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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