|
635 mm (25 inch) f/5 Dobsonian telescope
Below some pictures and descriptions of my 635 mm (25") f/5 telescope, a rather classical Dobsonian telescope.
The scope really is a joy to use. Movements are very smooth, even newbies that have never seen a Dobsonian can use the scope at 400X power.
- The assembled scope, ready for use:
 - The telescope weighs around 100 kg (220 lbs), but you never have to carry anything apart from the truss tubes and the ladder.
When not in use the scope sits on 4 castors and can be rolled for transport into a van. The ladder, the truss tubes (240 cm - 8 ft long)
and the ramp are put on top of the van.
The telescope is transported like this:
 - The upper cage fits inside the mirror box. It's all very robust, while not in use the telescope serves as a work bench in my garage.
  - The transition from wheels to ground: you do not have to lift anything!
   - The truss tubes, light shield and the upper cage are mounted. The whole assembly is done without tools and without loose small parts,
using large knobs.
  - To mount the upper cage, the telescope is pointed towards the horizon:
    - The finder and eyepiece light shield are added:
 - A view from the top:
 - Now it's time to remove the mirror cover.
The mirror sits on a 27 floating point cell, and is supported laterally by a steel cable, attached to the upper side of the mirror box,
exactly like the 406 mm telescope.
The cable ensures that the the mirror is supported at its exact center of gravity.
  - Collimation is a breeze with the large collimation knobs and usually takes about 1 or 2 minutes (collimation stays very close between observing
sessions).
The 25" primary mirror is from Torus (now OMI), 2002. Reported interferometric quality is: PTV 1/6 wave and RMS 1/33 wave on the wave front. This is
a good mirror that can be used at high magnifications; on many objects 450X power is used, with occasional use of up to 1200X on bright planetary nebulae.
 - The eyepiece height is about 305 cm (10 ft) in the zenith. To feel at ease at this height I use a very solid 6 step ladder with steps 25 cm (10") apart.
For maximum comfort an intermediate wooden step can be put on any of the aluminum steps of the ladder. This gives an effective step distance of 12.5 cm (5"),
perfect for observing at any altitude. Over-all I'm more comfortable using the 635 mm with the ladder than using the 406 mm from the ground
(where you have to bend down to reach the eyepiece).
 - A semi-circular eyepiece light shield can be put around the focuser for maximum contrast of the views.
  - Let the fun begin!
 - Waiting for magn 7 skies in the South of France.
The truss tubes and the interior of the mirror box have been blackened for increased contrast.
| |
