300 mm F/5.5 solar Newtonian telescope


Truss structure : Stellarzac Intrument, mirror polished by Skyvision


Optical design :

- 300 mm F/5.5 Newtonian telescope

- Radius of the diffraction limited flat field

l radius in arcmin diameter in arcmin diameter in mm
396 nm
430 nm
550 nm

Primary mirror :

Secondary mirror :

- Quartz secondary mirror by Antares Optics :

- Fully illuminated field > 20 arcmin

- Inox spider, thickness = 1.5 mm, impact on Strelh ratio : 0.98, amount of light in the diffraction spikes = 0.01%, length of diffraction lobes = 200 x diameter of Airy disk.

Truss structure :

- Truss tube and mechanical parts: Stellarzac Instrument.

- Motorized 2" focuser with tilt correction plate : Moonlite.

- Weight : 21 kg including mirrors, focuser and Astrophysics SBD16" dovetail plate.

- Solar Seeing Monitor : Airy Lab.

Thermal considerations :

- A lot of care was taken in the design process to minimize thermal turbulence due to the telescope (tube seeing and mirror seeing).

- Heating of the truss structure :

- Mirror seeing :

- Motorized 2" focuser with tilt correction plate : Moonlite.



A mirror for solar telescope : grinded or polished second surface ?

First ... the theory :

The polished front surface of the mirror reflects 4% of the incident light. The impact of the back face of the diffuse background depends on if it is polished or left grinded.

- a) If polished the second surface reflects 4% * (1-0.04) = 3.8% of incident light. In the present configuration, the light is reflected is a divergent beam of -7300 focal lenght

- b) If left grinded, the second surface diffuses light backward (= back to the focus) and forward. The following figure only illustrates backward diffusion. The amount of diffusion depends on the roughness of the surface.

The following figure illustrates the specular and diffusion at work at the second surface of the mirror (Bidirectional Transmission and Diffusion function, see http://docs.lumerical.com/en/particle_scattering_bsdf.html)

The light diffused backward from the second surface is then either reflected back (4%) by the front surface or transmitted (96%) through the front surface and contributing to the background..

In other terms, a grinded back face results is much lower diffuse background than a polished back face. This is because the diffused light is spread over a much larger solid angle.


Now, the comparison in images :

This effect is clearly visible in the following set of images.

A laser beam (coming from over the top center of the image) is reflected back by the mirror front surface (its edge is visible on the bottom right of the image) and diffused / reflected by the second surface of the mirror. The reflected spot, and the amount of diffused light is observed on a white screen set in front of the mirror..

Grinded second surface :

In this first image. The amount of light diffused back arround the main image is detectable, but very low compared to the main beam.

The main part of the diffused light is concentrated in two 90° solid angle lobes, one on each side of the mirror.

Polished second surface :

In this second image, a drop of water was deposited on the second surface, turning it into a "polished" surface with very low roughned (same exposure time).

Now the level of background light is really high, because it is concentrated over a small solid angle and not spread over a 360° solid angle.



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