Tripod Mod Last revised: 06/22/02 Pietro.org Astronomy

Tripod Tray Vibration Damper & Leg Stiffener

A quick project that can be accomplished with spare material and simple tools

Usefulness of a Tripod Tray 

Fig.1: Tripod and Triangle Tray

Placing a tray on top of the spreaders of a collapsible tripod does more than create a convenient surface to place hardware on. 

A great benefit is increased system stiffness. I've found that swinging a scope, through the usual RA and DEC motions, all-night on a tray-less tripod causes the legs to retreat toward the center of the tripod. This slight motion destroys polar alignment. A tray stiffens the spreaders (the parts that fold between the legs) and keeps the legs apart. Another benefit of a stiff system is the increased ease in swinging the tripod around . A common setup task for my tripod concerns the equatorial wedge and its limited azimuthal range of motion on the tripod head. Polar alignment cannot be done if the original placement of the tripod is such that the wedge is mounted pointing to far East or West of North. Proper alignment sometimes requires rotating the whole tripod/telescope system a few degrees  -- an easier task if the legs stayed spread out by themselves.

Lastly, I have found that the stiffening provided by a simple tray can improve observing by affecting an important tripod property: vibration damping.  Much advice can be found on the Internet for reducing tripod vibrations: placing vibration dampening material under the tripod feet, filling hollow tripod legs with sand, clamping a wood frame around each metal leg, replacing lightweight metal legs with solid wooden legs, etc. Besides vibration dampening foot pads, most of these projects will make your light-weight portable tripod less portable.  On the other hand, they may deaden tripod vibrations more than a tray. But building a tray is a good first-start in tripod vibration dampening: it is very inexpensive, easy to build, and does not compromise portability. If, after construction, the dampening effect is less that expected, at least you have created a convenient surface to place hardware on. 

Stopwatch-Slam Experiment

This tripod tray project grew from my crude timing experiments with wood boards clamped to the tripod spreaders.  These experiments indicated that damping time for telescope vibrations can be significantly cut when a tray is installed. You can test this yourself: setup your tripod mounted scope as usual and observe a bright star at high power. Put a stopwatch in one hand with a finger on the start button. While observing the star: slam that hand into the tripod and start the stopwatch on impact. Stop the chronometer when the visual star vibrations stop. It may take a few attempts to learn this "stopwatch-slam" technique. Note that the damping time is fairly consistent over a range of initial amplitudes (how hard you hit the tripod -- probably because greater initial deflections induce greater initial damping).  Now find a " (13-mm) thick or greater wood board that fits between the legs and clamp it to the spreaders. Use the "stopwatch-slam" to gauge what difference this tray prototype would make for your setup.

Meet the Tripod 

Fig 2: Tray corner butts tab on spreader; bolt is through spreader slot.

The directions here will demonstrate how to build and test the efficacy of a tripod tray on a common type of collapsible aluminum tripod with plastic spreaders. Because the physical details of the Celestron C5 Field Tripod tripod are very common (model #93591, weight 10-lb/4.5-kg, height adjusts from 31"/79-cm to 54"/137-cm) , this project can be adopted for any tripod of the same configuration. Note that this project is simplified by the fact that the tripod was designed to accept a triangular accessory tray. Each of the three plastic stems of the tripod spreaders (i.e., what Celestron calls "Leg Support Brackets") has a slot on the ends near the legs and a raised tab just past that. 

Fig. 3: Tray and back of spreader-system.

The tab is meant to help center the tray and the slots can accept protrusions or bolts that prevent the tray  from slipping. Spreaders meet at a central joint that has a hole in it. All holes and slots in the spreader system are actually tubes with thick walls -- making them ideal for holding bolts and other hardware.  Your project will have to be customized if your tripod doesn't have these features.    

Project Material 

Sizing information for this material is given below.

  • Flat wood board 
  • 3 end-bolts 
  • 1 center-bolt 
  • 8 washers - or 2 for each bolt
  • 4 wing nuts - or 1 for each bolt

The board material used here was " (13-mm) thick plywood of medium density with one side finished -- but any type of workable wood will do.  Select a piece small enough to fit between the legs when placed on the spreaders but big enough to simultaneously cover all the raised tabs. I choose a thickness that allows the tray to fit in an equipment case I use,  but thicker wood has a better dampening effect.

The wood will be worked into a triangle with flat corners.  Holes will be drilled at the edges and center of the triangle to match slots and holes in the tripod spreader assembly.  You may wish to experiment with other shapes.  More protruding shapes, such as a disk, may lead to tripod bumping, and cutting "knee" notches into a shape may reduce damping and rigidity. 

Fig. 4: Tripod is upside-down with wood placed on spreaders.  "A" is thickness of spreader + wood at end-slot.

Determine end-bolt length by measuring the combined thickness of spreader and the wood board at the slot location -- location is important because the spreaders may taper throughout their length.  Center-joint bolt length may differ from the end-bolt length -- so measure for that hardware separately.  Add to these measures the combined thickness of washers and hardware that will thread each bolt tip (next section). 

