Since the 1960s, star trackers have been determining attitude for satellites, deep-space probes, and crewed spacecraft, helping lock communication antennas to Earth, and point instruments at their targets. A sensor within the attitude determination and control system (ADCS), a star tracker captures an array of stars, matches patterns against an onboard catalog, and computes its orientation from those results. The accuracy of this process depends on the capability of optical components in demanding environments, where blur and stray light degrade the output at any stage.
Star Trackers in the Sensor Stack
Operating at a typical accuracy of 2 to 30 arcseconds, a star tracker is the highest-accuracy attitude sensor in the ADCS, frequently sitting alongside coarser instruments such as sun sensors and magnetometers. By combining their outputs, the instruments can compensate for each other’s limitations. Gyroscopes, for example, update quickly but drift over time, while star trackers are slower but absolute. Pairing the two means the star tracker continuously corrects the gyroscope, delivering high-precision attitude knowledge.
That performance hinges on the optics integrated into the star tracker’s design, and they must deliver this in a hostile environment.
Environmental Challenges
An in-orbit star tracker faces a series of environmental challenges. Stray light is the most persistent, with sunlight, earthshine, and moonlight (all far brighter than starlight) scattering onto the focal plane and drowning out dim star signals. Radiation poses a separate problem: charged particles hit the image sensor, creating bright spots that the system can mistake for stars. Temperatures can fluctuate between -120°C and +120°C per orbit, so optics must maintain their alignment throughout cycles. In addition, reaction wheels and other moving parts create micro-vibrations that, during a camera’s exposure, smear stellar images across pixels and reduce centroiding precision.
How Optical Subsystems Contribute to Orientation Accuracy
Because stars emit across a wide spectrum of wavelengths, a simple lens converges each color at a slightly different point. This effect, known as chromatic aberration, blurs the star’s image and shifts its calculated position. Achromatic lens groups combine glasses with varying properties to bring multiple wavelengths to a common focus, preserving centroid precision.
Optical filters, typically bandpass types tuned to the visible and near-infrared (NIR) range, limit which wavelengths reach the detector, reducing the residual chromatic blur left after lens correction and rejecting out-of-band stray light.
To maintain specified optical performance, these components often carry anti-reflection (AR) coatings which minimize reflection losses and in turn, improve the transmission of faint starlight, and suppressing ghost reflections that would otherwise raise the noise floor.
From LEO to Deep Space
Constellations are scaling fast. Operational satellites have grown from around 1,500 in 2016 to over 8,000 today, almost entirely in low Earth orbit (LEO). With mega-constellations, including Starlink and Amazon Leo, still expanding, each satellite requires precise attitude knowledge in order to function.
Autonomous navigation is also becoming increasingly important, and as more spacecraft – commercial, scientific, and military – operate with reduced ground-based intervention, the burden placed upon onboard sensors, especially star trackers, increases. Missions like NASA’s Europa Clipper are pushing star trackers into harsher radiation conditions and longer mission durations.
As these segments continue to grow, the optical subsystems inside these systems become fundamental to a mission’s success, making the specification and fabrication of precision lenses and filters ever more critical.
For more guidance on specifying lenses and filters for star trackers, contact our engineering team.