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Re: [ATM] strange (collimation?) problem
Dave writes
>The practice of offsetting secondary holders (so that the
secondary is
>offset to make the perimeter of the border of the field
that is vignetted
>symmetrical) is of highly dubious (and "very
slight") value: I have written
>about this extensively; you might look
in the archives. Everyone experiences
>a certain degree of vignetting
of field, with long focus eyepieces, or the
>film plane of a camera.
Who cares whether it is symmetrical or not?
>
>No one I know.
>
>Don't
offset your secondary holder. No advantage, only headaches in
>collimation. Just my STUPID 2 cents' worth. Do what the "Heavies" tell
you
>what to do.
-------
Andy replies
I'll bite..what's *wrong* with
offsetting the sec mirror..don't see what
the collimaion
headaches
are..
Some pro's to offsetting IMHO: <see below>
I think there is
often a bit of mixup regarding offset.
Optically, offset is defined
when positioning the secondary in-out of the tube - by using the common
sight tube, best with a "focal ratio" (length/inner dia.) equal to that
of the primary. Doing that will result in offset of the optical axis
relative to the center of the ellipse - the very thing needed to center
the "cone" of full illumination in the FOV of the eyepiece/detector.
I'd prefer the term "optical centering of the secondary" for this
situation, but I don't expect it to catch on soon... Once this is done,
the position of the optical axis is defined (after collimation of the
primary, the "outgoing" optical axis will hit the same spot on the
secondary, a little to the focuser side of the geometric center of the
secondary's elliptic face.
Please note that offsetting the secondary
away from the focuser or having it centered in the tube won't change
this - but it effectively determines if it is the optical axis, *or*
the secondary, that will be centered in the tube (at the level of the
secondary) - if the secondary is offset, the optical axis isn't, and
vice versa.
I have heard reports of difficulties with digital aiming
devices with a secondary not offset away from the focuser, so it might
be worth the bother for those using such a device (but note that the
optical axis need not be parallel to the center of the tube, instead it
should be perpendicular to the alt or dec axis!). With any design of
tube opening that isn't extremely narrow, vignetting isn't a practical
problem. But then again, when designing the upper assembly, why not
either offset the secondary holder relative to the center bolt, or else
offset the whole spider??
As for the asymmetry of the "shadow" of the
secondary: if you can see the shadow distinctly, you are much too far
away from focus to have much sensitivity left for star collimation.
Nils Olof
-one can use the actual minimum possible size secondery if
it's centered
in the imaging cone (which is not the center of the OTA)
-asymmetrical vignetting can be a problem with using CCD cameras for
differential photometry
-noticeable asymmetrical vignetting is in the
eye of the beholder,
there is no 'acceptable' minimum amount
-
personally, I prefer symmetry in the optical train vs in its
support
structure.
-and it's not hard to do, just build the offset into the
sec spider/mirror
holder.
Amount of offset will be dependent on size
& fical length of the primary.
Re the 'collimation difficulties'
caused by the asymmetrical location of the
secondary..easily fixed by
adding a masking disk larger than the secondary,
centered on the OTA.
This idea can be expanded to making the mask almost the
same size as
the primary, so during daylight collimation one sees a thin
annulus
of
light vs most of the primary. If the eye is centered in the focuser
via
a small
aperture, any miscollimation of the primary shows up as
asymmetry in the
ring of
light. Basically does the same thing as a
laser but can handle perforated
primaries.
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