Solar Disk Sextant

Investigators:

Objectives:

The primary aim of the Solar Disk Sextant is to measure the diameter of the Sun to a precision of several milli-arc-seconds. The resultant diameter measurements are to be used along with simultaneous solar luminosity measurements to determine the relationship between radius and luminosity changes in the sun. One can then use historical solar radius change data (obtained from solar eclipse data) to determine the amplitude of solar luminosity changes in the past. This will be of help in validating climate models. In addition, as a by-product of this research, we obtain a value for the solar oblateness, which is of great interest to those working in relativity and fluid dynamics, and several areas of solar physics.

The Instrument:

photoClick here for full image (127 Kb)

This picture shows the Solar Disk Sextant (SDS) intrument just prior to the September 26, 1994 flight from Ft. Sumner, New Mexico. Below the payload are the various NSBF antennas and the ballast hopper. The large brown squares are the crush pads that cushion the payload when it lands. Also visable is the WAOD (Wide Area Ozone Detector) instrument that flies with the SDS. The WAOD normally takes data during the ascent and descent phase of the mission.

The Solar Disk Sextant is an experiment which until now has been flown on stratospheric balloons, although we eventually hope to fly it in space. It consists of a 7" telescope, a 1/50 wave, optically bonded wedge assembly, and 7 linear CCD detectors. These components are mounted in a low thermal expansion INVAR tube that is mounted in an altitude-azimuth yoke arrangement. The prime guidance sensor is a LISS (Lockheed Intermediate Sun Sensor) coupled together with the output from the actual CCD information. Despite the motions introduced by the balloon environment this system is capable of guiding to a few arc-seconds absolute pointing, with a drift rate below 3 arc-seconds/sec. The 6-8 Gigabytes of data obtained per flight are stored in a special on-board VCR system. Some data are also transmitted to the ground via an 82 Kbit/sec line-of-sight radio link. This allows us to monitor the health of the instrument, and uplink any commands that might be needed. These flights take place from the NSBF Ft. Sumner, New Mexico facility. Typical flights take place in late September, and last for about 10 hours. Termination of the flight normally occurs in Texas or Oklahoma, although the 1994 flight landed less than 40 miles from Ft. Sumner.

Results:

Two flights of the SDS using the optically bonded wedge have taken place - on September 30, 1992 and on September 26, 1994. The data from these flights have been analyzed and give two important results:
  1. The absolute diameter of the sun has increased by about 10 mas during this two year period.
  2. The solar oblateness has remained constant at 9.1 x 10**-6, where the oblateness is defined as the difference in the equatorial and polar diameters divided by the equatorial diameter.
photoClick here for full image (11 Kb)

This Figure shows the most recent solar oblateness function found by fitting the 1992 and 1994 SDS data. Defining the oblateness as (Re - Rp)/Rp, where Re and Rp stand for the solar equatorial and polar diameter, yields a new value of for the oblateness of 9 x 10**-6.

The Future:

Currently we have made four flights of the SDS instument using the molecularly bonded wedge (1992, 94, 95 and 1996). The next flight is currently planned for 1999 or 2000. This represents a balance between data acquisition (more is better) vs. frequency of flight (and its attendent risks).

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Author: Larry Twigg
Last Updated: 27 October 1998
Web Curator: Leslie R. Lait (Hughes STX) (lrlait@ertel.gsfc.nasa.gov)
Responsible NASA organization/official: Dr. P. K. Bhartia, Atmospheric Chemistry and Dynamics Branch/Head