My previous post covered disassembling the David White 8300 transit and this one will cover rebuilding and calibrating it.
Some of the brass hardware would not be removed no matter what I tried. Eventually, they would break or get damaged enough to warrant replacing. The two assemblies that put friction into the altitude movement wouldn’t come out. I had to drill them out. I’m not exactly sure what made up these assemblies though I do know there was a brass plug on the bottom, spring in between, and a nut on the top. I gather turning the nut increased the friction. I drilled everything out and came up with a plan to fix it. Here’s a picture of me drilling out one of the assemblies.
The altitude fine adjustment screw/knob had also corroded itself to the aluminum frame. While trying to remove it, the knob started spinning on the shaft. I removed the knob and worked on drilling out the rest of the screw. It wasn’t perpendicular to the base so I had to elevate one side of the frame before drilling.
The cross hairs on my transit had long since vanished. There was a thick black disk with a hole in it (see below) that had very fine groves in it that held the cross hairs. I looked around for what to replace the cross hairs with and found that tungsten wire or Kevlar filaments would work. Some said the cross hairs used to be made of spider web silk. Specifically, the Black Widow spider was mentioned. I felt it’d be safer to go with the Kevlar. Chris from One Tool at a Time was able to get me enough Kevlar thread to do a million cross hairs. To make cross hairs out of Kevlar you pull a piece of thread apart until you get to the filaments that make it up. The Kevlar filaments are very fine and strong. I supported the cross hair ring and laid the filament across it. To keep the filament from sagging I weighted the ends with a couple paper clips. Once it was in place I put a few drops of super glue down to hold the filament in place. I repeated the process for the other hair.
Once the cross hair had dried I reassembled the telescope portion of the transit.
To clean up the other upper parts of the transit, I media blasted them with glass beads and gave them a cost of paint. The paint doesn’t match the paint of the telescope but it works well with it.
To repair the vertical axis friction assemblies I taped the holes for a 5/16″ bolt which I had hoped to find made out of brass. I was unable to and used some grade 8 bolts instead since they’re golden in color. For the rest of the friction mechanism I used a couple pieces of nylon and some springs. This works well and gives me the ability to adjust the friction.
The small screws that held the vertical vernier scale also had to be drilled out and tapped to a 4-40 thread. I found some small brass hardware to hold it on. I’m had to use one of the washers elsewhere but picked up another later.
I lightly polished some of the brass and chromed pieces before putting the altitude locks back on.
The vertical lock knob survived the disassembly and was reused.
I needed to replace two of the brass screws and knobs. They were both 10-32 size thread but the screws I bought were too tight to turn by hand. To fix this, I ordered an adjustable die that I used to trim down the screws. I didn’t have a die stock in the appropriate size so I had to use my vise. Its slight overkill I know.
The azimuth fine adjustment screw was missing its knob when I purchased the transit. I stumbled upon a brass piece that would work as a replacement knob at Ace. It doesn’t match the style of the other knobs but it is better than a black plastic knob. If (when) I get a metal lathe I’ll see about making some replacement knobs. Opposite of the fine adjustment screw is the part which is supposed to put pressure on the tab on the frame to hold the transit in place while you adjust it. This was also broken when I picked it up. I settled on a small spring (not pictured here) to pull the tab up against the adjustment screw.
Since the transit is permanently stuck to the tripod base I welded some wing nuts and all thread together so I can get the legs on and off quickly. The welds weren’t pretty but they’ll work.
At some point I will probably clean up the tripod.
Even though I’ve fixed and reassembled the transit it isn’t useful until it is calibrated. In the real surveying world instruments can be taken in to a shop and calibrated. I’m not sure I’d be able to find someone to calibrate my transit even if I had tried. It might have been funny. So, I needed to do it myself. Calibrating it proved to be a little tricky but I eventually worked out a way to do it without specialized equipment. As an overview, when a transit is set up it needs to be level in all directions when you swing it around in azimuth. To help with this, there is a level built into the transit. Normally, you plop the transit down, position the level over a pair of leveling knobs, and turn them until the bubble is centered. You’d then turn the transit 90 degrees over the other set of leveling knobs and center the bubble. (My transit has four leveling knobs. Some have three. Adjust procedure as needed.) You repeat this until the bubble doesn’t move as you swing the transit is azimuth.
As you may recall, I took the level off when I disassembled it meaning that the axis of the level is no longer parallel to the optical axis of the telescope. When they aren’t aligned the transit will give incorrect readings. After thinking about it for a bit I figured out how to solve this problem using the “Two Peg Test”. This test is/was used to gauge the accuracy of your surveying instrument. I’m going to use it to calibrate and then test mine.
My process was this:
1) Adjust the level so that it looks parallel to the telescope optical axis. It won’t be at this point but get it as close as you can.
2) Set up the tripod and do your best to get the top of the tripod level by looking at it. The leveling knobs can only adjust so much.
3) Adjust the altitude locks so that the telescope is held level compared to the rest of the transit. This is just a guess at this point. Engage the locks.
