Discovery vs. Zambuto Optical Shoot-out

By Tim Neubert

Over the past few years I have become a passionate visual observer and with such passion has come an undeniable desire to own increasingly larger optics produced by increasingly prestigious opticians. The simple six inch dobsonian that initially spawned my love of visual observing has recently morphed into a twenty inch diameter truss dobsonian that makes my wife seriously question the "for better or for worse" promise she made to me over ten years ago. My latest optics have all come from one optician, Carl Zambuto. Over the last decade, Zambuto optical company has gained a reputation of being one of the highest quality optical suppliers for the amateur astronomer. Carl Zambuto does not give any testing data with his mirrors to prove their optical quality. He simply provides a serial number, his signature, and the promise to accomplish a set of seven criteria which each mirror must satisfy before shipment. The seven criteria are:

1. The polished surface will have essentially a laser specification finish.
2. The large scale surface will be smooth and regular for a given focal ratio, with transverse aberration not exceeding the diffraction disk at the focal plane.
3. The small scale surface (microsurface) will be exceptionally smooth, with very low angstrom numbers.
4. Primary ripple will be functionally nonexistent, as in nearly undetectable to the eye under bench test conditions.
5. Spherical aberration will be reduced to functionally negligible levels both in higher and lower orders.
6. Astigmatism, both higher and lower order will be maintained at levels that are undetectable at the eyepiece.
7. The edge-zone and extreme edge of the mirror will be balanced.

Carl Zambuto claims that if a mirror satisfies the seven criteria, it will not be outperformed by any product money can buy. The astronomical community as a whole seems to agree that Carl Zambuto produces some of the best optics available. However, many amateur observers wonder if the seven criteria truly translate to an observable difference at the eyepiece over other reputable optics.

Goal of this comparison

Determine if the seven criteria translate to an observable difference at the eyepiece for a Zambuto optic over a similar Discovery optic.

Equipment

• Starmaster hybrid truss dobsonian with Zambuto optics: Diameter = 14.5 inches, Focal ratio = 4.3, Focal length = 62.35 inches
• Custom truss dobsonian with Discovery optics: Diameter = 15 inches, Focal ratio = 4.2, Focal length = 63 inches
• Eyepieces used: Televue Ethos 10 mm (100 degree), Explore Scientific 9 mm (100 degree).
• Collimation tools: Howie Glatter barlow and Catseye 2 inch autocollimator.

Overview

The two scopes were chosen because of their close specifications for focal length, aperture, and identical coating material. Also, Discovery telescopes has earned a strong reputation of providing premium optics at a reasonable price and seemed like a good contender for the comparison. The comparison took place at the San Diego Astronomy club site (TDS) on the July new moon weekend. Viewing conditions for the comparison were slightly above average with consistent seeing that did not vary over short time periods. Both scopes were collimated with the same Howie Glatter and Catseye tools and allowed to equilibrate to the outside temperature for 1 hour after dusk. Multiple classes of targets were selected and the identical eyepiece was used in one scope followed by the other by three individual observers. Comments were held until each observer had viewed a target multiple times in each scope and the comments were then recorded. The three observers have a combined astronomical experience of almost 50 years and have either viewed through or personally owned almost every major brand of dobsonian available commercially.

Targets and comments

• Open cluster M11 (Wild duck cluster): Both scopes resolved this compact open cluster nicely, but the Discovery image was noticeably brighter to all three observers. However, many of the dim stars that were just visible in the Zambuto scope could not be seen in the Discovery, even with averted vision.
• Globular cluster M13 (Hercules cluster): Both scopes resolved this cluster well and the Discovery image was again slightly brighter for all viewers. We looked at faint star trails in the Zambuto scope and then tried to find them in the Discovery. Again, many of these faint stars were undetectable in the Discovery image.
• Galaxy M51 (Whirlpool galaxy): Both scopes produced a nice image of this galaxy pair and the Discovery again produced a brighter image. The Zambuto scope was just able to resolve the full bridge structure for 2 of the 3 observers. The Discovery scope was not able to resolve the full bridge structure for any of the 3 observers. Overall the Zambuto mirror showed slightly better detail for 2 of the 3 observers.
• Nebula M20 (Trifid nebula): Both scopes produced wonderful images of this nebula. However, when looking at specific low level features, the Zambuto mirror was able to resolve slightly more detail.

Summary and Conclusions

We found both of these optics, the Zambuto and the Discovery, to be excellent. The Discovery consistently provided slightly brighter images due to a subtly larger surface area and coatings that were roughly 7 years newer than the Zambuto. Both optics produce near perfect star tests and hence any differences in contrast must be due to small scale surface features of the mirrors and not due to larger scale figuring or correction of the optics. Overall the slightly dimmer Zambuto optic unanimously produced more contrast on 3 of the 4 selected targets. So, for this comparison we conclude that Carl Zambuto's seven criteria must be providing the extra contrast that was observed at the eyepiece.

Other Considerations

We must point out that this comparison was only between two mirrors and we are only able to conclude that the seven criteria had an observable impact for these two mirrors alone. To make a broader reaching conclusion, this comparison would need to be repeated for numerous telescope pairs and across multiple optical manufactures.