Testing & Using your telescope

The Following steps listed below will enable you to get the best results from your telescope.

1. Testing your telescope

To avoid poor performance always make sure your telescope is properly collimated.
To test your telescope point it a  3rd magnitude star and rack the focuser in and out. The image should look like fig 1.

The perfect mirror (fig 1)

   Inside focus      At Focus       Outside focus

If the dark central spot is off centre then your telescope requires collimating. please adjust your telescope as specified in your telescope hand book.

Spherical aberration (fig2).

Inside focus      At Focus       Outside focus

The telescope mirror shown in fig 2 has one of the most common of faults.  The surface of this mirror is not figured correctly and is under corrected. If the opposite effect is seen, then the mirror is over corrected. Important. If your mirror gives this type of image, please make sure it has fully 'cooled down' and the seeing is good before condemning and returning it to your dealer.

Astigmatism (fig 3)


  Inside focus      At Focus       Outside focus

Astigmatism shown in Fig 3 can be caused by any number of defects only two of which are serious, the rest can be remedied. The most serious cause is that the main mirror is not a surface of revolution. This is caused by a defective manufacturing process when making the main mirror, fortunately this is rare. The other more common, serious cause is that the secondary mirror is not truly flat.
the other causes are:-

1. The secondary mirror has been incorrectly glued or clipped on to the secondary mounting cell and is distorted. Carefully remove the mirror from the cell and carefully remount it. If the telescope is under guarantee, contact your dealer.

2, The main mirror is not mounted properly in its cell. The mirror may not be making proper contact with the mirror cell or it may be pinched. If the telescope is under guarantee, contact your dealer, other wise remove the mirror from its cell and carefully replace it.

3. The telescope is not collimated correctly.

4The owner may suffer from astigmatism. In this case you may need to wear your spectacles when using low power eyepieces.

2. Setting up and using your telescope

when using a reflector allow plenty of time for the telescope to cool down. A warm mirror affects the image in two ways. Firstly the curve on the mirrors surface will not be correct until thermal equilibrium is reached and secondly and perhaps the most degrading of the image, is due to the thermals caused inside the tube rising from the warm mirror.

a. Set your telescope up on the lawn. Patios tend to take longer to cool down and the air currents caused above the site degrade seeing, also avoid looking over  roof tops.

b. Very crisp clear nights are excellent for viewing faint objects, the atmosphere is however usually very turbulent causing the stars to twinkle, observing fine detail on the planets or splitting close double stars is best left to another day. The best planetary viewing is  to be had when the  sky is slightly hazy. This is the best time to check the quality of your mirror or object lens with the star test.

c. Do not use a too higher power. The nights on which a power of above 300x can be used are few and far between. The contrast of a low power image may be improved by increasing the power slightly

Back to Testing the Mirror

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