Proiekt 345 (draft)
Is about one of the older dream of mankind – to explore the deep Universe. 5 designs are available from which only helium design is further described:
HELIUM design – mercury liquid mirror nested inside a protective helium atmosphere.
Baloon design – best for outer space – the best design out of Earth atmosphere
Spin casting liquid mirror – suitable ONLY for very high altitude balloons
Liquid – frozen mirror – suitable for mirrors less than 30 meters
Umbrella mirror – best for outer space
HELIUM design – mercury parabolic liquid mirror nested inside a helium protective atmosphere
Liquid mirror telescopes are telescopes with mirrors made with a reflective rotating liquid usually being used mercury. The surface of the rotating liquid takes a paraboloidal shape if the speed of rotation around a vertical axis is constant. This liquid rotating surface is suitable for use as the primary mirror of a reflecting telescope regardless of the container’s shape and size.
The main advantage of such kind of telescope is the price itself opposite to monolithic / honeycomb mirrors.
A monolithic mirror is very heavy and prone to shape changes due to thermal expansion and also the grinding itself for parabolic shape is a very slow and expensive process.
A multi-mirror honeycomb telescope is very expensive and requires high technology in order to align so many mirrors.
The main drawback of such design is that a liquid mirror telescope can only be used only to point the zenith (can only be pointed straight up).With other words the rotating mirror cannot be tilted to any direction. Such type of telescope has a permanently moving sky – stars from the sky effectively entering and getting out from the view field very fast.
The main idea of HELIUM design is to bring the “normal sky” over a rotating liquid mirror.
The design itself is also applicable for small size mirrors without using a dedicated helium chamber which is mandatory to be used for very large mirrors ( over 2 meters) where the rotational speed create ripples on the mercury surface due to it friction with air. For professional telescopes the acceptable maximum wind speed toward outside the rotating dish is 2.7 km/h (0.75 m/s) for an open mirror rotating at 20O C at sea level. That means in order to obtain brilliant images the maximum acceptable size of such kind of mirror is about 2 meters for a focal length of f/4.0.
Components:
On picture:
1 – the mercury liquid mirror – f/2.7
2 – the upper plane mirror (can be fix or can tilt)
3 – the outer tilting plane mirror normally rotating around the house of the telescope but also can be fixed
4 – the transparent window made either from a very thin foil or by rigid glass sheet hold on by a honeycomb mesh
5 – the sealed chamber (house) of the telescope filled with helium always rotating (following) the outer ground mirror
6 – CCD camera and lens or the second concave ellipsoid mirror (Gregorian telescope)
7,8,9 – stars disposed at 30O (7), 45O (8) and 90O (9) –
10 – the extension protective cover
11 – the second concave elliptic mirror
12 – the mercury liquid mirror on another position – f/1.2
Not presented in picture:
– mercury cleaning machine
– cleaning machine of helium by mercury vapors
– helium supplier / cleaning machine by dust and other gases
– rotating mechanical machine
– machine to remove the heat generated by rotating machine (thermoking)
– oxygen removal device
Pros:
– the most advanced design ever- unmatched by any other today ground based telescope design
– ultra-high precision of collimation, ultra-sharp images
– extremely large mirrors can be built – practically up to 240 m – the resolution is limited only by Earth atmosphere- an f/2.0 100 meters mirror have a maximum rotational outer speed of 25 km/h – 7 m/s but if the telescope is placed at 5.500 m altitude and due to helium density the relative effect is that of 0.5 m/s and for 240 m is about the equivalent of 0.75 m/s. Thus near no ripples appear on mercury rotating surface. Please note the helium density is about 7 times lower then atmosphere density for the same pressure.
– if second concave ellipsoid mirror is placed we have a Gregorian telescope which unlike the refractor telescope is free of spherical and chromatic aberration
– because of helium protective atmosphere no pollution with mercury occur on telescope nearby area.
– presence of inert helium means no oxidation of mercury layer – the reflecting surface remain clean for years
– the parabolic shape easily can be adjusted with temperature variation
– the upper mirror not require any protecting coating over aluminum reflective surface as long as helium is not corrosive
– if two outer plane mirrors are available in the same time the telescope out-of-work time is very short (just a couple
of minutes)
Cons:
– helium sealing – not easy at all such large chamber to be well isolated in order helium gas to not escape in Earth atmosphere
– the transparent window should be well aligned and clearly it will not be easy at all to made such window which is called also to seal properly the helium atmosphere. Rigid CaF2 glass sheet hold on by a strong carbon honeycomb mesh probably will be the best solution. The rigid glass sheets will be hexagonal – 30 cm in diameter and 4 mm thick sealed by a silicon rubber ring inside a hexagonal 2 meters diameter panel. Panels will be instaled one by one.
– a relative large area is required on site for such kind of telescope – this issue should be taken onto consideration in case of telescopes placed on high altitude peak mountains
– cumbersome and prone to ground vibrations (for mechanical rotational mechanism)
– the total reflectivity is about 75 % for visible spectrum which clearly is beyond other reflector telescopes due to lower reflective of mercury and also due to reflection via two aluminum mirrors
– the outer plane mirror requires special care = daily cleaning and monthly re-coating with aluminum but two such mirrors can be available onsite in the same time. Also if one outer mirror is used due to the space restrictions we can built a honeycomb mirror – where each shell (foil) will have both sides aluminum coated and the bottom side either can be clean up or can be replaced with new ones.
