Optical Tube Assembly

THE OPTICAL SYSTEM (OTA)

Our goal is to produce a state-of-the-art 21st century telescope with the latest technological advancements. In this regard Elektra Observatories has contracted Dream Telescopes & Acc., Inc. to design and manufacture the OTA. Every aspect of the OTA has been designed to deliver the highest Delivered Image Quality (DIQ). Great attention has been given to optimizing the local seeing so that the OTA operates at peak performance and yielding consistently sharp, highly focused images with the FWHM approaching 1 arc-second.

ACTIVE OPTICS

The OMI will use active optics on the secondary platform with a fixed primary. This will permit lateral motion, rotate, readjust tip/tilt and focus to 1u. Allowing automatic and precise collimation as the telescope moves around the sky.

The OMI Optical Tube Assembly (OTA) (3D CAD).

3D CAD of the OMI Prime Focus Assembly.

The OMI is a prime focus imaging instrument operating at a focal ratio of 2.5 with a linear field of 2.22º.

The key OTA features of the OMI:

1. 4.85deg², 2.22ºx 2.22º FOV.
2. Active optics for lateral motion, readjust tip/tilt, rotation for all sky collimation, enhanced pointing, reduced FWHM.
3. Fully corrected f/2.5 focal plane with triplet to 135 mm image circle with 4/6 micron spot sizes (80%).
4. Carbon Fiber Composite Core Serrurier truss for low mass, high stiffness and low temperature coefficient of expansion.
5. Borofloat Cellular ribbed open back primary mirror for ultra-low mass and superb thermal tracking.
6. 350-1000 nm useful spectral range
7. Enhanced aluminum coatings, 95% 450-650 nm, 75% 300-1200 nm with SiO₂/TaO₂ over-coatings.

The 1-metre primary is made of Borofloat and has a cellular rib structure with an open back. This gives us two key properties. A low mass of only 68 kg and access to the underside of the optical surface. This allows venting of the optical surface to ambient within a few minutes and thereby minimizing surface thermal issues. In addition the low weight reduces dimensional stresses on the mirror, telescope tube and the mount. The Serrurier truss OTA is built with a carbon fibre sandwhich core (CFSC). This material is extremely light and has a coefficient of expansion very similar to that of the Borofloat mirror. The end result is an OTA with low mass, high dimensional stability, high rigidity and a mirror that has excellent thermal stability. This will yield an optical train that is highly stable over temperature and produce sharp images over temperature without refocusing. The primary mirror will be coated with enhanced aluminum yielding 96% reflectivity from 450-650 nm and 75% from 300-1200 nm. It will be overcoated with silicon dioxide/tantalum dioxide SiO₂/TaO₂. The optical surface will be 0.25" in thickness and requiring low pressure with polishing. This will give it a unusually smooth surface.

The OTA is expected to weight in at 225 kg and thereby permit the use of a smaller, lower mass mount. The low mass and high stiffness of the OTA will greatly reduce all movements normally associated with a telescope structure.

FIELD FLATTENER

The f/2.5 prime focus will employ a 3 lens coma corrector/ field flattener with a spot (80%) radii if 4 micron at centre and 6 microns on the edge of the 3.1 degree field (diagonal) . The corrector will have good performance from 350-1000 nm focus planes and corrected for a 135 mm circle.

GUIDING, SHUTTER AND FILTER WHEEL

Guiding will be done using an off axis 35 mm, 16 Mpixel camera. Thus guaranteeing a bright guide star no matter where in the sky the telescope is pointed. A Bonn shutter will be used up next to the camera. These shutters can do exposures as short as 300us and millions of cycles without failure. The filter wheel will contain the u',g',r',i',z' filters.

OTA FULL SPECIFICATIONS

• 1.016 metre clear aperture prime focus imaging.
• Focal ratio = f/2.5
• Mirror thickness = 146 mm
• A= 0.79 m²
• 4.85 deg², 2.22ºx 2.22º Prime focus.
• AΩ = 6.0m²deg² étendue.
• Filtres: u', g', r', i', z', y', Hα
• Active Optics. Prime focus controlled with hexapod for lateral motion, readjust tip/tilt and focus to 1u.
• 96% reflectivity enhanced aluminum with SiO₂/TaO₂ coatings, >95% 450-650nm, 75% 300-1200nm.
• Fully corrected 135 mm image circle with 4/6.8u center/edge spot radius with triplet lens at f/2.5 focal plane.
• Limiting magnitude ~23 in 35s (Zθ =0º, s/n = 3, FWHM = 1.25", visual (r')).
• 1σ = 0.006 mag. in 1000s with mag 20 star (r'). 1σ = 0.003 mag. in 1000s with mag. 19 star (r').
• 5000 sq. deg. per night, 8hrs, mag=22.0 (v).
• Ultra light vented 68 kg primary Borofloat mirror, open back cellular rib structure with superb thermal tracking.
• Ultra light 225 kg total OTA mass.
• Serrurier truss carbon fiber composite sandwich ultra low TCE and very high stiffness.
• Active optics. Focus and collimation actively controlled throughout the entire sky. Enhancing pointing and focus.
• Interior prime focus camera with off-axis guider/filter wheel/focuser, single lens/mirror.
• Extensive use of baffles throughout the OTA to minimize stray light.
• Shutter doors on primary mirror.
• OTA length 2.756 m ( back of the OTA to top of spider hub)
• OTA width 1.270 m
• OTA front ID 1.206 m

The OTA shown on the one arm Equatorial mount.

Next: Optics