BACKGROUND

Reports from this list indicate that some need to tweek collimation
frequently while others do not. At first, this sounds like a paradox.
However, my notes tend to indicate that some may want and need
to tweak collimation more frequently than others depending on how
the scope is used and under what conditions it is used. However,
after several thousand hours of CCD and film imaging on SCT's (and
other scopes), I have found collimation to be one of the lessor hurtles
to produce good images. It is important to put efforts in the areas
likely to require attention (such as mount stability, alignment, focus,
and guiding) and minimize efforts worrying about other adjustments
that have less an impact on the final results so that imaging does
not become needlessly difficult.

For general astrophotography and CCD imaging, critical collimation
does not severely effect image results until one starts to approach the
theoretical resolution of the optics. Typical seeing of 4 arc-seconds
experienced by most does not allow one to record details at the
theoretical resolution of the optics. As a result, critical collimation
does not result in visibly better images because image blurring do to
seeing hides slight collimation errors and other optical anomalies.

I must say that with 1 arc-second seeing, any optical anomalies really
start to become evident. Field flatness and other aberrations are now
apparent to the experienced imager. That mega-special accessory
now produces or worsens distortions if it is not optimally corrected
for the position of the spherical primary SCT mirror. Good collimation
is noticeable under excellent seeing. I want to caution those that are
checking collimation with a dew cap in place that if the cap is slightly
askew, even a slight blockage of the clear aperture will result in
flattened diffraction rings on one side.



COLLIMATION FOR VISUAL WORK

Visual work requires different considerations. Visually, the eye and
brain act as a signal processor combining several images, discarding
the bad portions of the image while retaining the better portions, then
assembling a composite image. With observing experience, the human
signal processor is programmed to see more detail while discarding
what is not relevant. Likely, most of us have been to star parties where
an experienced observer sees things we cannot. We may not be able
to easily surmise whether the experienced observer is exaggerating
or is really seeing what he reports until we gain similar observing
experience.

Here, excellent optics will help with nice visual images under good
seeing, but somewhat less so under typical 4 arc-second seeing
experienced by most. Often, Cassini's division is used to "test" the
collimation and resolution. However, Cassini's division is an example
high S/N ratio (contrast), not resolution, because we aren't really
resolving it, even though we believe we see it, so the results may be
somewhat misleading.



COLLIMATION REQUIREMENTS FOR GENERAL IMAGING

Quite frankly, imaging is often less demanding of optical quality than
visual work because using amateur equipment, the signal is diluted
with noise of atmospheric blurring, reddening produced by absorption
of shorter wavelengths (extinction), and a host of other factors. In this
example, the benefit of precision collimation and diffraction limited optics
is buried in the noise produced by other sources.



COLLIMATION REQUIREMENTS FOR HIGH RESOLUTION IMAGING

When we are imaging resolvable extended objects such as the moon
and planets, it is possible to produce images with an apparent resolution
exceeding the theoretical limits of our optics. In this case, precise
collimation increases the signal to noise ratio (contrast) resulting in a
image that appears to be more resolved, when in fact it is not really.

The moon is a great object for high resolution astrophotography because
it's contrast and brightness provide the high S/N ratios we need to record
fine details at or better than the theoretical resolution of our optics. This
is why the best lunar images are not taken during a full moon where the
lack of shadows reduces contrast and resulting S/N ratios. Some of the
best lunar images have been and are made on small telescopes of 16"
or less, many are with SCT's not noted for high Strehl ratios. These scopes
are looking through relatively small columns of air resulting in images less
likely to be exhibit less contiguous blurring at typical amateur locations
as compared to a larger aperture telescope.

With the case of SCT's, the secondary obstruction and subsequent
contrast losses are small as compared to the overall S/N ratio. However,
good collimation increases the S/N ratio of the individual point sources
that make up the extended object and thus apparent resolution. In this
case, a small collimation error can produce enough loss in S/N ratio that
apparent resolution is reduced by 30-60% or more. In fact, the resolution
has not decreased as much as it would seem (as determined by viewing
close double stars for comparison), but the loss of contrast makes it
appear as so.



CAUSES FOR SCT COLLIMATION LOSS

It would seem that maintaining good to excellent collimation is a good idea.
Newtonian's often loose collimation as the weight of the diagonal increases
deflection of the spider vanes when pointing away from the zenith. Newtonian
primary mirrors may also move a bit in their cell. A counter weighted
secondary and/or thicker spider vanes helps.

SCT's such as the LX-200 that tend to loose collimation for mechanical
reasons that are quite correctable by the user. I often transport my LX-200
and collimation remains generally quite good. A SCT can be prone to
excessive collimation losses from a number of sources such as corrector
movement in the front casting, excessive secondary cell clearances in the
corrector, excessive primary mirror tilting (image shift) excessively loose
secondary collimation screws and differential optical tube expansion/
contraction, optical tube looseness, as well as a loose primary mirror.
If all the described sources are controlled, collimation in a SCT remains
good without frequent adjusting.



PREVENTING EXCESSIVE COLLIMATION LOSS

Corrector movement (more common in larger SCT's) is controlled by adding
thicker cork shims to the corrector edge, compressing the cork just before
insertion. This allows even corrector expansion but stops gravity from shifting
the corrector under the corrector retaining ring as the optical tube is moved
across the sky.

Excessive secondary cell clearance in the corrector is controlled by adding
masking tape to the secondary cell circumference for a snug fit. This also
helps prevent secondary loosening and movement during temperature
changes.

Excessive primary mirror tilting (image shift) is minimized with a somewhat
slippery and viscous lubricant. I have been testing a new lubricant that
appears better than anything I've used to date. The image shift on my 12"
LX-200 was reduced to a few arc-seconds in cold and warm weather- not
enough to effect collimation except for the most severe imaging requirements.

Excessive loose secondary collimation screws is minimized by tightening
all three completely, then adjusting collimation by loosening the desired
screws. This helps to prevent the secondary mirror from moving slightly
during thermal and mechanical stresses.

Differential tube expansion is controlled by waiting to start an image until
temperatures are stable and/or the use of gentle heat may help. The tube
may expand a bit on one side due to differences in cooling rates. This causes
collimation loss, focus loss, and guiding errors when using a guidescopes. I
have observed this anomaly in the Land Mode. Invar rods will control tube
expansion, but are not used in commercial or any known amateur SCT's
(other than Schmidt cameras) to my knowledge.

Optical tube looseness should be rare in Meade SCT's because more recent
models (as reported by John Downs) are cemented to the castings.

Loose primary mirrors should also be rare. Meade uses a cork gasket that
allows for expansion/contraction and a rubber o-ring (or silicone rubber) to grip
and hold the mirror on the baffle tube. My observations in the Land Mode
indicate very little image shift and collimation loss result from the Meade
methods of securing the primary mirror to the slider tube. Attempts to
support the primary in the thinned upper sections as often done with full
thickness mirrors are likely to result in astigmatism and should be avoided.



CONCLUSION

Good SCT collimation is always desirable and should be quite easy to
maintain in most SCT's in good mechanical adjustment. Those that find
they must constantly adjust collimation may want to review a few of the
items described above. Those with critical imaging requirements may
want to check collimation more frequently, however, only slight collimation
adjustments should be required.




--
Michael Hart
Husen Observatory

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