Repeating the Experiment that Made Einstein Famous
An attempt to measure the solar deflection of stars during the August 21, 2017 total solar eclipse to the highest precision ever realized.
– next scheduled update: February 2017
Commercial equipment today is better than ever!
Society for Astronomical Sciences, Ontario, CA, USA June 2016: Presentation Abstract
A feature article for Sky & Telescope Magazine appeared in the August 2016 issue, with additional description on their web site.
The Sky & Telescope update has a graph of test results analyzed without distortion. More work to include this effect resulted in the left-hand graph shown here: the RMS error is now 0.023 arcsec! The graph shown here was used to calibrate the optical distortion in the Tele Vue NP101is telescope, so stars were imaged near zenith, and 10 frames were averaged to reduce noise. While not a perfect comparison to the eclipse data, it shows excellent performance. Note that the entire outline of the graph is about the size of one pixel. Since no sun was present, the red circles should all be zero, and the average is fact only 0.00004 arcsec. The blue symbols show the deflections expected in 2017 on the same scale. The right-hand graph shows the results from the 1973 experiment by the University of Texas. Note that most of their stars are at larger distances, so their deflections are smaller. Analysis of the 1973 data gives an uncertainty of 11%, so I expect to do much better.
Site selection trip in August 2016
Steve Lang and I travelled to sites near Casper and Alliance to find the best spots to set up the telescope. Requirements include a good base for the tripod, clear view toward south, wind protection, electrical power, nearby lodging, and overnight security. We found an ideal site near Casper, so we will plan on traveling there well before the eclipse date. In case of a cloudy forecast, we plan on driving east to some place between Alliance and Grand Island on Saturday or Sunday, giving at least one night for polar alignment. We found one site in Alliance that meets most of the requirements, and will be looking for a better site before next August.
Optical Distortion in the Tele Vue NP101is Telescope
Distortion in the Tele Vue NP101is needs to be corrected to a very high precision, since it will not be averaged away by measuring lots of stars. In addition, the optical axis needs to be measured, since a slight change in the coordinate will result in a different correction for distortion. The graph here shows the allowed change in the optical train that will produce an error of 0.5% in the deflection. Since most of the stars are near the edge of the field of view, the change in the optical axis needs to be tracked within 10 microns.
The CCD camera is attached to the Tele Vue focuser, and gravity in different orientations will cause it to sag. Attaching and re-attaching the camera to the Tele Vue focuser, as well as the simple act of refocusing the camera, will also cause a small change in the optical axis seen by the camera. My precise measurements of the optical axis using an artificial star showed that the Tele Vue focuser is very rigid! The optical axis shifts less than about 5 microns when the camera is refocused, or the camera is re-attached, or the telescope points to different parts of the sky. With this excellent performance, the distortion calibration can be reliably used, with resulting systematic deflection errors less than 0.002 arcseconds.
In December 2016, all of the semi-automatic analysis routines were completed. I can now read the various calibration files and apply those coefficients to a simulated experiment. The optical distortion is well-defined and rigorously corrected. The plate scales and camera orientation can be found from fields near the central eclipse field, and those corrections applied to the eclipse images. This is the technique to be used for the August 21 eclipse.
In February, a technical paper was accepted by Applied Optics. This paper describes the use of Arago’s spot to monitor the change in the optical axis due to mechanical motion. I expect the paper to be published this spring.
I am currently working on a more detailed paper that describes the entire calibration procedure. This paper may not be completed until March, and will be submitted to the Proceedings of the Astronomical Society of the Pacific. If it gets published before August, any other interested astronomers can follow my procedure to their experiment. That will increase their chance of success.
Still to come:
Presentation at “The Lunch Bunch”, a group of retirees at Tierrasanta Lutheran Church in San Diego.
Presentation at AstroCON, August 2017, in Casper, WY. (astrocon2017.astroleague.org/speakers)
Al Nagler, Tele Vue Optics, Inc., for loan of the NP101is telescope and optical raytracing.
Greg Terrance, Finger Lakes Instrumentation LLC, for loan of the ML8051 CCD camera.
Stephen Bisque, Software Bisque, Inc., for loan of the MyT Paramount tripod.
George Kaplan and John Bangert, both formerly of USNO, for astrometric advice and help with NOVAS.
Norbert Zacharias, USNO, for astrometric advice.
This research has made use of FORTRAN version of NOVAS, the Naval Observatory Vector Astrometry Software package.
This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23.
Suresh Rajgopal, for help in setting up gfortran.
Corey Bruns, for help in automating the data analysis with linear algebra advice.
Ted Pecoraro, for help in improving the Paramount field tripod feet.
Steve Lang, for help setting up in Wyoming.
Jerry Kassebaum, for suggesting locations near Casper.
Greg Kinne, for help in scripting TheSky.
Ron Bruns, for calibrating weather instruments and operating an auxiliary experiment.
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Web page last updated February 10, 2017.