Optical Quality of the C5+

last updated 24 June 2006

During the 1980s, and especially around the time of the return of Halley's Comet, public interest in astronomy was at a peak. Despite the mediocre nature of the apparition, the comet's fame and the recent exploits of the Voyager spacecraft aroused the general public's fascination.

Along with this came the attendant need to buy a telescope for personal use, so people could see for themselves what the hubbub was all about. The demand grew so quickly that Meade and Celestron were either unable to meet demand without dropping quality, or else saw an opportunity to make a lot of profit while not tapping the budget for quality assurance. Whichever you believe, the upshot is that amateurs saw a significantly reduced level of optical quality from Meade and Celestron during the 1980s.

At the turn of the 1990s, both companies were beginning to improve somewhat, but it's generally understood that the one thing that turned the tide most dramatically was a Sky and Telescope article in which scopes from both manufacturers were reviewed and found wanting. I think it was in 1992, though I don't remember precisely, and it remains the first and last time that the two companies were lambasted that acutely.

The benefit for amateurs has been a revival of optical quality, to the point that most observers believe they surpass the scopes Meade and Celestron produced during the 1970s, at least in the consistency of their optical quality. Much of this is due to more advanced techniques, to be sure, but certainly some of it is because of improved quality control. The buzzword for the last several years at least has been "diffraction limited," and it would be difficult to sell a telescope to a knowledgeable amateur without it being at least declared as such.

But What Does It Really Mean?

A telescope is said to be diffraction limited when the quality of images seen through it is limited by the diffractive nature of light, rather than the precision of the shape of its optics. Because of diffraction, an image as seen through a telescope is "smeared out"; the manner in which this smearing happens is determined by the Airy pattern (see the contrast page). The Airy disc affects all telescopes, regardless of quality. However, in bad telescopes, it contributes a negligible amount to contrast loss, and in good telescopes, it is the predominant contributor to contrast loss. As a kind of compromise, we might consider that a telescope is diffraction limited when it is at what we might loosely call the Bode point of the contrast function—that is, when the difference between the actual shape of the optics and their ideal shape is causing less image degradation than the Airy disc.

This criterion is somewhat subjective, since how much image quality is being degraded is a matter of opinion that may vary somewhat from observer to observer. While this allows manufacturers to hedge their bets when it comes to optical quality, in practice they use another criterion, called the Rayleigh criterion. The Rayleigh criterion states that for critical applications, the optics must be sufficiently smooth that the light wavefront deviates from its ideal envelope by no more than 1/4 of the wavelength of light, where we choose a wavelength representative of what's being observed through the telescope. The middle of the visual range, which practically all amateurs use, is about 550 nm, so in order to satisfy the Rayleigh criterion, the optics must not be off by more than about 140 nm. This is approximately equal to 0.000006 inches. In comparison, shaving and vanity mirrors are typically off by as much as 1,000 times more.

Later note: I should add that the above criterion refers to peak-to-valley (abbreviated P-V) error. That is, the "high" and "low" points on the wavefront must not differ by more than 1/4 of the reference wavelength. This means a mirror with one peak and one valley rates the same P-V as one with lots of peaks and lots of valleys, even though the "average" quality of the former is quite obviously better than that of the latter. To measure this "average" quality of the mirror, there is another kind of error rating, called root mean square (RMS). As a guide, a mirror whose only problem is 1/4-wave P-V spherical aberration has a 1/14-wave RMS error. This amount is often used as the dividing line for good optics when measured by RMS error and is called the Marechal Limit.

RMS error is generally regarded as a better assessment of overall mirror quality than peak-to-valley. The only problem is that the RMS value is always smaller (usually significantly smaller) than the P-V error, and it thus has the possibility of confusing users who are unaware of the difference.

Celestron's Claims of Optical Quality

Celestron's claim (communicated to me in a phone call) is that their telescopes, and the 5-inch SCTs in particular, exhibit at most 1/4-wave total wavefront error at 600 nm, peak-to-valley. In other words, the optical surface is accurate to 150 nm. They also claim that typical quality is closer to about 1/8-wave error. They are silent about the RMS error.

C5+ owners are of course under no obligation to believe Celestron's claims. If you have any experience with your C5+ and star testing, and believe you have an accurate estimate of your wavefront error, please drop me a line at <brian.y.tung@gmail.com> and let me know.

Copyright (c) 1999, 2006 Brian Tung