Precision required
0.1 mm
- Caliper resolution
- 0.1 mm or finer display increments
- Caliper accuracy
- Manufacturer-rated ±0.1 mm or better over the span you measure
- Your measurements
- Measure and enter millimeters to the nearest 0.1 mm.
Resolution is not accuracy. Extra decimal places do not prove a measurement is more accurate. Use calipers large enough for the full diameter; do not use a ruler, tape, or circumference calculation.
ID and OD are different surfaces. Measure through the center at three orientations, roughly 60° apart. The inner jaws contact the bore; the outer jaws contact the external mounting band.
Depth starts at the flat front rim (A). Inside and outside usable depths stop at their own first step or obstruction. For a cap or kit, check both; they are not necessarily equal.
Measurement diagrams only, not your telescope geometry. No fit allowances or manufacturing dimensions are shown.
Prepare the telescope and calipers.
Remove the dust cap only. Leave retaining rings and screws in place. Clean the caliper jaws, close them gently, and check zero. Confirm the caliper’s stated accuracy for inside and outside measurements across your required span. If you cannot check the surface safely or trust the tool’s accuracy, contact us before ordering.
Find the two contact bands.
ID: use the cylindrical bore just behind the entry bevel or chamfer, where an insert would contact the cell. Do not measure the bevel, glass, clear optical aperture, or an internal retaining ring. OD: use the outside front-cell band that a cap would grip, not rubber trim or the tube barrel behind the rim.
Measure each diameter in three directions.
Use the small inside jaws for ID and the outside jaws for OD. Span the center and rotate about 60° between readings. Keep the measuring direction perpendicular to the tube axis and both jaws on the same band; tilting gives the wrong diameter. Touch gently without forcing the jaws. Remove and reseat the calipers, then repeat each location.
Check the contact band farther back.
Measure just behind the bevel, then farther back along the intended contact surface, as far as you can safely reach without approaching the optics. Use the same flat front rim as datum A to compare the locations. Keep any detailed readings for your own reference; submitting them is optional. Do not average away a taper, a step, or an out-of-round shape.
Measure the usable inside depth; check the outside land for caps.
Inside: from flat front rim A along the inner wall to the first shoulder, step, or obstruction. For a cap or kit, also check outside depth: from the same rim along the straight outer mounting land to its first step or obstruction. Measure at several positions around the rim. You can add the outside depth to your notes; we may ask for it before production. Use only a safe wall-supported method; never extend a caliper depth rod toward a lens, corrector, mirror, or filter. If the depth cannot be checked without risking contact with optics, stop and contact us.
Check consistency before entering the result.
Repeated readings at the same point must agree within 0.1 mm. If they don’t, check your technique and contact us if you cannot get consistent results. Different readings between orientations or measurement stations may reveal taper, a step, or an out-of-round shape. Use the optional notes to flag these details, obstructions, or heater bands, rails, and cables that stay in place. We review these details before production and may ask for more measurements, including secondary-holder dimensions for some masks. Never use glass as a measuring datum.
Cannot measure safely or get consistent readings?Contact us before checkout. Do not guess, substitute nominal telescope specifications, or hide conflicting readings by taking an average.
Your measurements
Three numbers. No fit allowances.
Use the smallest observed ID, largest observed OD, and shortest usable inside depth around the bore from your consistent readings. The outside engagement depth is a different surface; do not mix it into the inside-depth field. Detailed readings are optional. These are summaries of your measurements, not finished part dimensions: do not add or subtract a fit allowance.
