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qWhether you’re a visual observer or astrophotographer, focus masks
can take some of the guesswork out of focusing under less-than-ideal
conditions. Below is an easy-to-assemble 20° Bahtinov mesh mask for a
4-inch telescope that works well for both imaging and observing alike.skyandtelescope.com • JANUARY 2020 31
F
ocusing a telescope can take a great deal of fi ne-tuning
and fi nesse. It’s hard enough to focus a telescope or
lens by eye, even though most eyes are somewhat
forgiving. Once you’re within the range of a half-diopter
or so, the eye’s internal process of focus accommodation
automatically takes over, at least for young eyes. Of course,
there are mechanical constraints that have nothing to do
with the quality of the image viewed. For example, manu-
ally adjusting the focus knob causes the view to jiggle around
the fi eld. An image that jumps and shakes instead of stand-
ing still is impossible to focus, no matter how you measure
success. Motorized focusers mostly solve these mechanical
constraints — judgment of focus becomes nearly real-time, if
the focusing action is slow enough.
Most solutions for visual observing, however, do not cover
the full scope of the focusing problem. Astro-cameras possess
no biological eye accommodation, making them even more
diffi cult to focus. Poor focus can ruin an arduous exposure.
Many imagers rely on computer software to aid in the pro-
cess, employing metrics like the width and central bright-
ness of a stellar image (known as the full-width at half-
maximum, or FWHM) to determine quality of focus. Anyone
familiar with these methods will no doubt continue to use
them. But it’s handy to have an independent way of check-
ing the results, and it’s even better to have a backup plan if
software fails to deliver.Dividing the Field
Imagers and observers alike use various tricks to determine
perfect focus in an effort to convert the process from an
exercise in judgment to a purely geometrical comparison.
These methods have met with varying degrees of success;
some work better than others.
One of the oldest is to place a device called a Hartmann
mask or Scheiner disk over the telescope aperture that sub-
divides it into two smaller holes, producing two images of the
same star that merge into a single image as you rack through
focus. This method is repeatable if there’s a way of recording
the focuser’s precise position and returning to the observed
center of the pattern. The drawback to this mask is that it is
very coarse. The Hartmann mask signifi cantly reduces the
resolution of the instrument, increasing the size of the Airy
disk (the bright central core of a star’s image formed by a lens
or telescope) so much that it masks the shape of the star as it
approaches best focus.
In the mid-2000s, Russian astrophotographer Pavel
Bahtinov introduced a new type of focusing mask that soon
spread throughout the imaging community. Bahtinov made
a mask having three coarse diffraction gratings at shallow
angles to one another. These gratings were angled in such a
way as to induce an easily interpreted imbalance in position
between the diffraction orders of the various grids in defo-
cused images. This imbalance is visible in two ways. The fi rst
and most commonly used are long, overlapping (high-order)
diffraction spikes that are centered only at the point of focus.The drawback to this approach is the diffraction spikes are
only clearly visible on the brightest stars.
Bahtinov himself suggested a second method that works
well on dimmer stars. Instead of concentrating on the main
image of the star, use the fi rst-order diffraction spots seen
above or below the actual star image when using the mask.
The monochromatic Fourier diffraction model seen on the
facing page is slightly inaccurate as it doesn’t show the fi rst-
order spots stretched into stubby spectra, but with dim stars
the visible spectrum is very short. If you’re focusing a camera
through any color fi lter, monochromatic images of fi rst-order
spots are nearly perfect tools for achieving focus.
As alluded to earlier, the long diffraction spikes are great
if you can see them. The fi rst-order spots, on the other
hand, are always there, making many more stars avail-
able for use as focusing targets. More importantly, you can
employ a star in the direction of your telescope tube’s lean
angle to compensate for the effect of tube fl exure at differ-
ent points in the sky.Mesh Mask Alternative
Nearly all amateurs who know about the Bahtinov mask
have tried to fabricate one themselves by cutting long,
straight plastic strips, and some have succeeded after pains-
taking effort. Whether or not they were successful, all agree
that they don’t want to attempt it a second time. Isn’t there
a straightforward alternative that doesn’t involve such an
extraordinary effort?