The instructions here will place bolts through the top of the tray and push the tray against the spreaders by tightening wing nuts threaded at the bolt tip. For a low-to-the-ground tripod you may find it awkward to thread wing nuts from underneath the tray. Fortunately the completed project is easily modified for any other hardware configuration that is convenient to you.

Determine bolt diameter by dipping drill-bits (smooth end) through the slots and holes. Find a size that fits smoothly without rubbing the sides.

Fig. 5: Hardware sampling.

As long as they meet the size requirements any type of bolt will do -- so save money and a trip to the store by scavenging for hardware around your home. For convenience all four bolts could be of a uniform size that fits all the slots and holes in the spreader system. Because most of the length of the bolt will be through the wood and spreader, try to use carriage bolts: smooth bolts that are only threaded at the ends.

Size and Mark the Wood 

Fig. 6: Marking the center hole.

The first step is to mark the wood with the positions of the raised tabs, slots, and holes that exist in the spreader system. This is easily done if the wood board is bolted against the underside of the spreaders.

With tripod legs spread on the ground, place wood against bottom of spreaders and positioned to extend past all the end tabs. Use a pencil to trace the center hole (fig. 6). Remove the wood from the spreaders and drill at the marked center hole.  Use a drill-bit that matches the center bolt you measured for.

Fig. 7: Tabs marked and board bolted and clamped to bottom of spreaders.

Before bolting the wood underneath the spreaders, you must translate the raised tab positions down the sides of the spreaders. As seen in figs. 7, 8, and 9 this is done by abutting masking tape against the inside of the tabs and folding the ends of the tape down to indicate the tab position along the sides of the spreaders. Do not let the tape block the bottom of the slot

Place the wood underneath the spreaders and bolt in place using the central bolt hardware. You may need to clamp the wood to the spreaders, or have someone hold it, to keep it from spinning. This is shown in fig. 7. As shown in fig. 8: use a pencil to mark the wood on each side of every spreader where the tape marks the tab position. Trace out the slots in each spreader as shown in fig. 9

Fig. 8: Mark tab position on wood. Fig. 9: Trace out slot.

Saw and Drill the Wood 

Fig. 10: Connect the dots.

Remove the wood and lay it on a flat surface. Use a straight edge to connect the six pencil marks that represent the tab positions (fig. 10). Saw along these lines. It is not critical that the cuts be exactly straight, so an unguided hand-saw can do for the cutting.  

Drill appropriate width holes at the centers of the marked slots. 

Test mount the tray on top of the spreaders. Look through the drilled holes and make sure they line up with the spreader holes. Put a washer on each bolt and set them through the drilled holes. At some point your astronomical observations may require mounting the tray in the dark -- now is the time to make sure it goes on easily. If the fit is tight, or the bolt holes don't line up, file down the wood corners. 

Raised sides may be added if you fear objects will roll off the tray.  You can glue or nail thin strip of wood along the sides.  Any thick adhesive backed material you having lying around, such as Velcro or weather stripping, may be able to do the job.

Vibration Damping Tests 

Fields tests were done with the clock-drive running on a polar-aligned mount with telescope aiming at an eastern star 45-degree above the horizon. A diagonal, short-barlow, and 5-mm eyepiece were in the C5+ visual back for an approximate magnification of 500x. The rubber-tipped legs of the tripod -- on asphalt -- extended a few inches -- my usual setup. Visual timing tests were done using the "stopwatch-slam" method described at top. Six measurements were made for three configurations:  

  • No tripod tray: Results time without the tray served as a baseline for this experiment.  
  • Tripod tray loose: Tray in place with bolts, but wing nuts are not tightened. Discovering significant vibration damping without having to tighten the bolts would simplify setup -- especially in the cold-dark of winter. 
  • Tripod tray tightened down: Tray in place and wing nuts tightened.
Damping Test Results
  No Tray  Tray Loose  Tray Tight 
Average damping time (seconds)  2.09  1.78  1.36
Damping time Standard Deviation (% of avg)  13%  9%  5%
Damping time Decrease from "No Tray" (%)  0%  15%  35%

As we see from the results table, the damping time without the tray was over 2 seconds. A loose tray brought that time down a bit, and the tightened tray showed a significant 35% decrease in damping time -- to about 1-1/3 seconds.  Deviation in time measurements, as a percentage of the average time, decreased with damping time.  Your results will vary based on tripod, surface, and tray material characteristics.

Afterthoughts

The addition of the tray makes the tripod I've used for the last 4-years seem like a brand-new item.  I knew that thicker wood would reduce damping even further, but the " board made a significant difference in damping time, while being thin enough to store in an existing equipment case.  Most of my observations take place at much lower power -- about of the 500x eyepiece setup used in the timing tests -- so vibrations will likewise be less magnified in actual use.  The biggest beneficiary of of vibration dampening may be long exposure photography.  Cutting oscillations from diverse sources, such as street traffic and wind, should make for sharper images. 

My long-term concern is the integrity of the plastic tripod spreaders where the bolts are tightened. In winter the temperature drops rapidly at my location and may cause the already tight bolts to contract down on the spreaders. Perhaps rubber washers would be better. 

Post your vibration dampening ideas, test results, and other comments related to this project on the message board.
Fig. 11: Top view of finished tray on tripod Fig. 12: Bottom view of finished tray on tripod.
 


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