Jura Observatory
Swiss quality

The Bernard Comte Telescope "TBC61"
The Bernard Comte telescope is an imposing instrument with a 61cm - 24inches - primary mirror !
General description
The TBC61 is a Cassergrain/Nasmith telescope which can be permuted to a Newton mode only by exchanging the secondary mirror.
The primary mirror has a diameter of 610mm (24 inches) and weights about 150kg with its cylinder. The total weight of the instrument is near 1.2 tons.
The equatorial mount is fixed inside the dome on a 15 tons concrete slab. The Serrurier truss tube of the TBC61 is 2.5m long and 1.3m wide, mount included.

The telescope is European made and was constructed by the Valmeca society in Puimichel, Haute-Provence, France. The optical part was designed and constructed by three different collaborators : The optic laboratory of the College de France (Grasse, France), Astroptic (Mandelieu, France) and Zen (Venice, Italy).

The name of the Bernard Comte telescope comes from one of the most passionate and generous builders of the observatory. The construction of our observatory would never have been possible without his good temperament, knowledge and skills. Bernard, you'll always stay in our memories.
Instrument specifications
Primary mirror diameter / outer width 610 mm / 50,8 mm
Primary mirror weight 28 kg
Aperture in Newton mode / Focal length 3.4 / 2067 mm
Aperture in Nasmith mode / Focal length 15.3 / 9210 mm
Tangent wheel diameter / number of teeth 500 mm / 480 teeth
Screw thread of the worm / pitch Module 1 / 3.142 mm
Wheel / worm material Stell-bronze / Stainless steel
Tube flexion for delta=0 and an hour angle of 3h max 20 sec
Time for 1 turn of the worm, sidereal speed 3 min
Time for 1 turn of the wheel, sidereal speed 24 h
Command electronics MCMT II
Motor / type Sanyo Denki / Pas-à-pas
Motor reference 103H7823-0740
Number of steps / Angle by step 200 step / 1.8°
Number of micro-steps by engine step 128
Total number of micro-steps by worm turn / wheel turn 25'600 / 12'288'000
Theoretical precision of 1 micro-step 0.105 sec
Measured precision (without PEC) +- 5 arcsec
Televue Ethos 6mm (100°)
Televue Ethos 13mm (100°)
Televue Ethos 21mm (100°)

The primary mirror and its cylinder (120kg)

The secondary flat mirror is used for the Newton mode (left picture).
The secondary hyperbolic mirror is used for the Nasmith mode (middle picture).
The ternary flat mirror on its mount, also used for the Nasmith mode (right picture).

The bottom cylinder of the TBC61 is hold by three fix screws and 9 astatic levers disposed in such a way to avoid the primary mirror deformation due to its own weight.

A good mount is mandatory for a good tracking. Precision first depends on the tangent wheels. The TBC61 possess a 50 cm diameter wheel upon each axe.
We can see on the picture below that it is composed of two different materials. The central part is made of steel to insure a better deformation resistance. As for the outer part, it was decided to make it with bronze because of its good usury and friction resistance.


The heart of the telescope command is a MCMTII which controls two steps motors of 3 A/phase with a torque of 2.1 Nm (Sanyo Denki 103H7823-0740). Its power come from a 48VDC with 300W supply (Vicor FlatPAC VI-MU4-ES).

A USB extender 4-ports is a module Gefen Extend-it USB400 (USB1.1) used for the CCD and the webcam to the PC. The 4-ports converter RS232 is an ExSys EX-1334-IS-HM used for the focuser. The CCD and its accessories are supplied with a module EAD100US12 from XPpower (12VDC/100W).

The whole is protected by a battery backup power of 500VA type APC CS500.

Step motor 103H7823-0740Document PDF [PDF, 568kB]

Short picture history (July 2007) of the making of the telescope…
…and of its installation in the cupola (August 1998).
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Last update: 2017-03-24 - Webmaster - Mentions lgales - Infos techniques