You can't substitute B for V. B and V are pretty far apart, but you can do B only, but you won't have any color correction terms (meaning catalog B-V vs your B-V) which is your color calibration correction. You also won't have a color index vs airmass correction. B is also down in the not very sensitive part of most camera's color response. Red stars have B-V of more than 2 magnitudes (meaning B is 2 mag fainter than V, and for very "blue" stars B-V is slightly positive, maybe -0.5.)
What you can do is this:
1) take test exposures of a field of catalog stars hopefully close to zenith (airmass near 1). You want a wide range of color index from B-V being over 2 down to about 0.
2) Subtract your instrumental mag from the B mag of the catalog stars.
3) Plot that vs. color index (B-V). That plot is the error of your B measurement vs. the color of the star you are measuring. Since you aren't measuring the color of the star with a V filter, you don't know how the star's color is affecting the result. If you have the V filter, then you at least know the color of the star (color changes as the star changes brightness) and so that gets corrected. The whole point of needing 2 filters for one magnitude measurement is that you are calibrating the effect of the unknown star color on your single filter measurement, so you need a second filter to get a measure of star color.
You could in a pinch substitute a green visual (non-photometric) for V and use that to measure an instrumental color index and then use THAT to correct the B measurement you make. You just need something that is giving you a metric of the color of the star you're measuring, to correct the B measurement you make. The reason you need that is that everybody's system has some color sensitivity even with only one filter.
Edited by 555aaa, 15 February 2021 - 05:30 PM.