Astrophotography Calculators Use our free online calculators to work out your best exposure times and other indicators for your gear. Calculators & Tools NPF Rule Calculator Work out your best exposure time for sharp stars with our free calculator 500 Rule Calculator Use our free calculator for the 500 Rule for exposure time Focal Ratio Formula and Calculator Use our free focal ratio calculator to work out how fast your telescope is Download our FREE Astrophotography Equipment Cheat Sheet This free guide tells you what gear you need to get started photographing the Milky Way, Planets, or Deep Sky Objects. By entering your email address you agree to receive emails from Skies & Scopes. You can unsubscribe at any time. Get the Free Cheat Sheet Loading… Thank you! Check your inbox for your free guide!