El Cheapo Interferometer/22" Dob Assembly

It's amazing what somebody can cobble together that works and is dirt cheap:
First, the interferometer:
http://www.instructables.com/id/CHEAP-INTERFEROMETER-FOR-OPTICAL-TESTING-OF-TELESC/
Then, a 22" dob structure:
http://www.instructables.com/id/Making-22-Inches-Dobsonian-Telescope-Body-Under-10/
I bet that dob structure works a lot better than the stuff they sell to us...

Very cool! On the interferometer, I Think I see what he is doing there? There's a Twyman Green configuration and (I believe) a Fizeau alternative. The Laser Pointer would be coherent enough to get good contrast fringes in the TWG, provided the arms are of approx equal length, and that is indeed adjustable. The Fizeau would be for testing powered mirrors, not sure how he would get coherence, could be hit and miss. There's a beat-frequency requirement that would need another level of adjustment, otherwise you might or might not get contrast, a very sensitive function of path length. BUT, nice and fundamental!

The Dob Build. I had just coincidentally, over the past couple of days, watched John Dobson's EXCELLENT YouTube tutorial on building a 16-inch Dob, from scratch, on the cheap. Runs 90-min. And takes one from finding a piece of glass thru grinding, polishing, figuring, testing, coating, building the entire mount, and Using it! Amazing.

It's been 45 years since I made my last mirror... and that got me to wanting to make another one, or TWO?! Like a 6-inch F/12 and then a 16-inch F/5. Hmmm... hmmm... ?!

55 years ago, I made everything CHEAP, because I honestly had no $. Later, I gradually got spoiled, and started buying things... really too much. Should return to basics!  Tom

Tom,
I personally wouldn't want a 6" F12 due to the size of the supporting structure although you could leave the mirror spherical without the need to parabolize it and still have a primary at better than a quarter of a wave at that f-ratio. I wouldn't want to have an f-5 16" either due to the longer structure when an f4.5 like mine would do just as well. Unless you want to have a labor of love, I'd buy a GSO mirror from Agena astro at around 1200.00 and probably do well with it, similar to these tested by interferometry:
http://r2.astro-foren.com/index.php/de/11-beitraege/03-newton-systeme-und-verwandte-fragen/260-c065-zwei-gso-16-zoeller-f4dot5-im-vergleichgso
Then I'd put a cover over the mirror with a round hole in it about 5 or 6 inches in diameter, similar to a trap door installed on my astrosystems mirror cover, and open it for an unobstructed 6" f11 system or so, if I wanted that particular aperture along with the 16" primary without the cover. I think that's a better way to go...

Yes, I know! My 2nd mirror was a 6-inch F/12.5 that produced breathtaking images with a modest Orthoscopic eyepiece. Much later I had a (good!) 17.5 Coulter mirror in a scope that I made from... oil drums! And I DID have an off-axis 6.5-inch stop on that, which gave wonderful planetary images. I think I miss those scopes. My eyes were probably a lot better too.  Tom