StarryCamera is the best camera app for a starry sky.
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# How to use
1. Mount your iphone on a tripod.
2. Choose shooting mode.
3. Choose ISO, Shutter Speed.
4. Tap the shutter button.
# About 5 modes
Starry sky mode : Highlight bright stars, and increase contrast.
Star trails mode : Lighten blend mode for star trails.
Milky way mode : Increase contrast.
Night view mode : No image processing.
Lighten mode : Lighten blend mode. (for firefly, light trail, etc)
Changed shutter speed at lighten mode. (30, 60, 120, 180, 240, 300)
VariableCalendar3 Pro 
HalfPressCamera 
SerialCalPro (Scroll Calendar) 