TELESCOPES

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A CLOSER LOOK AT REFLECTORS

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Newtonian Reflectors

Cont'd.

The aluminium coating placed onto the surface of the glass returns about 90% reflectivity. The overall reflectivity combined with the secondary mirror drops down to about 80%. Different enhanced coatings brought out by manufacturers can bring this figure back up to around 90%. It is possible to get more reflectivity from a mirror by using silver for the coating. The reflectivity can go up to 95% overall but there are disadvantages. Silvering although is cheaper it is messy to do if you do it yourself and will only last up to a year. In comparison aluminising will last up to ten years. If you wissh to siver your mirror , do it just before you have a big session of observing to get the most out of it before it fades.

TELESCOPE LINKS:

Basics

Mounts

Light Gathering Power

Closer Look At Refractors

An advantage the refractor has over a reflector is it has no secondary mirror. A reflector's seconday mirror acts as an obstruction because it's in front of the telescope's main mirror. This obstruction degrades the image slightly, especially if the telescope has a short focal ratio. The shorter the focal ratio, the larger the secondary mirror is in relationship to the main mirror.

Dimensions

Balancing

Eyepieces

Cleaning

Tips

Astrophotography

Overall because

light gathering power

is the most important thing to have

Build your own telescope

Build your own telescope
for schools

for a telescope, the advantages of having a reflector out weigh having a refractor.

Binoculars

The short focal lengths of reflectors allow them to see more light. Basically this is only because of their ability to work at lower powers which do not psread the light out so much. To work out what telescopes can see by

there size and focal lengths go to

dimensions.

It is important to note that as reflecting telescopes get larger in mirror size they also get longer. Anything above 200mm in diameter will be long enough to cause difficulties if you wish to transport it in the boot or back seat of your average size car to a remote dark site. The other important point is telescope length may cause you to have to stand on something to reach the eyepiece. If you do need something to stand on, make sure it's stable. This is normal procedure for big reflectors and ladders for huge reflectors. It's important to be careful when using these for observing in the dark.

The weight of a telescope goes up as the size goes up. Included in the size increase is the mount. The heavier a telescope's otical tube is, the greater the counter balance weight. The 406mm (16") telescope used for the viewing nights at Telescopes and Astronomy weighs 95kg on the equatorial mount and 78kg on the Dobsonian mount. The eyepiece at it's highest point is 173cm (5'10") on the Dobsonian and 213cm (7') on the equatorial.

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