What should a star look like when perfectly in focus?

Because light is a wave, when light from a star passes through a circular aperture (like a telescope lens) it diffracts -- light waves scraping the sides of the aperture spread out and interfere with each other. Rather than the telescope lens focusing the star to a perfect pinpoint, the best the lens can do is focus the light into a small disk ( called an airy disk) surrounded by faint diffraction rings like the image to the right. The airy disk can be made smaller (more pin-point-like) by using a telescope with a larger diameter objective lens or mirror but...

What does a star actually look like?

But in the real world there are effects that preclude seeing a perfect diffraction-limited airy disk. Some you can't control, some you can.



Auto Focus Hardware
DC Focus Motors & Controllers

Motorize your focuser with a powerful 12V DC motor driven by our nFOCUS controller for fine focusing without vibration more...

STEPPER Focus Motors & Controllers

For computer controlled auto focus, motorize your focuser with our 0.10 degree resolution stepper motor and usb_nSTEP computerized interface more...

Focusing & Imaging Software
Autofocusing requires 3 suites of software, although some vendors combine these and other observatory functions.


A consortium of software developers and hardware manufacturers devoted to vendor-independent plug-and-play more...

ASCOM Yahoo Group

Autofocus Software

FocusMax aids astronomers in the difficult task of focusing CCD-based telescopes more...

FocusMax Yahoo Group

Imaging Software

Sequence Generator Pro
Sequence Generator Pro is a powerful image capture application that supports SBIG, QSI, Canon EOS and ASCOM cameras more...

Sequence Generator Pro Yahoo Group

Nebulosity is designed to be a powerful, but simple to use capture and processing application for Windows and MAC OS X. A huge array of cameras are supported for image capture and processing more...

Nebulosity Yahoo Group

MaxIm DL
Complete observatory integration. Controls your camera, autoguider, filter wheels, focusers, camera rotator, telescope mount, and dome. ASCOM compliant more...

MaxIm DL Yahoo Group