Pro Bahtinov Masks

Pro Bahtinov Masks are adjustable focusing masks that we design and laser-cut in Shropshire from 3 mm black cast acrylic with a polished finish. Eight sizes cover dew shields from 40 to 360 mm outside diameter.

A Bahtinov mask turns a bright star into three diffraction spikes. When the middle spike sits exactly halfway between the other two, the star is in focus. On our own Sky-Watcher 200P we reach focus in under a minute. The first time takes a little longer while you learn what to look for.

How it fits

Each mask has radial slots for three steel thumb screws with rubberised gripping feet. Slide the screws in or out to suit your dew shield; each mask fits from 2 mm under to 2 mm over its size range. The three-point grip keeps the mask centred, even over a dew heater strap, and holds it in place while the mount slews.

The size range is etched on each mask, so you can tell them apart if you have more than one telescope.

Choosing a size

Measure the outside diameter of the end of your dew shield and choose the size that suits it best. Our guide shows how to measure your dew shield. If you’re on the edge between two sizes, ask us: we can send different-sized rubber feet so the mask fits snugly.

For dew shields under 40 mm or over 360 mm, contact us and we’ll make a mask to measure.

Fitting the mask

1. Push the thumb screws through the slots and screw on the rubber feet.
2. Set the screws so the feet just touch the dew shield, evenly spaced.
3. Tighten the top two. Leave the bottom one slightly loose for quick removal, or tighten it on windy nights.