Re: Why the low power?

It could possibly be more effective for finding super-faint diffuse objects. By lowering the power, you concentrate the same amount of available light into a smaller area, rendering a diffuse object 20% smaller in size but 56% brighter (per unit area) to the eye. So if it's at the threshold of perception in your scope you *might* be more likely to spot it in a 40mm. But it would increase background skyglow by the same degree, a filter might probably be needed to make the difference really effective.