Bruce MacDonald wrote: > I am unsure about my accuracy for the two obs of Lacrosse 3; > ObsReduce gave the path as being outside my first pair of > stars. I see that Ted posted new elements for this sat, and > it could be that I was using out of date elements. Would > someone please advise? You reduced the wrong star-pair, but I believe I can see how you may have become confused. Your star-pair is part of a small asterism of 4 stars. Less than 2 deg away, there is a nearly identical looking asterism. To make matters worse, both were oriented almost identically in your FOV. To guard against this type of error, always look at the Observation-Prediction differences displayed by ObsReduce after each reduction. It displays both the time difference and the cross-track difference. The cross-track difference takes into Earth's rotation during the interval of the time difference, so it should normally be a small value, < 0.1 deg or so, assuming your observation was reasonably accurate, and your reduction correct. Reducing the incorrect correct star-pair in question, results in a cross-track difference of about 0.9 deg, relative most of the elsets issued over the past few months - a strong clue that the wrong pair had been reduced. Reducing the correct stars, ObsReduce reports: 25017 97 064A 2751 F 20040414204938790 17 25 0738144+385474 28 S This was within 0.020 deg cross-track and 0.1 s early, relative may epoch 04105.37945643 elset. Your second point also is quite a bit off - mainly in time - about 1.9 s late. The cause is less obvious. Perhaps a time reduction error. I do see an alternative star-pair, that agrees much better in time, but the stars are more than twice as far apart, and somewhat fainter than those you reduced. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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