Ability to set even below the minimum brightness on iPad
Many people use the black on white mode to invert the color mapping for applications that will not specify more of the LCD to be grey or black and less of it white. Some blog articles call this "night mode" but it's a typical adjustment for people that have different visual abilities.

Settings App -> General –> Accessibility –> Triple-click Home -> “Toggle White on Black.”

Even though you can't reduce the illumination of the lights in the iPad further without a jailbreak, you can use this system setting to increase the amount of light that gets blocked in many cases by triple clicking the home button.

Also, many apps are being programmed to only draw in night mode where only red pixels are used. This also cuts down on the transmission of colors that our eyes use to constrict pupils and preserve night vision. This isn't a general solution, but for apps like Star Walk - this works very well in practice.