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Since it appears the (probable) "best" transformation of the Tycho
photometry to Johnson V and B-V needs to be sent around again, here is
Bessell's look-up table, followed by a very useful polynomial for the
VT-to-V conversion from Mike Linnolt.
Briefly, Bessell used the E-region primary standards data for his
analysis, showing that the standard Tycho-consortium transformation is
not very good and is easy to improve. He published only a look-up table,
and not an analytical function, but Linnolt has recovered this at least for
getting V. The linear consortium transformation is the source of the
systematic errors that Sebastian has noticed.
\Brian
===============================================================================
Corrections to put Hipparcos/Tycho data on the standard V and B system
from: 2000PASP..112..961B
BESSELL M.S.
Publ. Astron. Soc. Pac., 112, 961-965 (2000) [July 2000 issue]
The Hipparcos and Tycho photometric system passbands.
TABLE 2
RELATION BETWEEN BT-VT AND HIPPARCOS/TYCHO DATA FOR B-G
MAIN-SEQUENCE STARS AND K-M GIANTS
BT-VT V-VT del(B-V) V-HP
-0.250... 0.038 0.031 0.066
-0.200... 0.030 0.021 0.051
-0.150... 0.022 0.011 0.036
-0.100... 0.015 0.005 0.021
-0.050... 0.008 0.002 0.006
-0.000... 0.001 -0.005 -0.011
0.050... -0.005 -0.010 -0.025
0.100... -0.012 -0.017 -0.038
0.150... -0.018 -0.020 -0.048
0.200... -0.024 -0.021 -0.058
0.250... -0.029 -0.023 -0.069
0.300... -0.035 -0.025 -0.079
0.350... -0.040 -0.025 -0.087
0.400... -0.045 -0.026 -0.094
0.450... -0.050 -0.030 -0.101
0.500... -0.054 -0.035 -0.108
0.550... -0.059 -0.045 -0.114
0.600... -0.064 -0.051 -0.120
0.650... -0.068 -0.060 -0.127
0.700... -0.072 -0.068 -0.131
0.750... -0.077 -0.076 -0.134
0.800... -0.081 -0.085 -0.137
0.850... -0.085 -0.094 -0.142
0.900... -0.089 -0.104 -0.147
0.950... -0.093 -0.113 -0.151
1.000... -0.098 -0.122 -0.155
1.050... -0.102 -0.131 -0.158
1.100... -0.106 -0.142 -0.157
1.150... -0.110 -0.154 -0.160
1.200... -0.115 -0.166 -0.162
1.250... -0.119 -0.178 -0.164
1.300... -0.124 -0.189 -0.166
1.350... -0.128 -0.199 -0.166
1.400... -0.133 -0.210 -0.165
1.450... -0.138 -0.222 -0.164
1.500... -0.143 -0.234 -0.161
1.550... -0.148 -0.245 -0.157
1.600... -0.154 -0.256 -0.153
1.650... -0.160 -0.266 -0.148
1.700... -0.165 -0.277 -0.143
1.750... -0.172 -0.288 -0.137
1.800... -0.178 -0.299 -0.131
1.850... -0.185 -0.309 -0.125
1.900... -0.191 -0.320 -0.119
1.950... -0.199 -0.331 -0.112
2.000... -0.206 -0.342 -0.106
-------------------------------------------------------------------------------
Date: Fri, 10 Jan 2003 13:16:03 -1000 (HST)
From: Michael Linnolt <linnolt@hawaii.edu>
Subject: Re: Bessell lookup table
To: Brian Skiff <Brian.Skiff@lowell.edu>
Brian,
While working on loading the Bessel data table into my charting program,
I noticed that the (V-Vt) vs. (Bt-Vt) is almost perfectly fit by a single
cubic polynomial!
V-Vt = -0.02(Bt-Vt)^3 + 0.0549(Bt-Vt)^2 - 0.1334(Bt-Vt) + 0.001
R^2 = 0.99998
Unfortunately the del(B-V) vs. (Bt-Vt) cannot be fit so perfectly by a
polynomial, but a 5th order is within R^2=0.99965 with significant
departures around (Bt-Vt)~0.4 and (Bt-Vt)~0.15.
Clear skies,
Mike Linnolt
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