The Fachgruppe Kometen was asked to give some hints concerning the appropriate HIPPARCOS/TYCHO- magnitudes which should be used for the comparison of comets. This topic was treated by Matthias Achternbosch and Andreas Kammerer. The diagram shows the spectral sensitivity of the dark-adapted human eye in comparison with the transmission in the VT- and V(Johnson)-system. Concerning the overall transmission and the maximum, the VT-system shows the greatest similarity to the dark-adapted human eye.
Comparison of the different systems
But, just how large are the differences between VT und V? The authors of the HIPPARCOS/TYCHO- catalogues give the following correlation: V = VT - 0.09×(B-V). This means, that the difference between VT and V is less than 0.2m for stars with B-V < 2 (including spectral class K) - comparable to the average uncertainty of a magnitude estimate by an experienced observer. As a consequence it is save to state, that the differences between the V-magnitudes of HIPPARCOS (ICQ-code HV), V-magnitudes of TYCHO-Katalog (TJ) und VT- magnitudes of TYCHO (TT) are small compared to the uncertainties of a brightness estimate. If one wants the differences to be in the order of 0.1m this can be achieved by using only stars with B-V < 1, including spectral classes B, A, F and G.
On the other side, the fourth photometric system used by HIPPARCOS, Hp (ICQ-code HK), is not recommended by the authors. The reason for this is the fact, that Hp covers a large spectral region, including not only the B- and V-band, but also the near infrared. Thus this system shows rather significant differences not only in comparison to the sensitivity of the dark-adapted human eye, but also, in many cases, to VT and V.
Conclusion: VT-magnitudes can be used as well as V-magnitudes of both catalogues to visually estimate the brightness of a comet, especially if the comparison stars used are not too red (B-V < 1.5). But, because the VT-system is a specific photometric system, used solely by the TYCHO-instruments, whereas the V-system is well-defined and used world-wide, we recommend the usage of the V-magnitudes (ICQ-code HV and TJ). In the case of measuring the brightness of a comet via a CCD-camera, using HV and TJ is a must.
