Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making
Articles
(by Ross Sackett, begun October 30, 2002)
1. Worry about stiffness, not strength.
For conventional materials such as plywood and aluminum tube, if it is stiff enough it is very likely strong enough.
2. Keep your spans short and your sections thick.
Remember your simple beam theory
3. Choose materials that balance stiffness with lightness.
Lighter materials can be made into thicker sections, which brings great stiffness advantages for only a moderate
increase in weight. Some telescope makers choose metal because they think it builds stronger or stiffer scopes. This
confuses the inherent elasticity of material with the stiffness of the structures made from it. The stiffness of a
component depends on its dimensions as well as the elasticity of the material itself. For parts subjected to simple
bending wood can be stiffer than a steel part of the same weight.
4. Wood is probably the optimal material for most components.
For its combination of moderate stiffness and low density, wood generally numerically outperforms metals like steel and
aluminum and even exotics like carbon fiber (and beats fiberglass by a long shot). When we factor in low cost, ease of
fabrication, and aesthetics wood is by far the winner for many telescope components. Materials like aluminum tube
may have advantages for some components because wood cannot be readily fabricated in thin hollow shapes.
5. Orient the grain to the force.
For components stressed in simple bending, use solid wood with the grain parallel to the axis of bending. For parts
stressed in shear (or bending in two directions) use plywood, but keep in mind that it is stiffest along the axis of the face
plies.
6. Whenever possible use inherently stiff shapes.
Triangles and boxes are stiff, rectangles and cylinders are not. Remember: a box has six rectangular sides attached
stiffly along all their edges. Even with a large circular cutout in one of the faces a mirror box can still act like a
structural box as long as all six faces are rigidly connected. Many structures that look like boxes are really tubes with
only four rigidly-joined sides, and even deep gussets can’t turn these into structural boxes.
7. Attach structure at the most rigid places.
Attach structural members to triangles at the vertices, not along their sides. For boxes, the corners are the stiffest
points for attachment, followed by the edges. The center of the face of a box is the worst place to attach structure.
8. Make the mirror box as stiff as you can while remaining reasonably light, and the UTA as
light as you can while remaining reasonably stiff.
9. Don’t mess with the truss.
The overall stiffness of the telescope depends on the truss; economizing weight-wise on the truss can badly
compromise stiffness. It probably isn’t worth it since the truss is usually only 5-10% of the overall instrument weight and
cannot yield big weight savings anyway.
10. Joints must be rigid.
For stiffness attachment points should fit closely, one part registering against the other. Metal fasteners alone may not
be enough; joints should be bonded with adhesive (if permanent) or set with bedding compound. In general, rely on
metal fasteners for strength and adhesive for stiffness.
11. Sandwich structures combine great stiffness with great lightness.
12. Mind your degrees of freedom, and don’t make it more adjustable than it needs to be.
Every point of adjustment is a potential point of maladjustment, so keep the number of adjustable parts to a bare
minimum. Take the three collimation bolts on the mirror cell that control the tilt of the mirror. Only two of these need to
be adjustable to collimate the mirror; twiddling the third bolt merely complicates things, makes the focal plane float
forwards and back, and has the potential to accidentally knock the mirror out of alignment. Don’t put a knob on the
third bolt, and consider gluing it in place. Similarly, edge stops (or slings) need to be adjusted only once in a
telescopes lifetime. Once these are in the position needed to center the mirror over the cell glue them for stiffness and
screw them for strength—you gain nothing (and risk much) by making these adjustable.
The tradeoffs:
Aperture vs. cost
Aperture vs. portability (=weight?)
Stiffness vs. weight
Looks like aperture, cost, stiffness, and weight/portability/convenience all interlocked
The intelligent tinker…
Prototypes everything
Uses inherently-stiff shapes
Builds like a boatbuilder, not an engineer
Balances innovation with established designs
Keeps it as simple as possible
Makes it no more adjustable than it needs to be
Copyright Ross Sackett 2009
1. Worry about stiffness, not strength.
For conventional materials such as plywood and aluminum tube, if it is stiff enough it is very likely strong enough.
