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barkis

Newtonian Collimation.

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This procedure is best done with the telescope tube off its mount, and placed in a secure horizontal position with the mirror end slightly higher than the front end. This is to prevent anything falling down onto your lovely mirror.

This technique requires that a black spot is placed on the exact centre of the main mirror. I hear cries of anguish all over the land. But it is easily done if you are carefull.

If you can find a piece of optical tissue and cut it to the same size as the mirror, and having placed a hole at the exact centre, place the tissue over the mirror and make sure it corresponds to the edges of the disc. This is purely to keep fingerprints off the mirrors surface. The small hole in the centre is where you are going to insert the self adhesive black spot. ( Beginning to sound like Treasure Island.?) Do not stick on the spot untill you are satisfied that the tissue disc is fully inline with the mirror.

Placing a marker on your precious objective will do no harm at all. It will lie in the shadow of the secondary, and therefore have no detremental effect on the optical system. Do not put the mirror back into the tube for the moment.

Remove the the spider and flat.

Make a disc of fairly stiff card of a diameter suitable to fit snugly into the mouth of the tube. Once you have a good fit, put a pinhole in the disc exactly in the centre.

If you used compass to mark out the disc, the hole made by the point should suffice. Then place a target cross onto the disc so that the cross intersects the pinhole. The disc is placed into the tube again with the target facing the mirror end of the tube. You can now put the mirror back into the tube.

The next step is to illuminate the card by placing a light source such as a small torch inside the focusser. Or any means to illuminate the card. Next, by looking through the pinhole you will see the reflection of the target in the main mirror. Simply adjust the mirror cell adjusters until the spot on the mirror is intersected by the cross on the target disc. Once you have centred the mirror spot onto the pinhole on the cross, you now know that the main mirror is pointing up the optical axis of your tube. It only remains to place your diagonal and its support back into the tube. Do this whilst the tube is still in a downward position, and therefore eliminating anything falling onto your mirror. Bear in mind also, that the secondary mirror is not necessarily placed in the centre of the tube, but it is sometimes offset away from the drawtube, this is so that the reflected beam of light is caught by the whole of the secondary mirror. The beam is wider at the mirror edge of the elliptical flat, and therefore loss of light can occur.

The only adjustments you need make now, are to your secondary mirror, and if your focusser tube axis is not at right angles to your tube, that too will require adjustment.

Collimation is extremely critical especially in short focal Newtonial reflectors. The procedure above assumes that the mirrors figure of revolution is precisely at its centre. If it isn't, then minute adjustments of the main mirror whilst observing a star maybe necessary. The star test is best done on a night of good seeing and a steady atmosphere.

I am targetting this procedure towards those members who are new to astronomy, and have got a Newtonian telescope, which at some point is going to need adjustment. I implore them not to to attempt it if they have any doubts about their ability to carry it out in a safe manner. The last thing I want is for them to perhaps damage their instrument. This is a method I use myself, and is not intended to overide any other method.

I hope it is of some help.

Cheers

Barkis.

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