Practical astronomy projects for every level of expertise
DIY Astronomy
Build a Bahtinov mask
Make an astrophotography focusing aid to fit any telescope
Achieving focus is arguably the most important aspect of a successful image. This month’s project is a focusing aid to help you achieve pin-sharp stars and therefore pin-sharp images. The design, developed by astronomer Paul Bahtinov, is a mask that makes use of diffraction, an optical effect that happens when light passes through small openings. Diffraction effects can be observed with a Newtonian telescope that has a ‘spider’ support for the secondary mirror, or a camera lens with an iris that has straight edges – in both cases, bright stars appear to have lines (called spikes) radiating from them.
Our mask has three sets of diffracting slits through which the starlight passes. On one side, two sets of angled slits produce a cross pattern, while the straight slits on the other side create a line that passes through the cross. When the image is properly focused, this line passes exactly through the centre of the cross, but when unfocused it is offset to one side.
Cutting it fine
Most DIY versions of such masks are made from card, but this design is slightly more robust and should last longer. However, it’s still easy to make, using just a few basic materials, a craft knife and glue. The mask panel is cut from a sheet of thin craft foam and the sleeve is built up from strips of cork. These were bought from our local hobby and craft store. A very sharp knife is essential for a neat mask, so pop a new blade in, use a cutting mat to protect the tip, and make the mask panel before cutting the cork strips.
There is no need to work out the design of the slits because there’s an online tool that does it all for you. Go to bit.ly/bahtinovmask and type in your telescope’s focal length and aperture. If your telescope has a central obstruction, add the diameter of this too. The tool generates a CSV file that can be printed from various applications, including MS Word and Google Chrome.
Once your mask is completed, you can use it by taking a series of images of a convenient bright star (or use live view) and adjusting the fine focus. Once you have the line and cross aligned, lock your focus, remove the mask, and reposition the scope to take an image of your desired deep-sky object.
What you’ll need
► Scalpel or craft knife, cutting mat, safety rule, pencil
► Thin, black craft foam (size to suit tube diameter), thin cork sheet
► Black spray paint, PVA glue, masking tape, low-tack glue or spray mount
► Rubber bands, clothes pegs
Step by step
Step 1
Remove the dew shield to work on, if possible. If not, protect the optics with a clean, soft cloth and make sure that the telescope can’t roll off the table. Measure the diameter of the dew shield or main tube. Protect the tube with some masking tape.
Step 2
Enter your telescope’s specifications into the online mask generator. Download the file and print. Use low-tack glue to fix the printout to the craft foam. With a very sharp scalpel or craft knife cut out the slits. Peel off the remaining paper.
Step 3
Cut enough 40mm-wide cork strips to wrap around the tube three times. Trim and fix the first strip around the tube, holding the ends together with tape. Apply PVA glue to the next two layers and wrap them round. Secure with rubber bands until dry.
Step 4
Slip the whole ring up by about 15mm. Cut three 10mm-wide cork strips. Carefully use PVA to glue these inside the cork ring, forming a lip. We used clothes pegs to clamp them. Wait until all the glue is dry before removing the ring from the tube.
Step 5
In a ventilated space, add a few layers of black paint to provide some protection against moisture and make it more hardwearing. We left the inside as natural cork where it touches the paintwork of the tube, making it easier to slip on and off.
Step 6
Once the paint is dry, you can insert the mask into the underside of the lip. You may leave this loose in case you want to experiment with different mask patterns, but when you are happy with the result, add a few drops of glue to secure the mask in place.
Mark Parrish is a bespoke designer based in West Sussex