Spiders

The structure that holds the diagonal mirror in a Newtonian telescope is called a spider, because it typically has several legs. 

Last night, I was finally ready to do collimation of the optical path, and I discovered that no matter what, I could not get the diagonal mirror centered in the tube!  Why?  Was it this far off in the old tube?

No.  I had to cut down the mirror cell bottom plate because the inner diameter of the tube is 19.875" instead of 20.25"; I forgot that the spider required similar surgery.  It was easy to forget, because the legs of the spider are made of .030" aluminum (in the interests of minimizing weight and diffraction of light), and they are flexible enough that they just bent to fit.  But they did not bend symmetrically.

My first concern was that I was going to have to start from scratch on this, but it turns out that there was enough spare room where the legs attached to the center point that I was able to redrill the holes in the legs 3/16" over and solve the problem.  The picture below was taken with my little HP PhotoSmart camera, and it isn't terribly sharp, but you can see where I moved the holes:

I may buy a ring roller at Harbor Freight today to make tube rings.  One of the reviews indicated that one of the plastic parts broke almost immediately, which doesn't surprise me.  I will be rolling 1/8" or 3/16" thick aluminum, so I doubt that I will be stressing it much, and it sounds like other than that one part (which I can probably machine a replacement for out of aluminum), it is an adequate tool.

Advantages of Fiberglassing the Sonotube

I had mentioned that I was going to put the polyester resin on the edges of all the holes, big and small, that I made in the Sonotube, to reinforce the edges and prevent fraying.  This worked like a charm.  I discovered that it also made the holes just a little small -- which meant redrilling the holes for the small ones.  I actually found the bolts that hold the mirror cell in place were threading into the holes!  I would not count on threads in this stuff holding any real load, but at least the bolts weren't flopping about in there!

The eyepiece focuser hole, however, required some filing -- and unlike Sonotube, which doesn't file well, the composite material I produced actually filed very nicely indeed -- better than the paint, which chipped a bit.  A very nice result.

Big Bertha, 3.0

I still need to do a bit of adjusting and collimation, and some touch-up paint, especially on the straps.  I was hoping to get to that tonight, because we have a clear sky, but I had orders to fill, so....

But everything is back in the tube -- and it appears that it is slightly heavier than it was before.  Oh well.  At least it should be rigid.

The following two pictures show what happens when you use a flash, and don't use a flash.  From the first picture, you would never know the inside of the tube is black.

Individual End Mill Prices...

I was looking online to order up a replacement for the 1/8" end mill that I broke yesterday, and I am startled to see how expensive individual end mills are.  Just the 1/8" end mill at Grainger costs almost as much as this entire collection that I bought several years ago from Little Machine Shop.  Admittedly, the end mill from Grainger is U.S. made, and might well last forever with the amount of milling I do.

Lack of Blogging...

My wife had a shoulder repair operation yesterday at St. Luke's, and pretty much the whole day was spent with getting her checked in, surgery, post-op.  I probably should have brought my laptop, but I wasn't expecting the entire day to get consumed.  I will say that I am always very impressed with the medical and support staff at St. Luke's (and every other hospital here in the Boise area).  Friendly; courteous; concerned.

She is doing okay, but considering that they went in and removed bone spurs inside her shoulder, she is in a bit of pain.  I stayed home yesterday and today to take care of her; I just can't imagine what single people do in these situations.

The evenings have been lost in the great telescope rebuild.  I discovered that all the work I put into trimming the C-channel pieces down was a mistake; there was not enough clearance for attaching the steel straps without a lot more precision in cutting than I could easily do with a bandsaw, so I started over, and it was much faster.  When drilling the attachment holes in the steel straps, the straps are thin enough that they briefly went red-hot as I drilled them!  (More oil next time.)

The new versions were faster to make, in spite of breaking the 1/8" end mill that I was using to cut the slot.  I finished with a 1/4" end mill, which looks less elegant for an .030" thick piece of steel strap, but it isn't like you can find a 1/8" end mill in Horseshoe Bend.

The upper picture shows the static end of the strap; the lower picture shows the screw that tightens it down.  Unfortunately, the thumb screws I bought at Grainger just weren't long enough.  A 1" long screw thread sounds good enough, but once the captive nuts were on it, that was not enough travel to be useful, so it now has a conventional hex head bolt.  I may replace those when I can order a thumb screw with a longer thread.  They work well -- although it takes a while to crank them down enough.  Of course, that also makes it less likely that I will overcompress the tube with the straps, so I guess that's a positive.

The mirror cell has turned out to be a "I was too clever for my good" moment (as have several in the project).  It turns out that having the mounting brackets separate from the bottom plate was a great idea -- except that the bolts holding the brackets to the bottom plate can't get past the mirror once the brackets are mounted inside the tube.  So I guess I will do what I was trying to avoid -- tap the mounting brackets so that I can screw the threads in from the outside of the tube.  This is a blind operation, but at least I can see the brackets from the rear.  Some other mirror cells have you trying to find the threaded holes in a circular casting where there are no real clues as to location.  The slots in the mounting brackets were for 1/4"-20 bolts, so it is easy enough to tap these for 5/16"-18 bolts -- which, fortunately, I have some in black oxide finish that I can use for this.

But that will be tomorrow night, or maybe the night after.  I actually have ScopeRoller orders rolling in, and I need to get some of these filled.

I Have A Philosophical Problem With Seat Belt Laws

But here's a warning that if you really prefer bouncing around the inside of your car in the event of an accident, be prepared for increased enforcement of the law.  From May 20, 2013 KIVI channel 6:

The Caldwell Police Department and other Idaho law enforcement agencies will be stepping up seatbelt enforcement starting May 20th through June 2nd.

Over 70 Idaho law enforcement agencies will begin dedicating extra hours and patrols to educate and enforce the state’s safety restraint law during a 14-day period including Memorial Day.

New PJMedia Article

Universal Background Checks: Shouldn’t We Review the Statistics?