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Some Real Work
Since I have been cluttering up the site with off topic help I thought I better do some real work.
The two plots are for 7 million stars for which I have at least three v measurements. The plots are the sigma of the set of each star, binned and averaged by mag.
I like the log plot but have included the other plot for linear fans.
We see plainly the "noise" floor at 0.05 mag. As we collect more and more photons, the measurement does not get better. Sigh! I think the reason for this is the construction of the CCD. The front side gate causes differing sensitivity depending on where the image falls on the gate structure That is what you get for trying to cover the whole sky in a lifetime. The tiny star image is small compared to the pixel and so the measurement cares where the image is sitting. This limits the noise to the 0.05 mag range. This is *not* the accuracy measure for a large number of points. That is something else. It could be much better for a large enough set of measurements, it could be much worse. It is the subject of another set of calibrations.
Using the log scale (which is why I like it) we can see a linear increase in noise above mag 10. This is I believe due to be photon statistics. This data says how close you can expect a single measurement to be to the mean value. It indicates how hard it will be to see a variation. It again does not speak to the accuracy of anything but how close a single measurement will be to the measurement mean. As above, the mean of many measurements can be better than this number.
I see tass as being mostly useful in the mag 10 to 13 range. Below 10 it is not so interesting because these stars have many other measurements. Above 13 the statistics will never be very good. But there is a useful range. Michael Richmond wrote a note "In praise of magnitude 12". In the useful range it appears that we are limited by photon statistics. That is good. We are doing, I think, as good as we can for the apparatus. I think we would not do much better at a great location. A great location should extend the range. One should be able to detect fainter stars since a dark site allows longer exposures before the signal sinks into the noise. The result would be to measure a much larger number of stars. This would come at a price of taking much longer to cover the full sky at the 100 measurement level. But if I understand my statistics, a better location would not make much difference in the 10 -13 range where I think
With enough data we will be able to see some eclipsing binaries to mag 13. We will see some Miras to mag 15 but that is about the limit of where we will be able to dig them out of the noise. That will be useful, I think.
Thomas Droege
Why Wait? Move to EarthLink.
log_sigma.png
sigma_mag.png
