Remember that your eyepiece can't exceed the maximum magnification of the telescope. If it does you'll end up with an out of focus image.

What's the 'f' number?

The 'f' number refers to the telescopes focal ratio or put another way how much light the telescope will let in. It can be calculated by dividing the focal length of the telescope by its aperture eg my telescope has a focal length of 750mm and an aperture (mirror size) of 150mm giving a focal ratio of f/5.

                            750mm / 150mm = f/5

A high focal ratio of say f/9 to f/15 gives a narrower field of view but good contrast, so the image produced tends to be magnified well with bright objects showing more detail. A telescope with a a high focal ratio is best for viewing objects like the planets and the Moon. 

A low focal ratio of around f/4 to f/6 produces a wider image but of low contrast so it's best for a telescope designed for deep sky viewing. I found the detail on Jupiter and Saturn difficult to make out with my f/5 focal ratio. I got around this by using an aperture mask. This literally makes the opening at the end of the telescope smaller and so increases the focal ratio.

I cut a 100mm hole in a piece of card and fixed it to the front of the telescope. The new focal ratio of my telescope now becomes:

                            750mm / 100mm = f/7.5 ie it lets in less light and so increases contrast.

One other thing to watch out for with a telescope that has a focal ratio in the f/4 to f/6 range - collimation (the alignment of the primary and secondary mirrors). These telescopes are much more sensitive to collimation, a telescope with a higher focal ratio is more forgiving if poorly collimated. A telescope that is poorly collimated will show stars with comet tails and the planets will look like there's two of them! Focusing will also be difficult!

What type of mount do I need?

The type of mount that comes with a telescope can also affect its price. There are two major types of mounts used today for reflector or refractor telescopes, the German equatorial and Alt-Azimuth (Dobsonian). Most refractors and reflector telescopes are mounted on a German equatorial mount. The equatorial mount consists of two axes at right angles. The one of the axis is called the polar axis, this axis is set to the same angle as the latitude of the observer's location or put more simply it points towards to the pole star. By sighting this axis on Polaris, you make the axis parallel with the imaginary shaft on which the earth rotates. Therefore as the earth rotates towards the east, you make a corresponding movement to the west. In this way you can track an object as it moves across the sky with the polar axis only. By attaching a clock drive motor to the polar axis the telescope can move it at the same rate as the earth rotation therefore it will track an object as it moves through the sky. 

The Alt-Azimuth types of mounts are the simplest mounts used today. They consist of two axes at right angles. One axis moves up and down and the other left and right. This system is sometimes call a Dobsonian (named after John Dobson, the father of amateur telescope making). The advantage of this mount is its low cost and ease of use. Dob's as they're some times called can hold very large telescopes from 6" to 30" tube diameters and are very stable. Another advantage of the Dobsonian telescope is that there's no setup involved, simply put the telescope on the mount and you're ready to go. 

What accessories do I need?

The essential accessories have to be your eyepieces. When you buy a telescope you usually get one or two eyepieces included. They're usually the cheaper types of eyepiece but they can be effective. I use a 25mm and 9mm kelner that was supplied with my telescope. In my opinion they perform very well, giving good clear definition and contrast.

I then decided to buy a 6.4mm Meade Series 4000 Plossl and a Meade 2X apochromatic Barlow lens (this doubles the power of an eyepiece). I found that the higher power eyepiece reduced contrast significantly which makes it useless for viewing small faint objects. On nights of good visibility (low atmospheric disturbance etc) I can get good views of the planets and moon with a pretty clear image. However, by using the barlow lens with the 6.4mm eyepiece it becomes impossible to focus clearly on any object. I've been advised that this is down to the limiting magnification factor of my 6" mirror. The general rule of thumb is that the maximum magnification you can achieve with your telescope is 50X magnification per inch of aperture. So my 6" telescope gives me a maximum of 300X magnification. In reality this works out at about 230X magnification before I start to lose quality in the image.

Other accessories such as stepper motors, piggy back mounts, camera adapters, CCD cameras etc are also available to enable the astronomer to automatically track objects, take astronomical photographs, locate objects easily etc etc. I'd recommend getting used to the basics with your new telescope before you start splashing out on other accessories.

When I've got a telescope then what?

Now you need to figure out how it all works. Easy you might think! Well just remember that when you look through the eyepiece the image you see will be inverted. This makes it tricky to hop from star to star if you get mixed up about which way is north or south. I found it very difficult to find anything on my first few observing sessions.

Definitely plan out your observing session, that way you don't waste too much time aimlessly sweeping the sky in the hope of seeing something. I use star chart software to identify objects in a particular area of sky and note the stars I'll use to guide myself towards the target object. I also use binoculars to help in this process. Star chart software or planetarium software if you like usually allows you to print an inverted copy of your chart. This allows you to see the same image on the chart as you see through the eyepiece making it very easy to star hop - you don't have to worry which way is north, south, east or west.

Then make notes on what you see, this will help you improve your observing skills. I found that I was missing details and objects because I wasn't looking properly. I managed to track down M65 and M66 (two galaxies in Leo). As I was searching for these objects I only saw them when looking at them. I failed to notice another galaxy just below the pair that was in the same field of view. The galaxy was very faint but if I'd looked harder I would have seen it. When I started recording what I saw I started to 'see' much more! Give it a go... One other thing that caught me out when hunting for objects was how small and faint some of these objects could be! I then started hunting for objects that were close to bright stars - made tracking down these objects much easier, as you gain more experience you can star hop across greater distances to your target.

If you would like your observations added to the site then complete the observation form and the details will be posted here. These reports will help others find out what can be seen with your type of telescope.

Wrap up warm and happy hunting.

Back home