INTRODUCTION TO ASTRONOMY

This article featured in the April 2003 Beginners Magazine

USING TELESCOPES

6. PREPARING TO USE A TELESCOPE

Whatever type of telescope you have, care must be taken when setting up for an observing session. It always pays to check the weather forecast before setting up. It can be very annoying to spend a considerable time setting up only to find it has clouded over before starting or shortly after. If clouds are forecast for later in the evening look at the sky in the direction the wind is coming from to see if there are any clouds on the horizon and if there are estimate how long before they reach you.

Carry the telescope to the position where you expect to observe from. Check that trees and buildings will not obscure your view of the objects you are expecting to observe. Remember the sky appears to move 15° per hour due to the rotation of Earth. Make sure you have all the things you will need before you start, it is annoying when you have to get something you may have forgotten especially if you need to go indoors and spoil your dark adaptation. Above all make sure you are well dressed for the cold, you can always take some clothes off if you get too warm but once thoroughly cold it is very hard to warm up again.

Make sure the mounting is stable. It will normally have three or four feet so it should stand steady on most surfaces. If it does wobble it may have a stone or other object under one of the feet. If you are going to view at night set the telescope up just before it gets dark. There are two reasons for this, first it is easier if you can see what you are doing. Secondly if the telescope has been kept indoors, in a garage or shed it may take an hour or so to acclimatise (cool down) before it will perform at its best.

Directly the telescope is set up remove the tube cover to allow air to flow through the Tube and around the mirrors. Do not fit the Eyepiece until you are going to use the Telescope because its lens may get covered with dew. Leave the Finder Cap on until you want to use it or it will also have its lens covered with dew. The dew can of course be wiped off with a cloth or tissue.

Some hints on positioning the Telescope. Use the patio or path they are comfortable and simple for casual viewing but there will be less air turbulence on the grass when you want to do more serious observing. The best position would be on a small path or on the edge of the patio near the grass. The patio stones or walls get warmed during the day and give off their heat in the cooler night air so try to avoid viewing across close paved areas or walls. Rising heat will cause the image to shimmer like the road on a hot day. Obviously try to set up away from trees or buildings but this may not be possible so set up in the best place to view your intended target you can always move to another position later. If you are viewing low objects sit on a chair or something, you will be more steady and a lot more comfortable.

To start viewing allow about five minutes for your eyes to get acclimatised to the dark. This period can be used to familiarise yourself with the sky and work out where everything is. Try to turn off all lights around you. If there is a street light bothering you it may be possible to erect a screen around yourself using canes, step ladders, washing poles, string and old sheets, curtains, towels or even news papers. Even lights that appeared dim, when you first began your observing session seem to get very bright when your eyes are fully adjusted to the dark. I will take about 10 to 15 minutes for your eye pupils to fully open so avoid looking at light or it will take the same period to re-acclimatise.

A Dobsonian telescope is an excellent instrument for casual optical observation and can be set up very quickly. It does not matter which direction it is place down in because to can be turned around on its vertical axis. All that is needed is to point the telescope at an object and start viewing.

A DOBSONIAN MOUNTED NEWTONIAN

An Equatorial Mounting needs more care in setting up. First the stand must be set level. On a tripod mount levelling can be achieved by adjusting the legs. With a pedestal there may be screwed feet if not it may be necessary to place packing under one of the feet. The Right Ascension (RA) axis must then be set to the correct angle for the latitude of the observing location, in southern England this will be about 51°. Some equatorial mountings have a small built in telescope that can be aimed at the pole star to align the RA axis. Shown below are two types of equatorial mountings. Once the 'Polar' axis is aligned to the pole the telescope can be aimed at the desired object. The advantage of an equatorial mounting is once the telescope is pointing at the object only the polar axis needs to be adjusted to track the object as it moves across the sky.

A PEDESTAL MOUNTING	A TRIPOD MOUNTING

To find an object first fit a low power eyepiece into the focuser. This is the eyepiece with the highest number (in mm) written on it, perhaps 25mm. Aim the telescope in the direction of the object to be observed by looking along the tube. The look through the finder to identify the object. Adjust the aim of the telescope to position the object in the centre of the finder. On a Dobsonian the adjustment is made by carefully moving the telescope by hand. Equatorial mountings are a little more difficult. First release the clutches so that the axes can be rotated but there is still a small amount of drag. Rotate the axes until the object is seen in the finder then retighten the clutches. Use the axis drives to centralise the object. The object should then be visible in the eyepiece of the main telescope.

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