# Emission Line Galaxies(ELGs) selection using narrow band

I selected ELGs using the z BroadBand and NB921 narrow band according to the condition of Hayashi+11

Then I plot the transmission curve of the filters used, I got this

As we can see NB921 and NB926 are not properly in the centre of $$z_{BB}$$. So, If I want to use y Broad Band for the selection, What I have to change/include in the selection condition?

In the case of $$z-NB912$$, the $$3\sigma$$ of $$z_{BB}-NB$$ is $$\sqrt{(f_{3\sigma,z})^2+(f_{3\sigma,NB912})^2}$$. How to estimate an error in the case of $$0.7z_{BB} + 0.3y_{BB} - NB$$?