Technical

Telescopes

The MAST telescopes are prime-focus Newtonian telescopes with a single parabolic mirror 24 inch (61 cm) in diameter and a focal length of 1830mm, resulting in an F/3 beam and a plate scale of 0.11 arcsec/micrometer. The telescopes are built by PlaneWave Instruments, with a solid fused silica mirror and a carbon fiber truss optical tube assembly (OTA) similar to the well-established CDK series. A central baffle with a Hedrick focuser is located in the telescope's prime focus, with a central obscuration of 12.5%, significantly lower than Cassegrain designs for telescopes of similar aperture. Each telescope is coupled to an L-550 direct drive mount and a stainless steel wedge aligned to the latitude of the observatory, resulting in an equatorial configuration. Each system is positioned on a 97 cm stainless steel pier with dedicated holes and fixtures for cabling, control computers, and other peripheral equipment. Each telescope is also equipped with an electric mirror cover to protect the primary mirror when not in operation.

The telescopes are designed for direct coupling to an optical fiber, and the fast F/3 beam guarantees minimum focal ratio degradation when working with few-mode and multi-mode fibers, with a minimum number of optical surfaces and no additional relay system required to speed up the beam. The minimalist design results in a diffraction-limited point spread function (PSF) on-axis and a highly comatic PSF off-axis.

The optical system of the MAST telescope is designed to be as simple as possible and only consists of a primary mirror that directly feeds the light into the fiber. The primary is a 60 cm (24 inch) parabolic mirror that focuses light at an F-number of F/3.3 which matches the numerical aperture of the optical fibers that lead to the spectrograph. The guiding camera will be located at the prime focus while a thin strip mirror will divert light from the target into the fiber. The simplicity of the design reduces the number of elements that can create optical distortions, eliminates unused degrees of freedom, and keeps the cost low.

Guiding System and Fiber Coupling Unit

Plugged into the Hedrick focuser of each telescope is a fiber coupling unit (FCU) designed and built at the Weizmann Institute, which performs the following:

  • Feed the on-axis beam to an optical fiber - the science target.
  • Feed an off-axis beam  to an additional optical fiber - sky background measurement.
  • Capture images of the on-axis beam, as well as the surrounding field of view for focusing, acquisition and guiding using a dedicated detector unit.
  • No moving parts and zero effective flexure between the detector unit photo-sensitive plane and the optical fibers facets.

Fiber Run

The fiber run is channeling the on-axis beam and a single off-axis beam from each telescope to the instrument room. It is comprised of 45 fibers with a maximal length of around 40 m. As mentioned, each telescope is coupled with two optical fibers: one for the on-axis science target, and one for an off-axis beam for sky background measurements. The 40 FC fibers are gathered in 4 braids and pass through dedicated pipes between each of the rolling roof structures and the instrument room.

DeepSpec

DeepSpec is a novel spectrograph designed from the ground up to classify transient and stellar objects. The design paradigm of DeepSpec is a multi-channel prism-based spectrograph: The beam is split into four channels, each with a custom prism tailored for a resolution of R~600 across the channel bandwidth (110 nm) – with the four channels covering a broad bandpass of 380 - 850 nm.

Following beam dispersion, each channel is imaged on an independent 1k x 1k CCD. DeepSpec offers an efficiency of >65%, and can reach an SNR=10 per resolution element for a 20.3 magnitude source (r band) in 10 minutes. DeepSpec will work in tandem with the ongoing large array survey telescope (LAST; Ofek & Ben-Ami 2020), and is expected to exceed in its capabilities the SEDM, a leading transient spectrograph (Ben-Ami et al. 2012, Blagorodnovna et al. 2016).

HighSpec

HighSpec is a high-resolution (R=20,000) short band-pass (10nm) spectrograph. It is based on custom binary mask ion-etched gratings working in Littrow, with an efficiency of ~90%. HighSpec will have three such gratings for the CaII H&K, Mg b, and Hα lines, mounted on a rotating turret. The spectrograph will enable systematic studies of stellar flares from various exoplanet hosts by monitoring M-dwarf CaII lines equivalent width (e.g., Melbourne et al. 2020), the study of the Rossiter–McLaughlin effect for exoplanet transiting a fast-rotating star (30 km/s), as well as radial velocity measurements of white dwarf binary systems (e.g., Maoz & Hallakoun 2017). The proposed system will be one of the key ground follow-up facilities supporting the ULTRASAT mission, the first space-borne Israel-led telescope expected to be launched in Q1 2025.