The telescope: OSIRIS
a customized open truss full carbon optical tube with a fused Boro 33 sandwiched glass plates.
Aperture: 750mm with a focal ratio of 2.8
Being a "fast" astrograph remotely operated led us to choose the prime focus design.
The advantages are multiple:
-no secondary mirror is needed.
-reduced obstruction: much lower than competing CDK, iDK, or Newtonian designs, our plan is to maintain obstruction below 25% including light baffle, filter wheel,off-axis guider, rotator, and focuser ( versus 55% to 60% of competing designs), just the extra light gathering gain is equivalent to at least an additional 60mm in the aperture for the same signal. In this case the equivalent of an 800 mm CDK optical tube ( look into "Interesting links" English or French concerning optical quality and obstruction).
-easier collimation procedure both at the primary mirror and at focus.
-Reduced overall weight with about 80% of parts made of 100% carbon fiber.
The camera will need water cooling as air cooling would produce excessive turbulences just at the front end of the optical tube and likely resonance vibrations due to fan rotation interfering with the tracking accuracy of the mount: taking this optical design into consideration, we made the choice to use the new CMOS QHY 600 BSI mono pro version with water cooling and excellent performances with a pixel size of 3.76 microns. Sampling will give a nice 0.37 arc second/pixel which matches the average quality of our observing site ( approx 1arc second at night).
-concept is optimized for the larger generation of new sensors, 70 mm of the corrected field will cover easily the full-frame sensor of the backlit sensor of the QHY 600 Back Lit CMOS with <4microns /pixel across. ED glass will be part of the imaging train of this corrector to minimize spot size and internal reflection.
160 mm front lens will be chosen to minimize vignetting: as technology and price will evolve we will, at a later stage, purchase a larger sensor.
Primary mirror: 750mm F2.8 is made of fused Borosilicate 33 glass, with an expansion coefficient slightly higher than quartz but this issue will largely be compensated by careful design of the primary mirror cell and the reflecting side of the mirror will only be 20mm thick at the center.
The sandwich design and beveled back support were chosen amongst other designs choices mainly for the cooling time concern we had of this large primary mirror: full cooling will reach the desired outside temperature within approximately 20 minutes with properly located fans around blowing through the mirror thickness itself which should help to maintain a laminar flow above the reflective surface of the primary mirror and maintain the full seeing potential of the instrument.
We chose light carbon and composite materials, aluminum for smaller parts to maintain weight below 120 kgs due to our 10 microns GM 4000 photographic weight restrictions.