2. Keep your spans short and your sections thick.
Remember your simple beam theory
3. Choose materials that balance stiffness with lightness.
Lighter materials can be made into thicker sections, which brings great stiffness advantages for only a moderate
increase in weight. Some telescope makers choose metal because they think it builds stronger or stiffer scopes. This
confuses the inherent elasticity of material with the stiffness of the structures made from it. The stiffness of a
component depends on its dimensions as well as the elasticity of the material itself. For parts subjected to simple
bending wood can be stiffer than a steel part of the same weight.
4. Wood is probably the optimal material for most components.
For its combination of moderate stiffness and low density, wood generally numerically outperforms metals like steel and
aluminum and even exotics like carbon fiber (and beats fiberglass by a long shot). When we factor in low cost, ease of
fabrication, and aesthetics wood is by far the winner for many telescope components. Materials like aluminum tube
may have advantages for some components because wood cannot be readily fabricated in thin hollow shapes.
5. Orient the grain to the force.
For components stressed in simple bending, use solid wood with the grain parallel to the axis of bending. For parts
stressed in shear (or bending in two directions) use plywood, but keep in mind that it is stiffest along the axis of the face
plies.
6. Whenever possible use inherently stiff shapes.
Triangles and boxes are stiff, rectangles and cylinders are not. Remember: a box has six rectangular sides attached
stiffly along all their edges. Even with a large circular cutout in one of the faces a mirror box can still act like a
structural box as long as all six faces are rigidly connected. Many structures that look like boxes are really tubes with
only four rigidly-joined sides, and even deep gussets can’t turn these into structural boxes.
7. Attach structure at the most rigid places.
Attach structural members to triangles at the vertices, not along their sides. For boxes, the corners are the stiffest
points for attachment, followed by the edges. The center of the face of a box is the worst place to attach structure.
8. Make the mirror box as stiff as you can while remaining reasonably light, and the UTA as
light as you can while remaining reasonably stiff.
9. Don’t mess with the truss.
The overall stiffness of the telescope depends on the truss; economizing weight-wise on the truss can badly
compromise stiffness. It probably isn’t worth it since the truss is usually only 5-10% of the overall instrument weight and
cannot yield big weight savings anyway.
10. Joints must be rigid.
For stiffness attachment points should fit closely, one part registering against the other. Metal fasteners alone may not
be enough; joints should be bonded with adhesive (if permanent) or set with bedding compound. In general, rely on
metal fasteners for strength and adhesive for stiffness.
11. Sandwich structures combine great stiffness with great lightness.
12. Mind your degrees of freedom, and don’t make it more adjustable than it needs to be.
Every point of adjustment is a potential point of maladjustment, so keep the number of adjustable parts to a bare
minimum. Take the three collimation bolts on the mirror cell that control the tilt of the mirror. Only two of these need to
be adjustable to collimate the mirror; twiddling the third bolt merely complicates things, makes the focal plane float
forwards and back, and has the potential to accidentally knock the mirror out of alignment. Don’t put a knob on the
third bolt, and consider gluing it in place. Similarly, edge stops (or slings) need to be adjusted only once in a
telescopes lifetime. Once these are in the position needed to center the mirror over the cell glue them for stiffness and
screw them for strength—you gain nothing (and risk much) by making these adjustable.
The tradeoffs:
Aperture vs. cost
Aperture vs. portability (=weight?)
Stiffness vs. weight
Looks like aperture, cost, stiffness, and weight/portability/convenience all interlocked
The intelligent tinker…
Prototypes everything
Uses inherently-stiff shapes
Builds like a boatbuilder, not an engineer
Balances innovation with established designs
Keeps it as simple as possible
Makes it no more adjustable than it needs to be
Copyright Ross Sackett 2009
| Eyeball engineering rules of thumb |