I have previously had a go at transforming a green channel magnitude estimate from my DSLR camera to Johnson V and with the new data I obtained on the 7th May, I can repeat this process. Below is a graph of V-v against B-V for the new data:-
This is strikingly similar to the graph I obtained previously. The data points for the stars 84, X and Y have been marked. The straight line drawn through the points has been obtained by linear regression on a calculator. The gradient of this line is -0.170 (to 3 d.p.) and is very similar to the gradient of -0.186 obtained from the 2nd April data. So our transformation coefficient in this case is T = -0.170 and I can use this to transform our green channel magnitude estimates to Johnson V.
Our D(B-V) values are the same as before. In this case I find that T*D(B-V) is -0.114, -0.061 and -0.155 for the stars 84, X and Y respectively. Given that the green channel magnitudes for Z from these comparison stars were 7.349, 7.324 and 7.420 respectively, then the transformed Johnson V magnitude estimates for Z are 7.235, 7.263 and 7.265. The mean of these values is 7.254 +/- 0.017.
That night I had a go at estimating the magnitude of Z UMa using 7x50 binoculars. My visual estimate was 7.3 which is very close to this photometric observation. In all there were 3 visual estimates for this variable star from the BAA and AAVSO on the night of the 7th/8th May (including my own) which gave 7.23 +/- 0.12 in very good agreement with my transformed V estimate.
All text and images © Duncan Hale-Sutton 2023
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