Sky & Telescope - USA (2020-01)

FOCUS TOOLS by H. R. Suiter and W. P. Zmek

30 JANUARY 2020 • SKY & TELESCOPE

Top row: A simple Hartmann mask

helps to get close to focus, but stop-

ping down the optical aperture signifi -

cantly reduces the system’s resolution

compared to the unmasked stellar

image, making it a challenge to see

the slight oval shape when nearing the

focus point.

Middle row: The popular Bahtinov mask normally

works by producing diffraction spikes that appear

symmetrical when the optic is in focus, but the

spikes are only visible on the brightest stars. You

can use fainter stars by balancing the fi rst dif-

fractive order images above or below the stellar

image (circled). The optic is in focus when these

side-by-side orders look evenly spaced.

Bottom row: Replacing the straight vanes

of the Bahtinov mask with a 7-count mesh

grid produces lateral diffraction detail. Not

only are the fi rst-order diffractive images

apparent above and below the stellar

image, the side dots can be employed for

checking the mask alignment.

This easy-to-assemble imaging aid can take the guesswork out of focusing.

Mesh Focusing Masks

Hartmann / Scheiner No mask

DEPTH OF FOCUS

DEPTH OF FOCUS

DEPTH OF FOCUS

Focused +1 +2 +3

Bahtinov No mask −1 Focused +1 +2

Bahtinov Mesh No mask −1 Focused +1