Kevin's Observing Table
Page last updated: 2002 April 15
Observing Table
Completed as of 2000 August 07
Start of the Project: 2000 March
Every year I and many other Astro-Dudes partake of the Star Party known as StarFest,
held at Mount Forest Ontario. And every year we come back full of ideas and energy to do
something about them. This observing table project came out of those ideas.
Of our group of 8-12 transgressors, only 2 had observing tables and they weren't happy with
them in any event. So about the beginning of March I started hunting the net
for project ideas. I found some
here and in other
spots.
Benefits
Design
First step was to see what others thought of their tables. Too small and too low. OK, the internet table was 29 x 20 inches, but if we want to make a bunch of these the dimensions should fit better into standard pieces of wood (ie 4x8 or 4x4 or 2x4). So I picked 24" x 18". 6" deep looked good so I split it between 5" on the bottom and 1" for the lid to allow for a pouch in the lid. Having builtin legs was a convenient must so I added another 2" depth to the frame only of the box, leaving the bottom of the box 5" from the top.Looked at material and chose 3/8" plywood for the box sides and 1/4" plywood for the lid, base, internal partitions and leg braces. The leg design would not allow for a straight fold out leg of more than 22" or so, add another 6" to that for the box height and the lid was 28" above grade. Not enough. So we came up with a double folding leg design that would take the boxtop to approx 36"
Inside the box I wanted space for at least 4 eyepieces along one side, a large area big enough for the biggest charts I could find around here (14"x19") (Star Atlas 2000) with the Observers Handbook, magazines, binoculars and whatever else, and another spot for flashlights, pens, pencils, etc. A plexiglass cover plate would be neat to put charts under to protect from dew. Initially this was in the lid (with no pouch), but later was moved into the box itself over the 14"x19" chart area to allow for a pouch to be built into the lid. Built in Red LED's with variable lighting would also be nice.
The lid could be propped open (with an arm?) to allow access to the eyepieces and charts or closed to be used as a wiriting surface.
Assembly
The 3/8" plywood was cut up into 7"x24" (front), 7"x24" (back), 7"x17.25" (left side), 7"x17.25" (right side). The sides were on the inside of the join, with the front and back extending on the outside (standard box). Two 1"x24 and two 1"x18" pieces were cut out for the lid (note in the future I would make this 2"x24 to make it easier to assemble). A router was used to route a 1/8" groove where the bottom plywood would go in and rest, 5" down from the top, allowing 1.75" underneath for the legs. More grooves were also inserted for the internal partitions. The bottom was then cut out of the 1/4" plywood (24"-1/8-1/8=23.75") by (17.25"-1/8-1/8=17") and another 18"x24" for the lid. The bottom was not quite cut right and was too small when in the grooves. So a ledge was added in on both front and back so the bottom would not fall out. Carpenters Glue and 3 screws (#6x3/4")(predrilled holes) went into each side and the basic box was assembled. The lid sides were assembled first with 2 screws (#6x3/4") and glue, squared up and then the 1/4" plywood lid was glued and nailed on using tiny finishing nails (1/2"). The lid and box were attached using a 24" long piano hinge. Holes were predrilled and nuts and bolts were used to put them together. On the front, two clasps were installed to close the front.
On the inside, two partitions were installed, one running top to bottom, approx 20"
from the left side, leaving a 3"x17" partition. Another 20" partition was installed running left to right
approx 15" from the top, leaving a 3"x20" partition on the bottom. They were glued
in. The partition height was 4", leaving a 1" gap.
This about used up the 24"x48" 1/4 plywood sheet so I went hunting for scraps from
other projects to build the eyepiece holder. This was a 3"x15"x1/4" piece of wood
that had two 5/4" holes drilled (for eyepieces) and four 7/4" holes drilled for
eyepieces in plastic cases that I use for storage. The holes were handdrilled with
a funny looking big-hole-making-bit. The eyepiece holder needed to be held up off the bottom, so
a small square (2.25"x2.5")of 1/2" plywood scrap was made for each end and glued and
nailed on.
