemble it.
The next step is the really exciting part, and forms the basis of the second Wex Guide to Astronomy – setting up and using the thing! Although that of course won’t be its title, so don’t try looking for that, it’ll be something far more esoteric or involving some witty pun, or maybe not… Either way, the truth is out there!