Unlike all the great popular rumors, collimation, behind the somewhat mysterious name, is an indispensable optical adjustment step in imaging, whether film or CCD, much less complex than it appears?

Often, on an instrument that has never had a regular optical maintenance, this setting is sufficient to transform the performance and capability. It is estimated 2/3 of the market, developing reflector telescopes poor performance, probably mostly because a defective collimation. Like all amateur astrophotographers planetary surfaces and Moon, I quickly understood the importance of such an adjustment on the images obtained in the end. The quality of a planetary image is very fragile collimation as the focus. The optimization of the collimating alters markedly resolution.

My collimating methodology is mainly for owners of Schmidt-Cassegrain but can adapt to that of a Newton telescope, the most decisive collimation being made on the secondary mirror.

This methodology can be applied in two ways: either visually with an eyepiece or digitally with the use a webcam and a computer software control.

1 - Instrument Commissioning temperature to avoid the instrumental turbulence

Whatever the means used for collimating, this step is essential. The mechanical elements of a Schmidt-Cassegrain are very sensitive to temperature and the change thereof causes expansion or compression thereof, causing also sizeable variations adjustment. It is very unwise to collimate a telescope that is not temperature, since it is enough to cause actual collimation.

Before a collimating telescope, it takes an average 1:30 to 2:00 so that the latter is placed in temperature with respect to that outside.

It is more advisable to put his temperature instrument, in all ways, instrumental turbulence is significant enough to make them invisible Airy disks in an incessant thermal disturbances ballet, making it impossible collimation.

2 - Concentricity secondary mirror shadow 3 - Discs Airy
In the case of a highly misaligned reflector telescope, which has never undergone regular optical adjustments, it is common to find that the collimation is so bad, that the shadow of the secondary mirror is no longer concentric with the bright disk primary.
This verification is achieved by using a magnification of about 100X and strongly by defocusing the image. The star appears as a luminous disc, obstructed by the shadow of the secondary mirror. The objective of this first adjustment, if necessary (Figure B), is to refocus the shadow of the secondary mirror relative to the bright disk of the primary (Figure A). To do this, simply locate the 3 screws to 120 ° at the front reflector telescope, just behind the secondary mirror (sometimes a clip portion can protect these 3 screws cross trees / tirantes) and acting on a of them or three. WARNING !!! Never completely unscrew the 3 screws at the same time as this poses serious problems and can even damage the instrument in some cases.

After a rough collimation by important defocusing, collimation remains very rough and it is necessary to perfect further. To do this, we focus this time the star perfectly. To increase the visibility disk Airy, it is necessary to use a higher magnification (at least 300X) and also use a green filter (WRATTEN KODAK No. 58). Here is a quick comparison between a telescope collimated and misaligned :

Telescope collimated

Telescope misaligned

Collimation

04/15

 

OBSERVATION AND IMAGERY

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Arnaud FIOCRET
 

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