The Mount

The McCreary Mount is the first truly novel equatorial mount design in this generation. It is intended to provide superior smoothness and accuracy in tracking when compared to any previous design.


Why Use a McCreary Mount?


* Dramatically inexpensive to build or purchase

* Silent in operation, allowing you to listen to

  "the music of the spheres"

* Inherently strong - supports far greater weight than

  other comparable designs

* Precise - subarcsecond accuracy possible in

  a well designed and constructed implementation

* You can build it yourself with simple tools

* Produces superior astrophotography results

* Observatories can realize tremendous cost savings

  and significantly improved accuracy when compared

  with other designs


The Prototype: Some Preliminary Specifications


Two Major Components

- Base (equilateral triangle 4'x4'x4')

- Platform - handles rocker box up to 22.5" square

- Each component easily carried by one person


Tracking Angle - 20 degrees

Tracking Time - 1 Hour, 20 Minutes


Periodic Error - None

Tracking Accuracy - < 1 arcsecond during 80 minutes

Tracking Adjustment - RA only (i.e. rate of motion)


Maximum Instrument Weight - 200 pounds


Polar Alignment - Altitude and East/West Bearings are adjustable with 10-32 pitch adjustment screws for adjustments in arcseconds rather than arcminutes

How and Why?

How is the McCreary Mount fundamentally different, and why is it better than anything that came before?


The fundamentals of the McCreary Mount are extremely simple. This simplicity is the secret of the mount's basic strength and accuracy.


Two Points


In simple geometric terms, two points exactly specify a line. In the McCreary Mount, these are two literal points - in the initial concept and "proof of concept" implementation, these were two steep, sharpened points of two metal bolts, mounted on a base or on piers to establish a polar axis. In newer and more refined implementations, these points are the centers of two extremely precise bearing balls.


Each of these two bearings is finely adjustable in position along a single axis, resulting in the ability to produce extremely accurate polar alignment.


Gravity Driven Fluid Motion


An incompressible fluid (typically water) is used to regulate and control the motion of the McCreary Mount. The weight of the platform and the instrument drive the fluid through a valve that precisely, smoothly, and silently regulates the mount's motion. There are no motors, gears, roller bearings, or other sources of mechanical "noise."


Design Variations


There are many variations possible, limited only by your imagination. The design inherently lends itself to great strength in construction, making it suitable for supporting very heavy observatory instruments much more economically than earlier existing technologies.

My Claims

What Am I Claiming as Original?


I claim the following as being, in their combination and synergy, the essential elements of my invention. To the best of my knowledge and belief, this unique combination of elements has not previously been invented or disclosed.


The McCreary Mount is a telescope mount that falls in the general category of "equatorial mount" in that it counteracts the motion of the earth's rotation in order to keep a telescope or other astronomical instrument accurately pointed at a celestial object. However, it differs radically from all prior devices in that it brings together for the first time the following combination of design features:


Specific Combined Elements of the Invention


The McCreary Mount employs a base comprising two literal, firmly fixed but finely adjustable geometric points which uniquely define a polar axis of rotation that can be precisely adjusted to specify a line exactly parallel to the earth's axis of rotation. In the preferred embodiment, these points are the geometric centers of two extremely precise spherical bearing balls mounted in any suitably stable supporting structure with their upper surfaces exposed to bearing sockets in the associated instrument platform.


It incorporates a platform or mounting for the telescope or other instrument or instruments, having two bearing sockets arranged so as to mate with the two support bearing points of the base. The shape and design of the platform may be infinitely variable and adaptable to the needs of the device that it supports.


The bulk of the platform is situated on the west side of the two supporting point bearings. This platform need only be sufficiently strong and rigid for the particular application and to prevent changes in the amount of sag or flexure during the range of motion desired for any particular implementation. The placement of the platform on the west is deliberate for good and sufficient engineering reasons.


Towards the west end of the McCreary Mount a hydraulic means of regulating the motion of the platform is employed. In its preferred embodiment, gravity is used as the motive force during tracking, and the flow of an incompressible fluid such as water may be finely adjusted through any suitably precise valving mechanism to regulate the motion.


Obviously, a curved fluid cylinder with a constant flow valve  will provide the most regular platform motion, free of tangent error. Also, in applications at great distances from Latitude Zero (the equator), modified versions of this hydraulic means may be employed while still retaining the basic identity of the McCreary Mount.




An infinite set of variations of and enhancements to the above described essentials are possible, but any system incorporating these elements in combination is fundamentally a "McCreary Mount."


It should be obvious to those skilled in the art of telescope making that such a device can be adapted to a wide range of applications, and can also benefit from devices and methods previously developed for existing equatorial mounts, such as setting circles and such as closed-loop guiding feedback mechanisms.


The inventor explicitly discloses and releases this invention into the public domain with the express desire that the use of his invention might further enable and facilitate the revelation of the Glory of God, Who through Jesus Christ created the heavens that we as astronomers all enjoy.


The McCreary Mount eliminates virtually all sources of periodic error.


All the usual error generators are eliminated, such as:

- Worm

- Worm gears

- Worm wheel

- Spur gears

- Auxilliary gear trains

- Motors

- Polar shaft

- Roller bearings

- Clutches

- Counterweights

First Principles

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

  ever possible

- 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.




While superior in many ways, the McCreary Mount admittedly has a few drawbacks.


- The mount is (deliberately) "larger than your average mount" - a price I believe to be well-spent for simplicity, accuracy, and great strength and stability.

- Portable versions are also "tall", often requiring larger ladders to reach the eyepiece.

- The direct gravity-driven fluidic drive is best used at latitudes of +/- 50 degrees. There are ways to adapt the design for use outside of this range, involving alterations to the hydraulic drive design

- The bearing design also works best at lesser latitudes, but there are also ways to stabilize the polar axis bearings for use outside of this range.


Problems Shared with Other Designs


Like other "platform type" equatorial drives:

- There is no "convenient" Declination tracking adjustment

  - A dec adjustment could be implemented in other ways


Like ALL equatorial drives:

- Does not directly correct for refraction

- Requires the telescope itself to not flex or bend

- Requires reasonable care in design and construction


You Want This Mount!


Once you understand the principles of it's operation, and recognize the potential specifications of a well-built version of the McCreary Mount, you will want one for your serious astrophotographic efforts.


The McCreary Mount is sure to become the "sina qua non" for all serious ground-based astrophotography.


The Prototype McCreary Mount