Guides
The ideas behind each tool, explained with worked numbers you can check. Read one before a session, then run your own gear through the calculator it links to.
How long should your subs be? Sub-exposure length explained
Why sky brightness and read noise set the shortest useful sub, with worked numbers for a dark site, a suburb and a city, broadband and dual-band.
→Read the guide7 minute readBack focus explained: getting the 55 mm right
What back focus is, why a filter changes it, and how to work out which spacers close the gap, with a full worked example.
→Read the guide7 minute readShooting the Milky Way without a tracker: the NPF rule versus the 500 rule
How long you can expose on a tripod before stars trail, why the 500 rule fails on modern sensors, and how ISO fits in.
→Read the guide6 minute readWhy your telescope dews up, and how to stop it
Dew point, why glass gets colder than the air on clear nights, and when shields, heaters and controllers each make sense.
→Read the guideReference
For when to photograph a particular object, see the target guide and the best targets by month. For dark nights, see the new moon calendar. How the site's numbers are worked out is on the methods page.