Stellarvue Telescopes

The mission of Stellarvue is to inspire a healthy interest inscience and astronomy by handcrafting precision telescopes.

Shop Our Telescopes Learn About SVX Optics

SVX Optics: Accuracy & Quality

Stellarvue's optics have evolved over the past 25 years with the addition of advanced testing equipment. Here is how we currently produce our SVX objectives.

Zygo Phase Shifting Laser Interferometer

Measuring Accuracy

The accuracy of an astronomical optic may be expressed in terms of Strehl ratio, named after the German physicist Dr. Karl Strehl. A theoretically perfect lens would have a Strehl ratio of 1.0, so under this system of measurement, optics are rated using a fraction of the number 1. An objective with a Strehl ratio of .95 is much more accurate than one that measures .90.

Strehl ratio is only one indicator of optical accuracy. We do not meticulously figure our objective lenses just to get a better Strehl number. Rather, we spend a great deal of time figuring the glass and testing the results on a daily basis to minimize aberrations. Our goal is to minimize aberrations below the theshold of detectability, improving the performance of the objective. A polished objective may have several aberrations and we use the Zygo interferometer to detect these. We map the optical wavefront, then we figure the glass to minimize these issues.

So the Zygo Interferometer is not used to simply show how accurate an optic is, it is a tool we use daily to improve the performance of our optics by minimizing aberrations.

The Process

Lens designs are developed to meet our requirements, for both visual use and imaging with CCD or CMOS cameras. We use super-low dispersion glass for the best color correction and contrast. For more information on glass, visit our page on Glass Types.

After glass is ground and polished, we use our Zygo Phase Shifting Laser Interferometer and our Double Pass Autocollimator to figure each objective carefully. This process eliminates common issues we find in machine-polished objectives, including astigmatism, coma, and spherical error. We also address trefoil by replacing any elements that exhibit this aberration during the process.

Optical figuring to improve performance is accomplished in two stages:

Reducing optical aberrations that impact performance (astigmatism, coma, spherical error) to the lowest level possible.
Making the final spherical correction.

Figuring - Stage One

In stage one we reduce aberrations. To understand how this is done, let’s look at one of these aberrations:

On-axis astigmatism happens when a lens (or its wavefront) is asymmetrical after being polished. This may make the optical wavefront look like a potato chip, as seen to the first oblique plot. Since the wavefront is not symmetrical, the focal length will vary as we move from one location to another within the circumference of the lens. Light from one plane comes to focus at a different location than light from another plane. This will turn a point of light (a star) into a distorted line or a cross.

This is an oblique plot taken using our Zygo showing a greatly magnified view of a wavefront from an objective assembled after machine polishing. These high and low areas are greatly exaggerated so they become obvious. See how misshapen the wavefront appears on this particular objective? Ideally this wavefront should be more even, but in this view the characteristic "potato chip" appearance is still obvious.

To fix this, we rotate elements to improve symmetry, then work the glass to further reduce on-axis astigmatism to an extremely low level. At this stage, the master optician improves the wavefront by making it more symmetrical. During this lengthy process, we reduce astigmatism through dozens and dozens of individual figuring sessions (rubs).

This work is extremely tedious. Each time a rub is done, the lens must be cleaned, cooled overnight, reassembled, and re-aligned. Then the next measurement is taken showing how much more work is needed. This process is repeated dozens and dozens of times over until we reduce astigmatism to the lowest possible level. As astigmatism is brought under control, the accuracy increases and other aberrations that were hidden now begin to appear. Each one of these issues will be corrected in order to bring the objective lens up to the highest level of optical accuracy we can attain.

The second oblique plot is the final test of the same objective shown above after months of hand figuring to reduce aberrations to the lowest possible level.

Figuring - Stage Two

In the summer of 2023 based on customer requests we implemented a slight change in our process to move the final optical correction toward the middle of the visual spectrum. During the spring of 2023 we tested this process extensively to ensure that photographic performance would not be compromised due to this change. Both red and green figured, high-Strehl SVX objectives delivered the same excellent color correction free of the dreaded "blue bloat." Since most of our customers are imagers, it was important that we took the time to test the results before we implemented this change. Based on our findings, we modified our final spherical correction slightly to ensure that our objectives are now more accurate in green light.

SVX Telescopes

Stellarvue has been producing refractor telescopes since 1998. Our new SVX telescopes are made with optics we hand-figure at our shop here in Auburn, California. These are the finest and most accurate telescopes we have ever offered. When you purchase an SVX series refractor, it will come with its own Zygo interferometric test report taken at full aperture in red light in our shop, documenting the accuracy of your individual objective. To learn more about what we do, watch the following video: Sky & Telescope's Dennis DiCicco visits Stellarvue.