Standards
The standards behind what fits what
Most of what makes two pieces of astronomy equipment fit together comes from documents or habits other people settled: a barrel diameter, a filter thread, a dovetail profile, a viewing standard. Some of those are published documents and are precise; several things this hobby calls standards are conventions with no publisher at all, and the difference matters when something does not fit. Each page says which it is, in four parts — what the document or the convention fixes, what it does not promise, what it works with, and which families on this site rely on it.
Optical fittings
The barrel diameters an eyepiece is made in, the filter threads cut into them, and the T2 thread that joins almost everything else — what each fixes, and what each leaves open.
Mount interfaces
The two dovetail profiles a saddle is cut for, the finder shoes that were never standardised, and the tripod threads underneath — the fittings that decide what attaches to what.
Filter designations
What the names on a filter actually designate, where a Wratten number comes from, and why two filters sold under the same three letters can pass different bands.
Solar viewing
The standard a solar filter or a pair of eclipse glasses cites, what the document covers, and why the maker’s own inspection instructions sit alongside it rather than under it.
Market conventions
Tube diameters, focuser travel and the informal names a whole hobby uses: designations that fix a shape or a habit, and leave everything optical open.