Optics

The INO340 telescope is a Ritchey-Chrétien f/11 which provides unvignetted 20 arcmin filed of view at the main Cassegrain focus. In addition 3 bended focus are also provided each with a field of view of 8 arcmin. The primary mirror is meniscus shaped and the secondary mirror is convex and together they form a well-corrected focus.

 

Main Optical Parameters
Optical configuration
Ritchey-Chrétien Cassegrain
Wavelength range
325-2500 nm
Primary mirror diameter
ID/OD (Nominal): 3400/700 mm
Primary mirror focal ratio
f/1.489
Exit focal ratio
f/11.24
Entrance pupil location
On primary
Back focal distance
1670 mm
Unvignetted field of view diameter
20’
INO340 Telescope
INO340 Primary Mirror
Primary Mirror

The main mirror (M1) is 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. M1 is a meniscus shape 18 cm thick made of a Zerudor ceramic with a 700mm central hole. The M1 is polished to a surface micro-roughness of ~1nm root mean-squared, one of the thinnest in the world allowing short and long wavelength observations. The M1 is supported by 60 actuators which are actively or passively controlled to keep the mirror shape undistorted under its own gravity at different pointing.

 

Primary Mirror Parameters
Glass
Ceramic ZERODUR®
Shape
Meniscus
mass
~4000 kg
Diameter
3401 mm
Optical diameter
3300 mm
Thickness
180 mm ± 10 mm
Central hole diameter
700 ± 1 mm
Radius of curvature (fronthyperbola)
10200 ± 20 mm
Radius of curvature (Rear face)
10380 mm
Focal ratio
f/1.5
Surface roughness
< 2 nm RMS
Conic constant
-1.006472
Scale
0.18731 mm/arcsec
Secondary Mirror

The 600 mm secondary mirror reflects the light from the primary mirror and is also controlled by a hexapod positioning system that can move and tilt the secondary mirror in all degrees of freedom. The mirror has is made of a Zerodur ceramic and has a hyperbolic surface geometry. The mirror supports are active and can correct the shape of the mirror, for example correcting errors that may be caused by the changing zenith angle and temperature.

 

Secondary Mirror Parameters
Glass
Ceramic ZERODUR®
Shape
Convex
mass
~60 kg
Diameter
602 mm
Optical diameter
582.4 mm
Thickness
90 mm (center)
Radius of curvature
1812.4 mm ± 20 mm
Surface roughness
< 2 nm RMS
Conic constant
-1.7667
Secondary mirror if the INO340 telescope inside the top unit.

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