How to… make a Bahtinov mask

A Bahtinov mask is easy to make and will help you take pin-sharp astro images with your DSLR camera

By Steve Richards

There are many important factors that go into a good astro image, but focus is by far the most crucial. No matter how colourful, well framed or well exposed your photos may be, if they’re out of focus they’ll be spoilt beyond ‘repair’.

Now, you might assume focusing a DSLR camera would be easy – simply look through the viewfinder and adjust the focus; after all, that’s how a single lens reflex camera is designed to work. However, if you try this method you’ll be rather disappointed, because a DSLR camera’s viewfinder is for daylight use; it just can’t cope on small, dim, night-sky objects. Things are getting a little easier though: many of the newest DSLR cameras have a ‘live view’ facility that lets you see the view through the telescope directly on the camera’s built-in LCD screen (see the DSLR Group test in issue 48).

If you don’t have a ‘live view’ function, one method of finding focus is to use special image capture and star analysis software to download the image from the camera and examine it on a laptop computer. However, it isn’t always convenient to have a laptop outside with you. One solution is an excellent focus tool that you can make called a Bahtinov mask. The mask is a clever focusing device invented by the respected Russian astrophotographer Pavel Bahtinov. If focus is an issue in your astrophotography then a Bahtinov mask is for you.

The mask is simply placed over the front of your telescope, which you point at a suitably bright star. It works by adding a specific pattern of ‘diffraction spikes’ to your test star image, rather like those produced by the spider vane of a Newtonian reflector. However, unlike the simple spikes produced by a spider vane, the diffraction spikes from the Bahtinov mask produce a bright elongated cross with a third line running through it. The aim is to adjust the focus with the scope’s focuser until the middle diffraction spike is perfectly centred in the cross formed by the other two diffraction spikes.

These ‘Before’ and ‘After’ images above show the difference between a view that is out of focus (top) and one that is in focus. Once a star is in focus, the horizontal diffraction spike is exactly centred between the cross made by the other two

To get an accurate focus with your DSLR attached to the telescope, you’ll need to take a continuous series of images that are of sufficient exposure length to show the star brightly in the camera’s LCD preview screen after it has been captured. Or if your camera has ‘live view’, use it to examine the star’s diffraction pattern. 

To make the procedure as quick as possible you should use a higher ISO number for focusing than you would use for the actual imaging. This makes your DSLR more sensitive to light and so will allow you to take a star image suitable for focus purposes in a shorter period of time. Remember to set the ISO number back to your preferred setting and remove the mask when you start your actual imaging run.

Blue Peter style
Making your Bahtinov mask is a true Blue Peter-style project involving paper, cardboard and sticky-back transparent plastic. First of all you need to enter your telescope’s parameters into this on-line template generator. This is because, importantly, each mask has to be tailored for your particular telescope. This isn’t a ‘one size fits all’ solution, so you will need to know the following dimensions for your telescope in millimetres: the clear, unobstructed aperture, the diameter of your scope’s front end and its focal length.

Once you have this information, visit the website above and simply fill in your data to download a customised template. This produces a Scalable Vector Graphics (SVG) file, which can be opened by many graphics programs including the popular free image editing package Gimp, and printed out to make your Bahtinov mask.

 

A step-by-step guide to making a Bahtinov mask

STEP 1
Print off your template and add stiffness and moisture proofing by sticking transparent Fablon material to the printed sheet. Cut out all the slots with a craft knife and steel ruler, being careful not to damage either yourself or the mask.

STEP 2
Cut out the circular shape of the mask, leaving a 1-inch margin from which you can cut out triangular tabs, with the base of each at the printed edge of the mask. These tabs will be used to attach the mask to the cardboard sleeve.

Make the inner sleeve that slides over the front of the telescope by cutting a 1-inch wide cardboard strip that is slightly longer than the circumference of the scope. Wrap it round the telescope, overlapping and gluing the ends to form a ‘collar’.

STEP 4
Leaving the sleeve in position, place the maskover the front of the telescope and bend the triangular tabs around the rim of the mask over the cardboard collar. Glue the tabs to the collar, taking care not to get any glue on the telescope!

STEP 5
Cut a similar strip of cardboard to the one in step 3, but about 10mm longer. Carefully glue this around the first collar that now has the mask attached to it with the triangular tabs. The idea is to trap the mask’s tabs between the two layers of cardboard.

STEP 6
Reinforce the new structure by wrapping electrician’s insulating tape around the outside of the collar. Don’t stretch the tape too much as it could contract and distort the collar, making it more difficult to slide it on and off your telescope.
 

This article appeared in the August 2009 issue of Sky at Night Magazine