– such design is practically only for stars positioned higher than 30O over the horizon – for lower stars larger and larger outer mirrors are required and also the total area of the telescope site will increase considerably.
– the lifting power of helium should be taken onto consideration
– the high price of helium
– large quantities of mercury are required for very large mirrors – an 240 m diameter mirror required at least 600 tons of mercury for a mercury layer about 1 mm thick.
– for Gregorian design the focal length could very long especially for very large mirrors (more than 100 meters) because the maximum monolithic concave elliptic mirror cannot exceed 8 meters – thus the second mirror in practice could be 10-30 times smaller than the main mirror (the liquid mirror)
– a large area of outer mirror remain unused if it is tilted from basic position (30O)
One of the best telescope locations on Earth is the 7.509 meters peak Muztag-Ata from Central Asia (east China).
Here – of course if China agrees – can be placed a very large (120 meters) HELIUM Gregorian telescope f/1.2 where two outer fixed mirrors can be placed on nearby Kalaxong peak (7.277 m) located at 1 km distance and on Kuksai peak (7184 m) located at 2 km distance. The outer mirrors will be fixed ones (not mobile – if located on Kalaxong/Kuksai peak) that means it should be able to tilt but also the upper mirror from telescope chamber should tilt in order to point the desire star or galaxy – in such case the diameter of telescope chamber will be larger as long as the upper mirror will be larger than the rotating liquid mirror. Also two windows are required in case of two fixed outer plane mirrors. Due to very large available area on the top of Muztag-Ata this mountain is the best place on Earth where to place a large telescope – unmatched even if by Antarctica – but the number of nights with good grazing conditions is under 60 per year. On the top of Muztag Ata mountain we have 1/3 of Earth atmosphere above our heads and sometimes less than 2 % of atmosphere water vapors. On this mountain can be placed an advanced design of HELIUM telescope – named binocular HELIUM – on which the window is made by columns of prisms and the light from stars are coming directly from the ground. With other words two outer plane mirrors can be placed side by side in closed proximity of the telescope chamber – both sending to the prisms the stars light – and making a mirror of 40 meters with the power of 57 meters one. That means both mirrors (rotating and outer) can be placed directly on main peak of Muztag- Ata mountain at 7.500 meters altitude but in such case the liquid mirror cannot exceed 40 meters diameter. In such case the total diameter of telescope platform will be around 120 meters. Also a non-binocular (only one outer mirror) design can be used with the price of limited view range.
Another possible solution is to place at a very high altitude ( 30 km) a HELIUM telescope under a very large and long multi-chamber balloon filled with hydrogen (at least 300 m diameter and 2 km height – 100.000.000 cbm). Very long exposure images can be obtained and the clarity of these images will surpass 300 times the Hubble telescope. Due to small friction with air the sealed chamber it will no longer be required but both plane mirrors need a protective transparent layer. The rotating mercury surface will be soon covered by a very thin layer with the mercury oxide.
This solution is extremely difficult due to complex issues which appear – the rotating liquid mirror should be counter-balanced by another rotating plate, the arm holding outer mirror itself should be very long (at least 150 meters) and counter-balanced, the outer temperature is very low, the pressure is abt. 1 % from normal pressure, ultra-light and very powerful materials are required and so on. Practically a sealed pressurized chamber is required where 3-4 astronomers and technicians to do their jobs during a period of 4-5 months and high care should be paid for the health of the staff. The main advantage of course is the lack of atmosphere – only 1 or 2 % of the Earth atmosphere can be found above the mirror and practically no water will be present. Unfortunately the sky right above the telescope will be occupied by the balloon which is a drawback of this design. Anymore such solution will be very useful in case of an Antarctic / Artic positioned balloon. A better alternative against this possible solution is the spin casting (depositing) f/1.0 liquid mirror Gregorian design. Such design allowed ultra-light mirrors up to 120 meters diameter to work but a balloon of 200 million cbm is required and the cost are extremely – probably around 850 million euro (2013) – including the possibility of mirror recovery in good conditions. At such altitude a 120 meters diameter mirror will furnish images IMPOSSIBLE to be obtained by any ground based telescope – no matter on which mountain/plateau is placed that telescope. Such large mirror will catch 10 times more details over a HELIUM design mirror being lighter at the same diameter and more reliable. I don’t dear even if to dream what amazing information will achieve for mankind such a large mirror placed at 30 km above the ground. We can start with an experimental 12 meters diameter mirror placed under a 10 million cbm balloon launched on the beginning of Antarctic winter – will cost abt. 25 million euro (2013) – including German man-power for mirror – everything else made / assembly in Union European – and will be a true extreme Antarctic expedition which will prove in full the huge advantage of this ultimate design – an incredible crispy images of very remote galaxies surpassing by far anything furnished until now by Hubble.
For deep search in Universe hanging a large mirror telescope under a high-altitude balloon is the best option for mankind from the perspective of performance, costs and technological difficulties.
