Quality means fitness to purpose. Thick metal armor is fit for a battleship; not so much for a backpack. What was John Dobson’s purpose? To see and show the heavens to all of us with big-mirror telescopes. Countless people have been wowed looking through his eyepieces. Today, intrepid sidewalk astronomers continue to elicit gasps of surprise from people as they look through telescopes at the night sky.
Early altazimuth telescopes built using John Dobson’s design by the likes of Earl Watts, Doug Berger and Bob Kestner showed that large thin mirrors can pass the most rigorous optical standards, setting the stage for the ‘Dobsonian’ revolution.
John taught countless telescope and mirror making classes over the many years. It is really unfair to judge John by an anonymous student’s so-so first mirror attempt. Held to the same standard I must be a miserable mirror maker because the students in my classes did not always succeed!
John’s own mirrors tested quite well: I can personally attest to the high quality of his 24 inch, 18 inch and 12 inch mirrors. Keep in mind that John sometimes worked collaboratively on larger mirrors. But don’t take just my word: John Dobson won an optical award at a highly contested RTMC in 1971 as did one of his students. Carl Zambuto says that one of John’s students, Earl Watts, talked him into making mirrors. Steve Swayze told me that he started mirror making under John's tutelage. Other students of John like Bob Kestner went on to become professional opticians, working on the COBE to fix Hubble. Think of it: it took John Dobson’s students to fix the Hubble Space Telescope. Hard to imagine that someone only capable of bad mirrors would inspire this lineage.
In the 1960’s and 1970’s the Foucault test was hands down the test of choice. The Bath Interferometer and the computers and software needed to analyze the images were decades into the future. Fine and not-so-fine mirrors were ground, polished and parabolized in the past and fine and not-so-fine mirrors continue to be made today. John Dobson’s mirrors were very fit to purpose, and led to the Dobsonian revolution.
John Dobson and Delphinium by Brian Rhodes