Outside diameter against clear aperture
Nothing formal, and that is the point of recording it. A telescope’s aperture is the clear diameter of its optics, and it is the figure printed largest. Its outside diameter is the width of the tube itself, and it is the figure almost every clamping accessory actually needs.
The two are never equal and the gap between them is not predictable. Wall thickness, a fixed dew shield, a cell that stands proud of the tube and the construction of the front end all contribute, and none of that is standardised between makers.
Accessory manufacturers respond by publishing ranges: a filter cell or a mask is sold for tubes between one figure and another, and it closes through a band, a set of screws or a friction fit.
What it does not promise
A tube inside a quoted range does not promise a secure fit. Cells hold best in the middle of their range, and a tube at the extreme of one is the situation every maker’s warning about a filter working loose is written for.
The figure does not promise clearance for what is already on the tube, either. A finder bracket, a focuser knob or a handle can prevent a cover or a case from closing even when the diameter is right.
And it says nothing about the length that has to be accommodated, which is what decides whether a case closes and whether a cover reaches the base.
What it works with
The outside diameter governs everything that goes around the tube rather than into it: solar filters, dew shields and heater bands, focus masks, covers, cases and the tube rings a dovetail bar bolts to.
On this site it appears on all of those, and on the telescope families themselves — where it is worth measuring, once, and writing down before any of those accessories is ordered.
The questions that keep arriving
How much bigger than the aperture is the tube?
It depends entirely on the design and the maker. A thin-walled reflector tube sits close to its aperture; a refractor with a fixed dew shield can be considerably wider than its lens. There is no ratio worth applying, which is exactly why the figure has to be measured rather than derived.
Where do I measure it?
At the point the accessory will sit, which for a solar filter or a mask is the front of the tube including any dew shield that cannot be removed. That is frequently the widest point, and it is the one that decides.
Why do accessory makers publish a range rather than a size?
Because cells for filters and masks are made in a few sizes and are expected to close on a spread of tubes, usually through a band or a set of screws. A tube at the very bottom or top of a quoted range fits less securely than one in the middle.
Last reviewed 17 September 2026