What are the First Principles of engineering design that have been overlooked in prior art, but are implemented in the McCreary Mount?
The following is a set of notes and outlines that feature some of the principles employed:
Seriously Engineer Only What Is Most Essential
- Shun conventional wisdom
- Put strength in the supporting piers & adjustments
- Pay attention to platform rigidity, not shape
- Require ultimate precision only where most needed
- Bearing balls Grade 24 (1/6th arc-second)
- Bearing balls Grade 10 (1/20th arc-second)
- A smooth, accurate hydraulic cylinder
- Eliminate tolerance stack-up
- Use gravity, don't fight it!
The Ultimate Simplicity of the Polar Axis
A line is defined by two points:
- Mount bearings are reduced literally to 2 points!
- The center of a sphere is a point!
- Most important, most solid, most precisely adjustable!
- Not much else matters or is critical!
- Bearings should have 1-dimensional, tight adjustments
- Adjust Elevation with one bearing point
- Adjust East/West with the other bearing point
- Use successive approximation for polar alignment
- The distance between points will change with adjustment
- Accommodated for by allowing one-dimensional upper
bearing socket movement in a track in the platform
- Support bearings on two solid piers in a fixed observatory
- Use a broad, stable base in portable units
- Sharpened points (one implementation) are effectively
very small spherical bearings
- Observatories may use large spherical bearings
- Personal mounts can also use inexpensive bearing balls
- Available accuracy to 10 millionths of an inch!
- Grade 10 bearings are accurate to 10 millionths
- Theoretical results are about 1/20th arcsecond
- Grade 24 bearings are accurate to 24 millionths
- Results in about 1/6th arcsecond in my prototype
- Polar alignment becomes very easy because when performing adjustments you only need to move the two bearings, not an entire assembly consisting of a base, shaft, bearings, etc. The base remains stationary and the platform and instrument are coupled to the bearings that you are adjusting only with a bearing surface.
Geometry is Far More Important Than Weight!
- Don't think that making your device heavy will
make it better!
- Use triangles and trusses, resulting in low flexure
- Use larger rather than smaller dimensions where
- The wider apart the bearing points, the smaller
the alignment change with a given motion,
the more precise the resulting alignment
- A curved hydraulic cylinder gives a true and steady rate
- no tangent-arm errors
- pressure compensation may be necessary
Avoid Balance! Use Pre-Stressing Instead!
- A balanced mount is susceptible to "noise"... every little
stress will move it off target
- An off-balance design helps to pre-stress all
structures into a strong, stable, steady form
as well as to pre-load bearings and eliminate
any possibility of backlash
Eliminate Tolerance Stack-Up
- Be sure all stresses are "uni-directional".
- Stability and accuracy actually improve to an
extent with instrument weight. The weight of the
telescope removes all slack, stresses all bearing materials
in one direction, and pre-stresses the platform structure.
- Flexure self-eliminates during modest length exposures.
The McCreary Relaxation Drive
Gravity is a steady, stable, reliable force.
- It drives the motion of the mount.
- The weight of the instrument helps and stabilizes.
- It pre-stresses the structure and bearings.
- Gravity supplies both power and "transmission" - i.e.
there are no gears, etc., needed!
Water as a fluid medium:
- Smoothly adjustable, reliable rate
- No vibration
- Non-compressible - i.e. no "bounce" as would be with air
- Silent operation is possible
- Leakage is OK as long as it is "constant"
- A medical ventilator-like non-contact piston could
be employed for greatest accuracy, where there would
be a small but constant leakage around the periphery
of the piston.
- Engineering trade-offs
- Larger fluid cylinder gives finer rate resolution
- Longer cylinder gives more time, but greater "tilt"
Swamp The Noise!
- Weight far overwhelms miniscule cylinder and
support point friction.
- Overcome resistance (resistance is futile! :)
- Be sure driving forces far overpower the friction of
any slip/stick forces.