Perseids 2015

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Bill Ward


Much to my surprise the night of the Perseid peak was clear all night. From22.00ut to 03.20ut I recorded 377 perseids and 51 others/sporadics. Out of these I captured 28 spectra, 8 of which were reasonably good.

One issue that has been problematic is correct “instrument correction” of the spectra. A flux corrected stellar spectrum can be used but what class of star to use? The temperatures that exist with the meteor head are high, several thousand kelvin but using a corresponding temperture of star/black body equivalent means that the shorter wavelength won’t be right. Similalry the reverse is true, correct for short wavelength then the long wavelengths go awry. This is especially a problem as silicon based detectors are hugely more sensitive in the red/near IR than the blue.

I thought it might be interesting to do a instrument correction by dividing through by a nominal silicon photodiode response.

I got a curve from a large photodiode manufacturer and normalised the curve at ~700nm, about halfway between the lowest responsivity and the peak at 820nm. The actual curve varies slightly depending on how the juntion is biased but this mostly affects the longer wavelengths which are being lowered by the division so the errors are small over all (I think 😉 )

Anyway here are a couple of graphs from a spectrum of a Perseid from this years peak. The difference is huge!, they don’t even look like the same spectrum the red has been lowered by that much. This actually shows how INsensitive silicon devices are to the far blue. (film was much better, who’d have thought….)

Uncorrected “raw” spectrum.

and the “corrected” spectrum

The red end may be a little over done as the original shows a rise in the graph caused by smoke wafting across the fov raising the backgroud level. However the blue end was reasonably dark and is fairly representative. This was the only spectrum of the night that had some blue lines to test the idea on. Its not a perferct example but demonstrates the point.

Having said all that acutal flux calibration is still subject to much uncertainty and is quite difficult to do in an easy manner for us amateurs.

Food for thought….