Tools for checking this yourself
This site spends its whole length quoting other people’s millimetres. These are the four things worth owning if you want to check one — or read the length of an unmarked ring that arrived in a box of second-hand adapters.
The rule for this page: if it doesn’t help you measure, fit or verify a spacing, it isn’t here. No mounts, no dew heaters, no eyepieces, no gear recommendations. Four entries is the right length for this list.
Digital calipers, 0.01 mm
Every figure on this site is a manufacturer's claim. Calipers are how you check one — and the only way to know the length of an unmarked ring that came in a box of second-hand adapters. If you buy one thing on this page, buy this.
What to look for. 150 mm capacity is far more than enough, 0.01 mm resolution, metric. This is a commodity: the cheap ones are fine for reading a spacer to a tenth of a millimetre. Measure a part you already know the length of first, to see whether yours reads true.
Metric thread pitch gauge
M42×0.75 and M42×1 are the same diameter and will not mate. On unlabelled second-hand adapters there is no way to tell them apart by eye, and forcing one into the other damages both. A gauge settles it in seconds.
What to look for. Any metric set covering 0.5–1.0 mm pitch. Most astronomy threads are ×0.75, which is finer than general engineering sets always include — check the range before buying.
Bahtinov mask
Spacing is judged by the stars in your corners, and out-of-focus stars look wrong in the corners too. Get focus unambiguously right first, or you will spend a night chasing a spacing error that was a focus error.
What to look for. SIZED TO YOUR TUBE'S OUTER DIAMETER, not your aperture — a 65–100 mm mask will not fit a 105 mm tube. Measure the outside of the dew shield before ordering. This is the one item here where buying the wrong variant is easy.
A small parts case
Anyone who does this for a season ends up with a drawer of unlabelled aluminium rings that all look identical. Ten minutes with a case and a marker saves an hour of re-measuring on the next clear night.
What to look for. Nothing special. Write each part's length on it in permanent marker the day it arrives, while you still know what it is.
These are searches rather than single products, deliberately. For a caliper the model genuinely doesn’t matter, and a Bahtinov mask has to match your tube’s outer diameter — one curated link would be wrong for most people who clicked it. They are affiliate links and may earn this site a commission.
Next: how to tell whether a cheap spacer is actually the length it claims →