OK time for the legs. Originally I was planning on 1"x2" wood (pine) but upon inspection the stuff in the store was pretty flimsy, so instead I went for 2"x2" (pine). A small mistake but more on that later. We were looking for a leg height of approx 30" but the base was only 24" long. So we made one leg approx 22" long and mounted it with a block and a carriage bolt on one side, then added attached another overlapping piece and secured the two together with another bolt. When the 2nd piece was swung out it was 30" long. We drilled another hole and inserted a dowel and presto, a leg of the correct height! The other leg quickley followed suit. Now the tough part. Attaching a cross brace in such a way that the legs on the other side could interlock. The cross brace on the first section of leg went flush against the bottom of the box, the other went on the bottom (see the photos, this is tough to describe). That done the leg could fold out and lock in place. Remove the dowel and collapse it back. This left side legs were on the outside of the area. The right side legs had to both be on the inside to allow them to fold up. We used two block mounts per leg since there was no box side to take advantage of. Assembled the leg segments in the same manner and tested. They weren't even close to parallel and would not extend. More redesign and finally got them lined up. Added cross braces (again glued and screwed) and tested out the whole system. The box was at a great height! The right side legs were a little too close together but this was the prototype. On the next table, we will try a better quality of 1"x2" wood and use it for the legs. That should allow at least another 3" spread of the legs. When open the legs are past vertical and are held up against the box sides. Perhaps some string will be put between them to make sure the whole box/leg assembly doesn't fall apart.
Back Inside
Got the 1/8" plexiglass and cut it to size (15"x19") using a $4 plexi cutting
tool. Used it incorrectly the first time and put a chip on one corner.
Decided to mount it inside the box
by using a 18" piano hinge. Thought some more and decided that
there needed to be a surface under the plexiglass to hold the chart
up to the plexiglass. Went out and got some table counter stuff (formica)
and cut it to size.
Taped it to the plexi and lined up the hinge and predrilled the mounting holes.
Remember to drill slowly! I forgot once and the plexi cracked and shattered for about 1-2"
Removed the tape and the piano hinge matched up with the lid hinge and
attached to the same bolts.
Mounted the plexi and table material. Works great! Added some foam weather
stripping to the bottom so it rests gently on the 4" partitions. It overlaps just enough
on the bottom and right to allow easy lifting. It sagged a bit on the left
so a small scrap was tacked on to provide a ledge under the plexi.
Needed to hold the plexi in place when the box was vertical, so added some sticky
velcro to the underside of the plexi and some sticky strip on the partition underneath it
(and stapled it in on both sides). Plexi now stays in place quite nicely.
At this point the inside and outside were varathaned, sanded (medium), varathaned again and sanded (fine) again. The folding handle was added next but the screws were too long for the 3/8" front. So another small scrap of 1/4" plywood 3"x4" was glued to the inside of the box and then the handle attached (with the screws penetrating the 1/4 plywood). Inside, a thin layer of foam was glued into the chart area, a thicker foam went under the eyepiece area and in the front bay area. A foam strip was put into the lid to hold the eyepieces in when the case is closed and vertical. A velcro strap holds the plexiglass closed. Another foam strip holds the items in the front bay in.
 bottom view |
 legs partial |
 legs locked |
 table up |
lid open |
 side view |
When reasonably complete, peel off the plastic or paper protected cover from the plexiglass. The 4 eyebolts used to lock the legs went into a canvas pouch with a velcro closing, with more velcro on the canvas to hook it in place under the table with the legs, keeping them accessible and quiet. Added 4 felt pads on the back side so when the box is upright, the piano hinge doesn't get damaged.
