Image of the day

Captured by

Trifid Nebula

My Account

New to Astromart?

Register an account...

Need Help?

The Celestron Ultima 100ED

Posted by Doug Peterson   03/04/2005 12:00AM

At one extreme of the amateur telescope market are the high performance ED/APO refractors that deliver the maximum contrast per inch of aperture possible. They are also less susceptible to seeing conditions than their obstructed, and therefore less-aperture efficient, counterparts. These pricey toys are a joy to use, and once you've crossed over into the Triplet Zone, you are not coming back.

At the other extreme are the spotting scopes, generally designed for casual terrestrial use. My experience has been that they are a commodity for mass consumption. Rarely are these compact devices of high enough quality for serious astronomy in wavefront terms: in astronomical telescopes the Rayleigh limit of 1/4 wave is considered the upper limit of allowable error, in spotting scopes it appears to be a hopeful target. This is too bad. If you have ever examined a bird’s wispy feathers or squirrel’s multicolored pelt through a high-resolution color-corrected scope--while the animal is perched on a waving tree branch against a shockingly blue sky--you know bird watchers deserve the highest quality optics.

I have inteferometrically tested many quality spotters, Vixen ED80, B&L ED77 Elite, the technically beautiful Leica APO 77, and others and found that 3/8 of a wave is typical. More importantly, these errors are visible at the eyepiece. My “keepers” have been the achromatic Swarovski CT-85 collapsible at 1/5 wave, and a 78mm Nikon FS/ED/A, at a remarkable 1/10th wave--a true astronomical telescope.

In some cases, the residual high-order color aberration (spherochromatism, the variation of spherical abberation with color) is worse than the secondary spectrum these ED scopes’ designers are trying to cure. The reason? The typical spotting scope form-factor works well at an objective F# of 6, and focusing is accomplished by translating the porro prisms, which changes the optical path length. So far so good. But in order to have even faster focusing action, some designers use an objective of about F4.5 and a moving Barlow-type element of around 1.5x. Trouble is, the performance of modern lens designs start to deteriorate below F6, as higher order aberrations begin to appear. Without expensive stopgaps such as Petzval designs or heavy aspherizing, the result is color fringing and soft images.

In addition, the dozen or so surfaces in the light path all have to be flawlessly figured, very close to their nominal values for index, dispersion, homogeneity, radius and thickness, and have very little wedge, as the wavefront passing through the system accumulates defects at an alarming rate. What’s more at these fast F#s the tolerances for these errors shrinks dramatically. One such system has an airspaced triplet (6 surfaces), two porro prisms (8 surfaces), a Barlow (2 surfaces), a 45 degree wedge prism (4 surfaces), and a hermetically-sealed exit window (2 surfaces), for a total of 9 pieces of glass. The possibilities for disaster boggle the mind. And we haven’t even talked about loss of throughput and contrast with all of the energy-scattering in such a complex system.

Is there any hope an astronomical-quality spotting scope can be made at an affordable price?

Recently, affordable ED optics out of China have been exciting budget-hunting amateur astronomers worldwide. In the case of the 80ED and 100ED from Orion, and the 80ED from Celestron, the excitement appears to be for good reason. These moderate F# astronomical telescopes have been selling for 1/3 to 1/2 the price of their American, Japanese and European counterparts, and yet perform very well.

Now, similar optics have found their way into faster and more complex Asian spotting scopes, at the same startling price drops. Do these scopes put the fear of God into Pentax, Leica, Nikon, Swarovski and the others? More importantly, are they good enough for Astronomy? Read on.

Having worked in military optics QA for many years, I can tell you the Dirty Little Secret about optics: they vary from unit to unit. If the allowable deviation is tight, as in Takahashi and Questar, who have specified in print a 1/8 wave tolerance, the probability of a true lemon is rare, but not unheard of. If the allowable deviation is not specified, then it is anyone’s guess as to what you will get. At this point you must trust the manufacturer’s reputation. Even Meade and Celestron, who have a generally good reputation for quality in their larger apertures, have a different reputation for their imported products.

As we start looking at the control American and European companies have over their foreign suppliers, it is a mixed bag. TMB and importers of Russian optics have done a good job of controlling their imports, largely as a result of Markus Ludes specifiying Zeiss’ QA requirements on wavefront on maksutovs and apochromats. The assurance of an interferometric test report with guaranteed performance is a good start, and has generally been trustworthy. I have yet to see an optic out of Russia that was not essentially what the interferogram specified, even when I have gone to the trouble to synthesize in software what I see in the star test at the eyepiece.

China has been a different story. If we rewind 50 years to the state of post-Imperial Japan after World War II, we see a country similar to today’s China: enormous technical capability waiting to be exploited by the West. At first, a reputation for a cheaper alternative, a period of investment and tutelage under capitalist business interests, finally a reputation for high quality at a more reasonable price. Large companies like Meade and Celestron are currently tapping this reservoir in its first phase, smaller companies such as Burgess are attempting to create a niche in this new market place and fast-forward to the final stage.

Ultimately the question of interest to amateur astronomers is: how well are these investors able to control the output of these foreign producers? This requires on-site quality control.

To cut to the chase, the Celestron Ultima 100ED spotting scope is well made, well designed, and the three samples I examined were cosmetically very clean. Oh sure the tube is a sturdy plastic, and the zoom not nearly as good as the Vixen zoom that was its obvious inspiration. But the color correction was surprisingly good. Spherical abberation was not a problem. There was only one problem. They didn’t work.

The first sample would not reach focus at infinity. On nearby glints, I could see about 1 wave of coma. Rotating the front half of the tube showed that the problem was in the objective. I returned it.

The second sample had maybe a half wave or more of coma, causing flare on every object observed. Useless at 66x on the sky. Barely useable on terrestrial targets. It was passable at 20x as an RFT. It looked to me like the prism cluster was tilted away from the optical axis. I returned it to Celestron for repair.

It is not clear if the replacement, now the 3rd unit, was repaired, or a replacement. But it does not matter, because it was worse than the others. Coma again, and in gobs.

Not recommended--until Celestron can improve the quality control.