Optics section is actively involved in the instrumentation activities related to various projects in the institute. The section has various facilities and instruments for aligning, testing and integration of optical system. The details of facilities and instruments installed are as below:
Clean Room Facility: Optics laboratory has a class-10,000 clean room with laminar flow unit. The clean room has a vibration isolated optical table of 12 x 9 x 9 feet dimension. This facility is used for assembly of optics and electronics (CCD camera etc.) related to the instrumentation programs.
Figure1: The class 10,000 clean room facility with vibration free table installed at optics lab, ARIES, Nainital
2. Optical Profiler cum Phase Shift Interferometer:
Size of samples mirror that be accommodated: up to 500 micron
Vertical resolution : 0.1 nm
Measurement repeatability : 0.1 nm Rq
Z imaging range : <1 nm to 5000 micron
Z – Scan speed : > 3 micron/sec at a height
resolution of 0.1 nm.
Purpose: Surface roughness measurement, Shape profiles of optical elements, step height of coatings on mirror, 3D surface profile analysis
Figure2: Optical profiler set up (left), measurement sample kept at tip-tilt stage (right)
Model: StellarNet Inc.
Wavelength range (nm): 300-1100
Optical resolution: 1nm
A/d resolution: 16 bit
Connector: SMA type
F ratio: F/4
Stray light: <0.1%
Purpose: Fiber fed instrument used in laboratory experiments
Figure3: Transmission curve of filter measured using spectrometer
Wavelength accuracy for laser head : ±0.1 ppm over life time
Wavelength stability of the laser head : ±0.02 ppm over life time
±0.002 ppm over 1 hour
Fundamental optical resolution : 0.5 wavelength or better
Non linearity error : < ±5 nm
Dead path correction error : < ±0.15 times dead path distance
Purpose: micron level measurements (linear/angular alignments, vibrations etc.) of mechanical and opto-mechanical systems
Acceptance angle < 2 degree
Angle of incidence < 10 degree
Wavelength range: 365 nm- 97 0 nm
Global absolute accuracy: better than 0.6% for reflectivity> 50%; @400-900 nm
Repeatability: better than 0.02% for reflectivity>50%; @400-900 nm
Purpose: Reflectivity measurement of mirrors over visible wavelength range
Figure4: Reflectivity measurement of aluminium coated 3.6m mirror using reflectometer
Model: Edwards Turbo pump (T- station 75)
Pumping Speed (N2): 61 ls-1
Ultimate pressure: < 1 x 10 –9 mbar
Cooling method: Air
Run-up time: < 3 minutes
Interfaces: RS 485, Remote
Accessories: O-rings, clamps, Bellow pipe, vacuum gauge etc.
Purpose: Used for CCD evacuation
Figure5: Evacuating 4Kx4K CCD using Vacuum pump
7.CO2 Snow Cleaning Gun:
- Snow gun (on/off system)with 10 foot flexible stainless steel PTFE lines hose
- Liquid CO2 Cylinder fitting
- Pressure gauge:0-2000 psi (0-75bars) on a TEE by the CO2 cylinder fitting
- Sintered stainless steel filter rated at ~0.003 microns
- Venturi nozzles (Asymmetry venture nozzle design for constant enthalpy expansion) with suitable high and low flow speed nozzles/ orifices
Purpose: Used for large telescope Mirror cleaning and for precision cleaning of small optics like lenses, small mirror etc.
8. Zemax-EE Software: Software for optical design
9. 3.6m Aluminium coating Plant @ Devasthal
Initially the primary mirror of the 3.6m Devasthal Optical Telescope is uncoated polished zerodur glass supplied by Lytkarinskiy Zavod Opticheskogo Stekla , Russia/Advanced Mechanical and Optical Systems, Belgium. In order to do aluminium coating on primary mirror, the coating plant including washing unit is installed near the telescope, by Hind High Vacuum (HHV) Bangalore, India, in 2014. The coating unit consist of a vacuum chamber fabricated out of stainless steel, with 2 halves, torrispherical dished ends welded with flanges clamped together. The chamber has an overall dimension of 4000mm dia. X 1700mm height (i.e. from the center of bottom dish to the center of the top dish). The bottom chamber is supported on a 5-tubular support. A rotary pump, roots pump and two cryo pumps are connected to the chamber through valves to evacuate the chamber up to the level of 10-6 mbar. Pirani gauge, capacitance manometer and a penning gauge are connected to gauge manifold and their respective controllers are installed at control console. A whiffle tree structure is installed in the bottom chamber and its rotations can be adjusted manually with a maximum speed of 5 rpm. Mirror weight gets uniformly distributed among nine whiffle tree points. Load cells can also be connected to check the mirror loading. Top lid is lifted up using hydraulic pump and moving forward and reverse directions using two motors installed with it. All the system can be operated from the control consol manually.
Figure 1: Placing the 3.6m Mirror inside the coating chamber
The coating process is usually carried out under precisely controlled high vacuum conditions (10-6 mbar) using magnetron sputtering technique. Ion beam cleaning is done in rough vacuum conditions, later the sputtering process is carried out at 10^-3 mbar level. A mask is installed below the magnetron to match with the surface area of the mirror to maintain the coating uniformity. Shutters with two pneumatic doors are installed below the magnetron for protection during ion beam cleaning. Shutter will be opened only during the sputtering process. Sputtering process is done for about 100 minutes to get coating thickness around 130nm with good opacity.
Figure2: Ion beam cleaning process (left), sputtering process (right) during sample coating
10. 104cm Aluminium coating Plant @ Manora Peak
The mirror aluminising facility was installed at Manora Peak, Nainital in 1972. It is used to coat mirror sizes up to 104cm diameter. The chamber unit is made of mild steel having lesser porosity, lesser corrosiveness and low level of absorbed gases. The final surface finish of O-ring groves, base plate etc. and fabrication of few butterfly valves, glow discharge system, roughing and backing lines etc. were done within the institute. The vacuum chamber design is such that, it allows the coating of 104cm mirror in a vertical position. Rotary and diffusion pumps are used to achieve a vacuum of 2×10-5 Torr. Vacuum in chamber is measured by using Edwards Penning and Pirani gauges attached to the base plate and in the roughing and backing lines of the unit. Coating is done using thermal evaporation of aluminium. For this several tungsten helices carrying aluminium pieces are mounted on concentric copper rings. All the filaments in each set can be evaporated simultaneously. Power to the assembly is provided by a low tension transformer (12 Volts, 1000 Amps) with a variac in its primary. The variac is also used for high and slow rate of evaporation for better reflectivity and better adhesiveness respectively.
Figure1: Aluminium coating facility at Manora Peak, Nainital