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.
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.
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
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
2 - Concentricity
secondary mirror shadow||
3 - Discs
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.
!!! Never completely unscrew the 3 screws at the same time as
this poses serious problems and can even damage the instrument
in some cases.
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.
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 :