When using Astro-mode, you need to set your camera into BULB-mode. With this setting, the controller is able to control the exposuretime of your camera.
When set into BULB-mode, you are able to exposure for a very long time, which is very common when shooting astro objects. The exposuretime is define before the sequence start in the settings screen of the Astro-Mode.
You can also define the amount of pictures and a delay time between the pictures.