Sky & Telescope - USA (2020-01)

skyandtelescope.com • JANUARY 2020 33

diately display any mutual misalignment of the two smaller
tilted regions.
Another advantage is that mesh masks are extremely easy
to modify or repair. It’s as simple as turning the grids over the
holes or simply cutting out a new piece of plastic canvas.
Another diffractive focusing mask was popularized at
roughly the same time as the Bahtinov mask. Although Andrei
Oleshko’s mask made less impact in the amateur community,
it’s equally sensitive and even easier to make with mesh.
Using plastic canvas mesh, the most straightforward
way of making one is to place a 5-count grating parallel to
a 7-count grating over a mirror-image hole. The modeled
color image below shows the way the Oleshko mask works.
Because the 5-count grating wasn’t available in black, it was
constructed out of translucent plastic. A pleasant observation
was that the transparency of the mask material makes little
difference to the fi nal image. Poorly focused light transmitted
through the translucent material is scattered through such a
wide angle that the translucent mesh works as effectively as
black mesh.
Because strict alignment of any of these diffraction masks
over the aperture isn’t necessary, they can be loosely placed
on the front of the telescope using long nylon screws. These

The Oleshko two-frequency mask produces vertical spectra that line up when the optic is in focus.

don’t attach to anything; they simply extend like fi ngers

down over the edge of the tube. The complete structure is

light and won’t affect balance.

For visual use, diffractive masks require high power. At

560 nm, the 5- and 7-count meshes give fi rst-order dif-

fraction peaks separated by 32 and 23 arcseconds from the

zeroth-order (in-focus) stellar image at the center. Given

this scale, you can imagine Jupiter covering about half of the

frames modeled here. To show deviations of spacing or linear-

ity well enough, the magnifi cation for small telescopes should

approach 20× per centimeter of aperture. When focusing a

camera, the spread of the diffracted fi rst-order dots or spec-

trum separations should extend at least 7 to 10 pixels.

These masks are not magical. Because of many variables,

including mechanical stability and atmospheric turbulence,

focusing will still be a challenge. But it will be noticeably

easier than simply viewing a dancing point of light and trying

to decide where it’s smallest and brightest.

¢ DICK SUITER is the author of Star Testing Astronomical

Telescopes. BILL ZMEK is the author of Interferometry for

Amateur Telescope Makers. For more information on focusing

masks, visit https://is.gd/focusingmasks.

Oleshko Mask No mask −1 Focused +1 +2

23

arcseconds

C D E

DEPTH OF FOCUS