In late October 2005 I hoped to see major surface markings on Mars, but my 10-inch scope showed only a uniformly yellow ball. Do I have an inferior mirror? The optics were well collimated. Is there a good test for optical quality?

Mars Oct. 21-22
By the night of October 21-22 the dust storm had widened and moved south; it's the large, light yellowish patch on Mars's lower left portion here. North is up.
S&T: Sean Walker

Starting in mid-October, dust storms on Mars spread a wash of yellow haze over the planet’s southern hemisphere, reducing contrast. Add that to the usual effects of mediocre atmospheric seeing, tube currents in telescopes that have not fully cooled to nighttime temperatures, and the fact that Mars is just plain small, and it’s no surprise that Martian surface features can be a challenge. In addition, one side of Mars really is blander than the other. And don’t forget that you need to look for a long time in order to glimpse all you can.

To answer your next question, there is an easy but very strict test of a scope’s optical quality, which every amateur should learn. After collimating and letting the scope fully cool (which may take a couple hours), examine a moderately bright star at very high power. Turn the focus knob slightly one way, then a tad the other way. The slightly out-of-focus disk of the star should look identical on both sides of best focus. This star test is so sensitive that few telescopes pass it perfectly. But if the star disk looks plainly different on either side of best focus, the scope won’t give images as sharp as it should.

— Alan M. MacRobert

About Alan MacRobert

Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.

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