Lighting Design
An off/on toggle switch and a 5k potentiometer will be installed on the eyepiece ledge. A red LED will be inserted into the case front (power on indicator, or anti-collision light if you prefer), and 3 other red LEDs into the lid (left top and right). The wires are scrap 24 gauge cat5 cable and glued in place. A 100 ohm resistor is in series to allow the system to run from a 12vdc source. Originally a 12vdc battery was going to be built in but we ran out of room. Now there will be an external power jack so the table can be powered from the Battery Box Project (coming up next!). Other uses for power here is a clock, radio for time signals, and other timing devices for barn door tracking. Note: changed back to internal power source.Using the table
Carry it around and set it down vertically, unfold the legs. Get the bolts and lock the legs in place, replacing the bolts bag underneath. Pull the box upright. Open the box and start observing. The rigid lid arm was replaced with a peice of string as the box is far more unstable in a wind, so I'll try using the box with the lid down when not in actual use.After the Fact
The lid should be a little thicker (1.5-2") to allow for more accurate lid to box fitting & construction. The inside legs should be farther apart.So... I tore apart the inside legs and reversed the position of the inside and outside portions so that the legs in contact with the ground are now 3" farther apart. (Leg Set #2) Conceptually the outer leg set could have the same done but I am tired :)
When the lid is open, the system is unstable in a moderate wind (so keep the lid closed!) How to keep it more stable? A bungie cord underneath hooked into a ground stake?
Built in lighting - using a small gooseneck lamp is a plan, as well as illuminating the plexi from its edges using LEDs.
the box is still unstable in a wind with the lid up. The inside legs still have too narrow a stance. Let's think about replacing the legs with pvc piping. two sections, one fitting snugly inside the other. The first one about 20" long and the second about 20" long. Drill several holes through both pieces to allow a peg to go through and hole the lower section in place, at several different heights to allow for different users.
Electrical: added in a small 7Ah 12vdc battery, power switch, 5k variable rheostat, a 100 ohm resistor and 4 super bright red LEDs, illuminating various portions of the box for use. Also inline was a pressure switch that would turn on the LED's when the lid opens and turn them off when it closes. Glued the 22 guage wires down along the inside of the box and glued down foam padding overtop of that. The 100 ohm resistor is in series with the rheostat to protect the LED's from 12vdc with no current limiter. I may add in an external power jack for use in charging the battery or drawing off external power for something else (like maybe a motorized barn door tracker?).
Set #1 were the wooden pine 1"x2" folding pine legs - way too unstable
Set #2 were the same legs as above but rebuilt for a wider stance - still unstable
Set #3 were 1/2" aluminum tubing with variable height pegs - completely unusable -
cost about $25
Set #4 were 2x2 pine single part legs 30" long that "plugged" into blocks
under the table. There gave the best results too date but the legs would often
not fit in correctly, would sometimes fall out cost about $15
Set #5
 side view |
 angle view |
 foot assembly |
 top assembly |
This set is a folding TV tray style made of 1"x2" whitewood (harder than pine). Cost about $25 for an 8', 8' and 6' section. 4 legs were made 40" long each identical. inside table dimension = 23" height to table was 30" trig gives us a leg table angle of 49 degrees, so the tops of the 4 legs were cut to that angle. The top side had to rest against the table wall and were cut at 90-49=41 degress. The bottom ends were left until after complete construction for fine tuning..
Two crosspieces of 1x2 were cut, one for the top and one near the bottom. All holes were predrilled and all wood was glued together. The outside legs were assembled first, leaving a 1/8" clearance on both sides for the legs to fit into the table.
The inside legs were trickier. They use three crosspieces, top bottom and one in the middle. I tried to give it an 1/8" clearance with the inside legs but ended up somehow with over 1/4". So I added in some spare washers to pad the space.
After both legs were assembled the tricker part came in aligning them. So the box was placed upside down, both legs inserted and clamped. I was trying to measure for 36-37" from floor to leg. After many attempts the legs were close enough and a single screw was put through each side. The outside legs were piloted to a large size to allow the screw to freely rotate.
Put the legs out and the table on top and drew lines on the bottom legs to cut so they sit flat. Cut those angles and remeasured. A few more attempts and the entire box was sitting at 36" and flat within 1/4".
Two eyebolts were added and a string tied across to prevent the legs from spreading apart before the table was firmly seated on top. Then the entire assembly had 4 coats of varathane added on. Done! Working, stable, portable legs!