Occult 現時最新版本 4.0.6 (2009年3月8日發佈)
更新事項
This version contains a significant update relevant to lunaroccultations. As a result of some queries concerning a graze of ZC844predicted to occur on March 5 (in the US), a significant issue came toattention. In brief:
* XZ80P was compiled by Mitsuru Soma many years ago. In that exercise he put much effort into incorporating data from Hipparcos.
* XZ80Q adopted XZ80P positions without change, where the position was derived from Hipparcos.
* The issue that escaped my attention (although Mitsuru alerted me toit at the time ... oops!) was that where Hipparcos included positionsfor the components of a double star, the position used in XZ80P was amean position of the two stars. [The mean being derived on the basis ofrelative magnitudes.]
The consequence is that for all such stars, the XZ80Q position is notof either star, but of the mean position. And this creates a problemwhen the positions of the components of the double star are determinedfor accurate graze predictions, and accurate reductions - since doublestar offsets are traditionally referenced to the brighter star of apair.
Initially I thought there would only be a few 10's of stars affected. In fact there are 693 stars affected by this issue. And
they are generally relatively bright stars.
The issue was further complicated by the fact that for a number of these stars there is a separate entry for one of the
components. The usual situation is for the A component to have an entryin XZ80Q, and occasionally for the B component to have a separateentry. However (and as a result of this 'mean' position issue) itturned out that there were two stars where the second entry was in factfor the A component, but was interpreted as being the B component.[ZC844 was one of those two stars...]
As a result of this:
* the file XZDoubles.dat has been revised to flag all those doubles where the XZ80Q position is a mean position; and
* the program has been changed to properly handle the situation wherethe XZ80Q position is a mean position. This includes an enhanceddisplay of double star details in the 'View catalogue details of astar' form.
Finally, the measurement of double star details as a result of lunaroccultation observations is an area where useful observations can bemade. Small groups have formed in Australia/New Zealand, and NorthAmerica, to coordinate observing activity to record lunar occultationsof doubles at significantly different position angles - so that the PAand separation can be measured (as well as the relative magnitudes). Tosupport this activity, Occult now includes a form to do a least-squaressolution for the PA and separation of a double star using 2 or moreobservations of the pair. Those observations can be on the same night.But if the pair is not changing quickly, observations in the same yearcan be used. As an example, from 4 observations of the reappearance ofAntares on 2006 May 14, the following solution is obtained:
Sep = 2.694 ±0.014" PA = 275.92 ±0.71° RMS fit: 0.00 ±0.03"
One of the interesting aspects of double stars is that for closedoubles the magnitudes of the components is very poorly known. Andthere is an inherent 180 deg ambiguity in the measured position anglewhen observed with speckle interferometry. Last year 3 of us observedan occultation of chi Sagittarii (which has an orbit for the pair), andfound the stars to have a mag difference of 1.54 (WDS has thecomponents of the same magnitude), and that the A star of the orbit wasthe fainter star. To quote Brian Mason (of US Naval Observatory,responsible for the Washington Double Star catalogue)
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It is certainly possible that we've got the quadrant wrong on this one.It is an easy system for us to resolve with speckle, but delta-m isn'treally possible to any degree of accuracy (at least for standardautocorrelation techniques). I think this is a unique niche that LunarOccultation can make a significant contribution to double staranalysis. The pairs of William Finsen are especially interesting asthey are bright close pairs with, allegedly, small delta-mags.
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