4) Level the transit using the four adjustment screws on the base of the transit. Since the level isn’t parallel to the transit base you can’t just center the bubble using the adjustment screws. Instead seek to have the bubble displaced by the same amount of marks on the level when you compare the transit facing at 0 and 180 degrees (I’ll call this front and back). If you can’t get the bubble to do this you may need to adjust the altitude locks. Eventually, you’ll reach a point where the bubble is displaced an equal amount when you rotate the transit to face front and back.
5) Now, rotate the transit 90 degrees and adjust the other set of leveling knobs to get a bubble that is displaced equally when you compare it facing this way and opposite 180 degress (I’ll call this left and right).
6) Check front and back and side to side to make sure you still get equal bubble displacement. If not repeat steps 4 and 5. With steps 4 and 5 completed the base of the transit will be level and we can start the “Two Peg Test”.
7) The test has two parts and we’ll now do the first part. Mark out 100 or 200 ft (I used 100) and put pegs in the ground at both ends with your transit at the middle. Have someone hold up your surveying rod at the first peg (call it A) and take a reading with the transit. Move the rod to the other peg (call it B) and take another reading. Subtracting these two readings will give you the elevation difference between the two pegs.
8) Step two of the “Two Peg Test” required moving the transit and taking another reading. So, move your transit to peg B and repeat the leveling process covered in steps 4 and 5.
9) Measure the height of the transit’s eyepiece. You can now use your previous elevation difference between peg A and B to calculate what you should see when you take a reading of peg A from peg B.
10) Send your helper down to peg B and take a reading. Unless you’re really lucky, what you see and what you calculated won’t match. Thats ok. Unlock the altitude holds and rotate the telescope to get the reading you calculated. Now holding the telescope steady adjust the level to center the bubble. I tightened down the altitude friction bolts so the telescope wouldn’t move. With the bubble level adjusted you’ve now made the bubble level axis parallel to the telescope’s optical axis.
11) Now readjust the vertical locks to hold the transit horizontal (bubble centered) when they are engaged.
12) Repeat the “Two Peg Test” to test the transit. This time you’ll want to try to center the bubble when leveling the base of the transit. If you can’t, run through all the steps above again.
13) Repeat step 12 until the difference between the elevation in step 1 and step two of the test is very small.
I realize thats a bunch of instructions so I’ll use the numbers from my testing as an example. I may add some drawings at a later point to clarify things.
I marked out 100 ft with my transit in the middle. I leveled the base as best I could as covered in steps 4 and 5. I took a reading at peg A of 4′ 9-1/2″ and turned the transit 180 degrees. At peg B I got a reading of 4′ 7-5/8″. Subtracting these tells me that Peg A is 1-7/8″ higher than Peg B. (Technically, my pegs were garden pavers.)
I moved the transit to Peg B and leveled the base of the transit. My eyepiece height was 4′ 2-1/2″ and I measured 5′ 1″ at Peg A. This shows an elevation difference of 10-1/2″. Clearly, this is horribly off as expected. Instead, I expected to see 4′ 4-3/8″ (4 2-1/2″+ 1-7/8″). I then adjusted the transit as covered in step 10 and 11 to read this value.
Now I need to repeat the test. I placed the transit back in the middle and leveled it. This time I got a reading of 3′ 11-1/8″ at peg A and 3′ 8-7/8″ at peg B. Doing the math shows an elevation difference of 2-1/4″ with peg A being higher. (Yes, this is different than last time. The first time the rod was on top of the paver and the second time it was on the ground next to it. My helper left and I had to pinch the rod between two stones to hold it up. In the end it doesn’t matter because values from one “Two Peg Test” don’t carry over.
I then moved my transit down to peg B and leveled it. This time I got an eyepiece height of 4′ 2-1/4″ and read a value of 4′ 4-1/4″ at peg A. Subtracting the values shows an elevation difference of 2″. Comparing the elevations readings of 2-1/4″ and 2″ shows a difference of 1/4″ at 100 ft. At this point I was happy with the results. I did a test and determined each mark on the transit’s level corresponds to 1/2″ at 100 ft. So, measuring half a mark is probably about the best I can do by eye. For the record 0.25″ elevation 100 ft forms a triangle with 0.012 degrees at one of the corners. So, a little error goes a long way. Doing this gives you an appreciation for those who did this back in the day and who continue to do it today.
I now feel I have a transit calibrated well enough to do the work I need to do in my backyard. I think I almost go in over my head on this one but managed to pull it out. It’s not completely returned to original like I normally prefer but it beats it rotting away in a land fill somewhere.
Rambling on the “Two Peg Test”… The two peg test is pretty ingenious and simple trig shows it. When you take readings at equal distances in step 1, any error due to altitude misalignment cancels out (because both errors are the same) if the base of the transit is level. If it is, then you’re given the actual difference in elevation between the two pegs. When you head to one of the pegs and take a reading of the other peg you now can see the error in a higher or lower value than expected. Since you’re taking a reading at twice the distance the error would be twice that in step 1. If the base wasn’t level in step 1 then you have some error in your elevation calculation which messes up the test. So, level carefully!