I would like to take night shots of the sky, but the SX50 limits exposure length to 15 seconds and will not allow ISO settings for exposures longer than one second. Is there a way to increase exposure length and decouple ISO settings from shutter speed? Given the cameras max f-stop of 3.4, I need to decrease shutter speed to around 25 seconds while using a really high ISO value (maybe 3000 or 6000). Any help is appreciated.
You should be able to set longer exposures using CHDK overrides:
http://chdk.wikia.com/wiki/CHDK_User_Manual#Override_TvI suspect CHDK ISO overrides will will let you go beyond base ISO with > 1 second exposure, but I don't know for certain.
ISO overrides generally do not let you go beyond the standard Canon limits: If your camera supports 100-3200, don't expect CHDK to let you use 50 or 6400. In some cases the highest ISO step (3200 in this example) also can't be set using CHDK.
