Re: Laser versus Non-Laser Collimation

From: Axel (ritesh_laud_at_hotmail.com)
Date: 03/07/05 

Date: 7 Mar 2005 11:41:24 -0800

> In looking at available web resources, maybe collimation is an area
> that would benefit from a short webcam movie demonstrating the two
> collimation techniques (offset and non-offset).

That may be helpful but note that as you said, offset is a one-time
adjustment and the essential collimation procedure is the same
regardless of whether offset was performed or not. So the videos would
be practically identical. In fact, a laser procedure will have no
difference. The video would be able to show that a Cheshire will
display a slightly off-center reflection of the primary (but the
reflection of the secondary will be centered in the reflection of the
primary) in a non-offset scope.

The upshot is that if secondary offset isn't performed, all that
happens is that the secondary needs to be tilted slightly to get the
beam to the hit the center of the primary, and hence the reflection
angle goes a bit over 90°. The primary is then tilted slightly toward
the focuser to return the beam to its source. As Greer notes, the
wavefront remains perperdicular to the eyepiece axis and the two axes
coincide at focus. That's all that is necessary for optimal image
quality. This is basic collimation; it has nothing to do with
achieving better illumination, like offset does.

If we then tilt the primary a bit farther towards the focuser, as Larry
seems to suggest, we are *out* of alignment and image quality is
degraded. In this case, not only will the optical axis not be parallel
to the focuser axis, it will be displaced from the focus by a small
amount. Errors of both type 1A and 1B will be incurred, as in Nils
Olof Carlin's discussion:
http://web.telia.com/~u41105032/kolli/kolli.html.

If proper offset is performed, the reflection angle is exactly 90° and
the secondary reflects the maximum possible amount of the light cone.

All this is just simple geometry and can be quickly understood by
drawing it out.

Cheers,
Ritesh

Relevant Pages

  • Re: Collimation Question
    ... if you are interested in checking your collimation without ... There is only one way I know of to make a laser accurate enough ... Are you confusing secondary offset with being out of collimation? ... focuser as seen as the reflection through the primary. ...
    (sci.astro.amateur)
  • Re: Collimination Non Problem (I hope)
    ... I had the same concerns about the collimation of my 8". ... but the reflection from the inside of the ... my drawing, the offset was to the left in the picture), and they said ... They said the offset ...
    (sci.astro.amateur)
  • Re: Laser versus Non-Laser Collimation
    ... > offset v. nonoffset collimation: ... > "In other words, if you do not offset, the only implication is that the ... > primary mirror will not be pointed exactly down the center of the tube ... > for two types of views down the focuser tube when checking out my ...
    (sci.astro.amateur)
  • Re: Laser versus Non-Laser Collimation
    ... Vic Menard's collimation page ... "In other words, if you do not offset, the only implication is that the ... primary mirror will not be pointed exactly down the center of the tube ... for two types of views down the focuser tube when checking out my ...
    (sci.astro.amateur)
  • Re: Laser versus Non-Laser Collimation
    ... Larry, your drawing, when compared with Olof Carlin's, nicely ... illustrates the key difference of offset and not-offset collimation. ... secondary mirror is not centered, ... This is "offset" collimation. ...
    (sci.astro.amateur)