Our Bahtinov masks and star spike generators fit over the end of your telescope’s dew shield, so the size we need is its outside diameter. Measure carefully: a made-to-measure part is made to the number you give us, and a millimetre or two out can mean a loose fit or one that won’t go on.
What you need
- Best: digital or dial callipers that open to your dew shield’s size.
- Good: a soft tape measure (a dressmaker’s tape) or a strip of paper and a ruler.
- Avoid a steel rule held across the end: it’s easy to miss the widest point.
Where to measure
Measure the very end of the dew shield, the part the mask or generator slides over. Not the tube itself, and not the lens cap.
- If your dew shield slides out, extend it to where you use it for imaging and measure the end.
- If there’s a lip, ring or screw head near the end, measure over it as well and tell us about it.
- If the end isn’t perfectly round, which is common on older or dented shields, measure the widest point.
With callipers
- Close the jaws gently on the outside of the dew shield, square to the tube.
- Read the measurement to the nearest 0.5 mm.
- Turn the callipers a quarter turn round the shield and measure again.
- Use the larger of the two readings.
With a tape or paper strip
- Wrap the tape (or paper strip) snugly round the end of the dew shield, keeping it level.
- Read, or mark, where it overlaps. For a paper strip, lay it flat and measure to your mark with a ruler.
- Divide the length by 3.14 to get the diameter. For example, 471 mm ÷ 3.14 = 150 mm.
- Do it twice and use the larger figure.
Before you order
- Double-check the number and the units: we work in millimetres.
- If your measurement is within 1 mm of the edge of a price band, or you’re not sure, tell us before ordering and we’ll check it with you.
- A photo of the callipers on the dew shield is the quickest way for us to confirm it.