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.
Measuring Accuracy: The accuracy of an astronomical optic may be expressed in terms of Strehl ratio, introduced by 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. As this fraction declines you will see more and more issues in the objective that impact performance.
The Process: Lens designs are developed to meet our requirements both in terms of visual use and in imaging with CCD or CMOS cameras. We use super low dispersion glass for the best color correction and contrast.
Glass is ground and polished to meet our prescription. Once this is accomplished, we utilize our Zygo Phase Shifting Laser Interferometer (shown to the left) and our Double Pass Autocollimator (shown to the right) to carefully figure each objective. This is done to eliminate 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:
Stage one: Reducing optical aberrations that lower performance (astigmatism, coma, spherical error) to the lowest level possible.
Stage two: 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:
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 below. 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 a Zygo test report of an objective assembled after machine polishing. The image that is top, center on this test is an oblique plot generated from the interferometric measurements taken. This plot is a highly magnified picture of the wavefront. These high and low areas are greatly exaggerated so they become obvious. Looking at the oblique plot above, you can see how misshapen the wavefront appears on this particular objective. Ideally, this wavefront should be flat so correcting this requires hand figuring to make the wavefront more uniform or symmetrical. The master optician works the glass to eliminate this aberration through dozens and dozens of individual figuring sessions (rubs) as well as rotating elements to find the best final performance.
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 is the reason Stellarvue objectives take such a long time to complete. 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 need to be corrected in order to bring the objective lens up to a high level of optical accuracy.
Here is the Zygo test taken after hand figuring to reduce this aberration.
Figuring – Stage Two
In stage two we make the final spherical correction. If this is done on the Zygo, the correction shown will be made in red light since the interferometer uses a HeNe laser, which emits a red beam (632.8 nm). Spherically correcting for this final figure in red increases the accuracy in red. Many imaging cameras are most sensitive to red light and most of our customers are imagers.
The naked eye is most sensitive to green, so it makes sense that many visual-only observers would prefer the final spherical correction be done in green light. This will reduce the accuracy in red (in which we certify optical accuracy) and increase it in blue compared to an objective corrected in red. We can now make the final spherical corrections in green using the Double Pass Autocollimator in green light while simultaneously monitoring the change on the Zygo.
If you are interested in purchasing a Stellarvue telescope and have questions, call us at (530) 823-7796.