Get to Know the Primary mirror of the INO340 telescope

The primary mirror of the INO340 telescope resting on top of actuators


The optical system of the INO340 collects and focuses light from the universe to be analyzed by science and guidance instruments. The major component of the optical system is the primary mirror (M1) of the telescope. It took about six years from the first conceptual design of M1 until its manufacturing and final delivery to IPM Iran. The work of grinding and polishing of the primary mirror was done by the Opteon co. located in Finland.

M1 is a single segment fast f/1.5 mirror with a diameter of 3.4m, which makes IN340 one of the most compact telescopes at this size. The term “fast” for a telescope actually comes from the world of photographers and it generally refers to a measurement of how much light the mirror can put at the focus.  In photography, the more light you get the faster your image forms, and the faster your shutter speed can be, hence the term “fast”. A fast mirror reduces exposure time and allows one to observe low brightness images that might not be possible otherwise. A faster mirror means a shallower and narrower depth of field, allowing one to isolate a subject from the background.

A sample of ZERODUR® ceramic designed for a telescope mirror.

M1 has a meniscus shape and is 18 cm thick and is made of a ZERODUR® ceramic. ZERODUR® is an extremely low thermal expansion glass ceramic. It has been used for a number of well known telescopes in the world including both Kecks and Nasa’s airborne SOFIA observatory. It is also the choice of ESO scientists for their Extremely Large Telescope (ELT) which is expected to become operational around 2025.

Due to the overall design of the INO340 telescope which is Ritchey-Chrétien Cassegrain, the primary mirror has a 700 mm central hole. This design which is a variation of the Cassegrain has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors. You can see the basic diagram of how the light is transferred inside a Ritchey-Chrétien telescope in the following diagram. It is interesting to note that we share this telescope design with many of the world’s large telescopes including the VLT, Keck telescopes, the GTC, Subaru and many others.

As the Earth spins the telescope tracks objects  across the sky sometimes for hours. Despite all engineered precautions, this subtle movement results in slight deformation of the telescope structure which would actually make scientific observations impossible. This is where active computer-controlled corrections of the primary mirror enter. INO340 uses active optics to apply the necessary force to correct for gravitationally-induced deformations as the telescope changes its orientation. The system corrects all low frequency aberrations. We are going to look at the Active Optics of INO340 in another featured piece specially discussing this system.