Monthly Archives: December 2012

Foucault tester

As I mentioned in my last post, I was less than impressed with the views through my recently built telescope. I was blown away by the views of DSO’s (Deep Sky Objects) such as nebulas and galaxies, but when I pointed the scope at Jupiter I saw nothing more than a mushy blob with two lines through it (the northern and southern equatorial bands). Jupiter looks much sharper in my 90mm APO (Astro-tech AT90EDT) than it does in my 333mm newtonian! A star test revealed that the mirror was overcorrected. This is not surprising since Coulter wasn’t exactly known for their precision optics.

So what am I going to do about this? Fix it, of course! But before I take a pitch lap to the 13.1″ f/4.5 mirror I should probably know what I’m doing. All of the advice points that starting one’s mirror making career is best done with a 6″ f/8 or 8″ f/6. Since I have 2 6″ pyrex blanks, I’ll start with a 6″ f/8. I have been accumulating all of the books, mirror blanks, and mirror test equipment that I could get my hands on over the past few months. I recently built myself a foucault tester based on the Stellafane design. I used it for the first time last night to test my 13.1″ mirror (as well as a 10″ meade DS-10 mirror that I found on craigslist for $75!).

You might notice that the wood used to build the tester looks very similar to the wood of the original 13.1" scope. The circle of life...

You might notice that the wood used to build the tester looks very similar to the wood of the original 13.1″ scope. The circle of life…

I followed the Stellafane blueprints pretty closely. I made the base slightly larger to hold the dial indicator better.

I followed the Stellafane blueprints pretty closely. I made the base slightly larger to hold the dial indicator better.

Ronchrigram of the 13.1" using a 90 LPI ronchi screen confirms the mirror is indeed overcorrected.

Ronchrigram of the 13.1″ using a 90 LPI ronchi screen confirms the mirror is indeed overcorrected.

Spherometer built using Gordon Waite's instructions.

Spherometer built using Gordon Waite’s instructions.

All that’s left for me to do is to build a grinding stand, which I’m planning on doing today. I hope to update the blog more frequently rather than I have been.

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Rebuild complete!

I originally made this blog to track the progress of my rebuild, but obviously that didn’t happen. I have completed the telescope and have been using it for several months. Mechanically, I am very pleased with it; optically, not so much. I’ll follow up on the optical side of things in another post.

I used THE K&B Dobsonian book as a guide for this build. I think this is probably my first attempt at a woodworking project so I am more than happy with the results. I found that I enjoyed the woodworking aspect of this project as much as I enjoy actually looking through the telescope. During the project, I acquired many woodworking tools including a table saw, drill press, and router. I was able to justify buying these as the costs will be amortized over many projects and they will also be used for remodeling projects (my wife liked this reason more :).

The UTA (upper telescope assembly) and mirror box of the telescope are made of 1/2″ (12mm?) baltic birch plywood that I bought from my local Rockler. I would estimate that I used 5 2’x4′ sheets which cost ~$25 each. I noticed that the quality of these sheets varied considerably and that it is very worthwhile to take the time to find a good sheet (pretty face and not warped). You can see the jig that I made for dovetail splines above (based on the plans I found here). I am very happy with the way the dovetails turned out. The dark wood highlights are walnut.

The altitude bearings were made by gluing two sheets of 1/2″ BB together than using a circle jig to route out a circle. Next, the cutouts were routed. This was very tedious, but I learned quite a bit while doing it. I should’ve used the drill press to drill each of the end points rather than trying to route to edges.

I modeled the UTA truss clamps after the Astrosystem’s Telekit. The lower truss clamps were based off of Fiske Miles design. These work very well! The trusses themselves are from moonlite.

The mirror cell is made of aluminum box channel that has been bolted together. This worked just fine but I think on future projects I will have these TIG welded. The cell is an 18-point design that was PLOP optimized. I used a stainless steel wire with turnbuckles for the sling.

The focuser is a 2-speed moonlite CR2 and works like a dream.