The Tele Vue NP-101is was selected for an historic project by Dr. Don Bruns: to repeat the 1919 experiment of measuring star deflections during a solar eclipse that confirmed Einstein’s theory of Relativity.
The experiment will take place for the August 2017 solar eclipse, with Don using the NP-101is and FLI Microline 8051 CCD camera mounted on a Software Bisque MyT Paramount and field tripod.
In Don’s August 2016 Sky & Telescope article “A Do-It-Yourself Relativity Test — Using off-the-shelf equipment during next year’s total solar eclipse, you can prove that Einstein really was right”, Don commented:
For the telescope, I chose the highly portable Tele Vue NP101is. This apochromatic refractor’s 101-mm aperture can capture 10th- magnitude stars with 1-second exposures, and its diffraction limit is only 1.3 arcseconds at a wavelength of 630 nm — much smaller than the 2½-arcsecond daytime seeing I expect to encounter. The objective’s 540-mm focal length provides a 2º field of view with a medium-format camera.
Since I’ll need to pinpoint the centers of star images to within 0.02 pixel, those stars falling near the edges and corners of each frame must be as sharp as the ones in the center. The NP101is has a flat, color-free image plane, perfect for creating these very accurate star images, and I can adjust the focuser so that the camera remains perfectly square to the optical axis. Finally, the rugged, lockable focuser allows me to slew to multiple sky locations with minimum risk of camera movement, essential for good calibration.
In an 8 page paper for the June 2016 meeting of the Society for Astronomical Sciences, titled “Measuring Starlight Deflection during the 2017 Eclipse: Repeating the Experiment that made Einstein Famous”, Don explained the equipment choices and methodology with the following section on the telescope:
This experiment is made feasible by the availability of superb, commercially-available amateur astronomy equipment. Some of these items were not even dreamed of in 1973, and have been vastly improved even since the 2006 eclipse. After carefully analyzing all of the requirements for this experiment and comparing those requirements with a wide variety of telescopes and cameras, I selected what I believe is the optimum combination.
The ideal telescope is the Tele Vue NP101is refractor, shown in Figure 3 [www.televue.com]. This telescope is small enough to be portable, but its 101 mm aperture is large enough to capture 10th magnitude stars with 1 second exposures. Its diffraction limit is only 1.3 arcsec at 630 nm, much smaller than the 2.5 arcsecond daytime seeing I expect to encounter. The short focal length of only 540 mm allows a wide field of view with a medium format camera. Figure 3. The Tele Vue NP101is telescope provides essentially perfect optics over a flat, wide field of view, all necessary to provide good images during the eclipse. The short focal length provides an ideal size match to the imaging camera pixels.
I’ve used this same telescope model in previous experiments, and have always verified that its optical performance is essentially perfect. The image plane is flat and color-free, with no central obscuration or spider to add scatter. This makes the telescope a very-high contrast instrument.
In a recent communication, Don (an optical physicist) also said this about what he believes is possible with the NP-101is:
This small diffraction-limited telescope has a tiny geometric distortion coefficient, so minor data corrections are easily applied.
Digital image analysis, with appropriate corrections, will determine star positions to within 0.1 arcsecond over a 3º field of view. At least 20 stars should be visible between stellar magnitudes 5 and 10. Since 50 frames will be recorded during the eclipse, those 1000 data points should reduce the stellar position uncertainty down to 0.02 arcsecond. This will determine Einstein’s deflection to an accuracy of 1%, the best optical measurement of the deflection ever demonstrated.
We congratulate Don on his efforts and dedication, and are honored to be part of his project.