19 August 2005

Primum non nocere

First, do no harm

I would not make a good doctor.

When we were putting Some Assembly Required through its paces Peter noticed that there is play in the draw tube. It rocks from side to side. So this morning I thought I would perform what seemed to be an easy repair.

While I had the offset ratchet screwdriver out I figured I might as well take the drawtube off the secondary cage and clean up under it as well as see how easy it will be to replace the tube with a real focuser. It took a bit of work to get the back screw out. It was tied up tight against the inside of the cage. But it also appeared to be binding. The front screw wasn't in anywhere near as tight and was unscrewed by hand. That is where all the slop was coming from in the tube. It was good that it wasn't in tightly because that screw is under the front end of the mirror and there isn't space to get a tool in there to loosen it.

The drawtube is just a thick aluminum tube that is cut to a curve the same diameter as that of the secondary cage. It just sits over the hole and the two screws screw into it from inside the cage. It will be easy to get it off to replace it with something else when I find what something I want. But it will require drilling holes in the secondary as all the focusers I have seen mount with at least three holes and most four. And a lot of the focusers I have seen that are low profile like I need cost as much or more than I paid for the scope.

I started putting the tube back on the cage and the back screw went in much easier than it had come out. But I wasn't able to even get the front screw started. So I took the back screw out and screwed the front screw as tightly as I could turning the screw bit with my fingers. It went in as easily as it had come out. But once it was in I wasn't able to get the front screw started at all. I figured the holes might be off a little so I backed the back screw out and tried to start the front screw but it wouldn't go. It looks like the holes in the drawtube, and maybe the cage also, are off by at least half of the screw's diameter. That is probably why the screw wasn't drawn up tight to begin with. On one side of the drawtube you can see where the tap pushed the metal out rather than cutting it because to hole was too close to the outer wall of the tube.

So right now I have a spare part. I am not going to be able to get both screws in until I get the Dremel out and drift one of the holes a bit and I am not ready to do that yet.

I Went to the web to make sure "First, do no harm" was written by Hippocrates and the first site Google turned up burst my balloon. It is a widely held misconception that the phrase "First, do no harm" comes from the Hippocratic Oath that all doctors take. There are actually two misconceptions there.

The first is that all doctors take the Hippocratic Oath; they don't. They don't have to take any oath at all. It depends on where they get their degree.

The second misconception is that that phrase comes from the Hippocratic Oath; it doesn't. There is no indication that Hippocrates said those words although he did express a similar sentiment in Epidemics when he wrote "Declare the past, diagnose the present, foretell the future; practice these acts. As to diseases, make a habit of two things——to help, or at least to do no harm."
http://www.geocities.com/everwild7/noharm.html

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I went to Home Depot today and picked up a 1' section of 1.5" PVC pipe. I had wanted black pipe but they say it only comes in white. I guess what that means is that it only comes in white at Home Depot because my best tin whistle is made of black PVC pipe rather than tin. But, anyway, I got this section of pipe that will be long enough to make both of my finders. Then I went looking for end caps to use for the sun sight. They they didn't have except for 2" pipe. But they said I could get them from one of the larger Home Depot stores in the outer boroughs and recommended the one on Northern Boulevard in Queens.

I have been working on the design for a filter cell for the scope. The last time I was down on Canal Street I found a plastics store I had never been in before so I stopped off there to see if they could laser cut it for me. They don't provide that service and say that Industrial Plastics, who used to do it for me, just disappeared when their building was sold. So I tried Canal Plastics. They could do it without the laser for cheap until they heard I needed two identical pieces with screw holes that will match up. It is going to cost me about $15.00 for each half of the cell including the cost of the plastic. I will still have to buy some plastic bars and feet for it. I had my diagram in Adobe PageMaker and they need it in Illustrator so I need to rework it and submit it to them for a final quote. When I go back I will check about plastic for the sun finder.

While I was on Canal Street I looked around there for end caps but didn't see any place that looked like it sold plumbing supplies, mostly surplus electronics.

Next I went to Queens to the Northern Boulevard Home Depot. They didn't have exactly what I was looking for either. My concept was to use a push-on end cap. They have them for smaller pile but not for 1.5". I had to get screw-on caps which for 1.5" pipe have an inside diameter of 1.75" and aren't about to stay on my pipe. So then I had to get some kind of an adapter that is threaded on the inside and the outside. The inside threads will just slip over the outside of my pipe and the caps will screw onto the outside threads. It will all fit together without the need for adhesives.

While in Chinatown I looked around for small pieces of frosted plastic to put inside the back cap for the sun to shine through but didn't find what I was looking for. Initially I will use paper and continue to look.

I have revised my mounting system. The black string I had planned to use turned out to be elastic that I had taken out of the waist of my winter jacket because it had lost its elsasticity. But it had too much elasticity left to use it and not enough to use it to advantage. So I am using some 1mm perloin and a cord-lock. I passed both ends of the cord through the cord lock going in the same direction then slipped the bight or cord through the secondary cage under the finder scope shoe. Bringing the bight outside the cage at the rear I slipped the pipe through it then held up the front where the cord lock is and slipped the tube through that part of the cord. Now when the cord is pulled tight and the cordials slipped down against the tube it holds it fairly firmly in place and the cord does not add much obstruction to the light path and it is in an area the primary mirror doesn't see anyway. I may switch over to Velcro at some point but will start out this way.

Over the weekend I need to get the Dremel out again and use a cut-off wheel to cut it in half and drill a hole in the center of the front cap of the sun finder. I will hole paper behind against the back end of the pipe if the sun ever comes out again to judge how big of a hole I need to put there.

Item	Source	SKU	Unit Price	Quantity	Cost
12' PVC Tailpipe, 1.5"	Home Depot	042805012161	2.20	1	2.20
PVC pipe caps, 1.5"	Home Depot	039923137241	.93	2	1.86
PVC pipe fitting, 1.5"	Home Depot	520417	1.19	2	2.38
Cord	EMS		est .15	2'	.30
Cord Locks	EMS		est .25	1	.25
Subtotal					6.99
Tax (8.5%)					.59
TOTAL					$7.58