Telescopes

Simonyi Survey Telescope

Telescope Mount Assembly (TMA)

The TMA's compact steel structure was designed to be rigid yet relatively lightweight, in order to slew 3.5 degrees (i.e., the field of view) within 4 seconds and settle within 1 second. The azimuth cable wrap allows rotation -250 to +250 degrees from north. The telescope is capable of non-sidereal tracking with angular rates of up to 220 arcseconds per second in both azimuth and elevation.

"Large Synoptic Survey Telescope mount final design", Callahan et al. (2016)

Go to the TMA page for a public audience.

Telescope Mount Assembly and Dome Slew Rates

The following telescope and dome slew and crawl rate values are based on internal technical documents outlining system requirements. These values are still undergoing characterization, and the nominal (required) values listed here may differ from the final measured performance.

Dome crawling: The dome must support continuous crawling—a slow, steady azimuth motion during exposures to anticipate the position of the slightly oversized slit for the next field—at up to ±1.5°/s for 34 seconds, and must track the telescope in both azimuth and elevation at rates up to ±220 arcsec/sec to maintain alignment with the optical path.

TMA operational ranges

Elevation: 15 deg - 86.5 deg

Azimuth: -270 deg -- +270 deg

Telescope zenith avoidance zone

The telescope can and must reach 90° (zenith), for maintenance. However, science performance requirements (e.g., pointing/tracking precision) only need to be met between 15° and 86.5° elevation.

Active Optics System

The Vera C. Rubin Observatory’s Active Optics System (AOS) maintains sharp, seeing-limited images across its 3.5-degree field of view by continuously adjusting the shapes and positions of the M2 mirror and camera relative to M1M3. It corrects distortions from gravity, temperature shifts, and other perturbations using a combination of preset (open-loop) and real-time (closed-loop) controls, the latter informed by wavefront sensors on the focal plane’s edge.

References:

• Advancing the Vera C. Rubin Observatory active optics control system, Megias Homar et al, 2024, SPIE

• The Active Optics System on the Vera C. Rubin Observatory: Optimal Control of Degeneracy among the Large Number of Degrees of Freedom, Megias Homar et al, 2024, The Astrophysical Journal

• Using AI for Wave-front Estimation with the Rubin Observatory Active Optics System, Crenshaw et al, 2024, The Astronomical Journal

• The Vera C. Rubin's M2 support system integration and verification at the TMA

Hexapods and Rotator

The Simonyi Survey Telescope uses two precision electromechanical hexapods—one for the Camera and one for the Secondary Mirror (M2)—to actively maintain the alignment of the optical system. A rotator, integrated with the Camera hexapod, enables precise image de-rotation during exposures in the telescope’s alt-azimuth configuration, compensating for Earth's rotation, which causes the night sky to appear to move across the detector.

Reference: Final Design of the LSST Hexapods and Rotator, Sneed et al, SPIE, 2016, SPIE

Mirrors

The telescope has three reflective surfaces in the light path: the primary (M1), secondary (M2), and tertiary (M3) mirrors. M1 and M3 are formed out of a single piece of glass (M1M3) but have different curvatures. M2 has a 1.8 meter aperture in the center to allow light to pass to the camera.

Mirror sizes:

• M1: 6.7 meter effective diameter
• M2: 3.4 meter diameter (convex)
• M3: 5.0 meter diameter
• M1M3: 8.4 meter diameter

Etendue (integrated throughput, a measure of survey capability), defined as collecting area times the solid angle of the field of view, is 319 meters squared degrees squared for the design of the telescope and the LSST Science Camera.

M1M3 and M2 are coated with four layers. An adhesion layer of nickel-chromium (NiCr), a reflective layer of silver (Ag), another NiCr adhesion layer, and a final protective layer of silicon nitride (Si3N4).

Go to the mirrors page for a public audience.

Auxiliary Telescope

AuxTel has a 1.2 meter primary mirror and a slitless spectrograph. It will observe bright stars every night to obtain atmospheric transmission and improve the photometric calibration of the LSST data. Scientists will not need to process or analyze AuxTel data or apply the derived corrections themselves; this will be done as part of the processing with the LSST Science Pipelines.

Go to the AuxTel page for a public audience.

References:

• Vera C. Rubin Observatory auxiliary telescope commissioning as a control system pathfinder, Proceedings of the SPIE, Volume 11452, id. 114520U 16 pp. (2020).
• The Vera C. Rubin Observatory 8.4m telescope calibration system status, Proceedings of the SPIE, Volume 12182, id. 121820R 14 pp. (2022).