David A. Harbour presents a brilliant narrative for the Amateur Telescope Maker (ATM), on exactly how the "Foucault Test" for telescope mirrors is interpreted. Through a series of "foundation lessons" Mr. Harbour ensures the ATM understands what the Foucault Test is revealing, and what to do about it as he figures the telescope mirror to achieve the perfect paraboloid.Imagine if the makers of the Hubble Space Telescope had used this book as a guide to test the HST's primary before launch! The savings would have been in the millions! Well, that may be a stretch, but for the ATM, who has taken on the delightful but challenging task of making his own telescope mirror, how nice it would be to have a guide to ensure success! Success is at hand. Dave Harbour presents a thorough and informative instruction on just how to be sure the mirror is perfect! This book is for the ATM bookshelf to accompany the classics of Porter, Texereau and others.