RHESSI and the Transit of Venus I
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|1st Author:||Hugh Hudson|
|2nd Author:||Martin Fivian|
|Published:||1 June 2012|
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Transits of Venus across the solar disk happen only rarely, but in pairs. In this Nugget we describe how RHESSI benefits from them; the first of our pair was in 2004, and the next (and the last one for more than a century) occurs June 5-6, 2012. Note that this has nothing directly to do with RHESSI's main goal of X-ray and γ-ray imaging and spectroscopy, but instead with its optical aspect system - a set of three simple telescopes with 4-cm apertures.
Historically, we can point to the 1639 observations by Jeremiah Horrocks in the village of Much Hoole, Lancashire. Figure 1 shows this worthy gazing at a projected image of the Sun, and one can see the shadow of Venus creeping across it. The whole process takes several hours. The shadow is comparable in size to a sunspot, but - depending on the telescope and its environment - much darker. Horrocks apparently improved on Kepler's predictions, based on his novel adoption of ellipses for planetary motions. Horrocks had no formal training in mathematics or physics (but how much was there really, in 1639?); nevertheless he improved the equivalent of the orbital elements for Venus, thereby predicting the occultation, and then observed it with his own home-made telescope. Not much is known about any of this, but from this observation he was able for the first time to determine the vastness of the scale of the solar system - the distance to the Sun could not be known without this crucial observation.
There is a great deal of history involved in transit observations, but 1639 was a remarkable beginning.
Learning from the Transit
No longer do we need such astronomical observations, in fact our vehicles have actually gone out there and visited Venus (1962), and have gotten very near the Sun (1976) as well. But the two transits of the current cycle (2004 and 2012) are very significant as a convenient tool for calibrating telescopes, especially those in space (such as RHESSI). The main areas of calibration deal with learning about the optics of these telescopes, referring to the exquisitely precise knowledge we now have regarding celestial mechanics. For example, we now know the value of the astronomical unit to be 149,597,870.700 km, as compared with Horrocks's first determination of some 95,000,000 km. It also took him about 20 years to get his result published (alas, posthumously), a contrast with how we communicate science in the modern era.
Figure 2 shows a RHESSI aspect-system (SAS) observation from the 2004 transit. The SAS optics are read out by a 2048-pixel linear CCD detector, and so this image just shows a thin strip that intersects the solar limb in two places, and crosses over the shadow of Venus in between. The reason for the gaps in the image is the need to save telemetry; the system allows us to read out specific ranges of pixels. In this case we are interested in the edges of the images (Sun and Venus), in order to get a precise location; and we are interested in the bottom of the shadow, since this gives us a tool to explore the properties of the scattered light. SAS is not optimized for this, with the result that the shadow is not jet-black as it ought to be. With the transit, we can map this out with a range of image geometries as RHESSI rotates and nutates, and as Venus wiggles its way across the disk.
The "wiggles" mentioned above will result from RHESSI's orbital motion and parallax. Because we will obtain tens of thousands of images, we will be able to track this motion highly accurately. We do not actually have predictions yet of the expected time and location of the Venus shadow, partly because of the routine unpredictability of the exact orbit. Once this ephemeris information becomes available, though, we expect that our precise timing and centroid information will enable us to characterize the plate scale of the SAS telescopes repeatedly through the five orbits, and to an accuracy sufficient to give us interesting determinations of the absolute size of the Sun.
The RHESSI aspect system consists of a unique set of three small solar telescopes, which rotate round their axes at a period of about 4 seconds because RHESSI needs this motion to be able to make its X-ray and γ-ray images. As we have shown by measuring the solar oblateness with a precision considerably better than one milli-arcsec, these telescopes are capable of superb astrometry in a relative sense. Now, with an accurate determination in orbit of the plate scales, we may also be able to determine the value of the solar radius in an absolute sense. This will be a hard job, so stay tuned...