Back focus, explained

Five things that are widely repeated wrong, with the manufacturers’ own words and the figures from this site’s dataset.

• Why 55 mm is not a universal back focus55 mm is a convention, not a law. Some flatteners publish a different required back focus for every telescope they fit — here are four, from one manufacturer, none of them 55.
• Camera back focus is measured from different placesTwo cameras can both say 17.5 mm and mean different things. Most manufacturers do not state which plane the figure runs from — here is how to tell, and where it goes wrong.
• Does a filter change your back focus?Yes — and in the direction most people guess wrong. Glass in a converging beam moves focus further away, so a filter makes your required spacing longer, not shorter.
• Petzval astrographs do not have a required back focusRedCats, Askar FRAs and similar four-element astrographs focus over a range rather than at a point. Searching for their exact back focus is looking for something that does not exist.
• Generic vs branded spacers, and how to check oneUnbranded spacers are usually fine and occasionally not. What actually differs, why a caliper reading can mislead you, and how to measure the length that really counts.