The precautions needed to be taken in the nights of stargazing are notoriously two:
- The shield of any stray light that could be inserted in the optical observation path,
- The isolation of the inside of the telescope than the outside air currents that would promote the formation of dew condensation on the primary and secondary mirrors.
There is also a third trick, but it is to be taken into account only when you install the telescope on the grass of a meadow, or snow, where the soil moisture bearer of condensate, It could be traced back from behind the primary inside the tube of a telescope not closed on the back.
And in this case the solution is to draw on the ground a good camping carpet. Not so expensive, far-reaching, easy to clean and comfortable.
Both the first and the second device are satisfied largely, but not in total, from the presence of the telescope tube, or vice versa, in truss type telescopes, with a fabric shield that mimics the telescope tube.
But given the small distance which typically runs between the apex of the tube and the "secondary mirror group + focuser ", It remains to complete the work of shielding from light and moisture on the secondary, installing over the opening of the telescope, a further protection, that works best if it is tubular and not partial.
THE REALIZATION
It lends itself to be easily adaptable to many types of Dobson telescopes.
Such a hood tube foldable fabric is not difficult thing to achieve, and it makes use of the following material:
- Two or more bars (obviously depending on the diameter to be realized) of rod Ø 3mm carbon fiber or glass fiber (or metallic rod threaded M3). The first two can be purchased from sports stores like "Decathlon" and used for kites; the threaded rods one meter long, in hardware.
- They need then a number (with a minimum of four) of tees with inner light (hole) 3mm (brand "Claber" or similar plastics little tubes connectors from garden irrigaton, see photo gallery) for drip irrigation systems, sold in blisters in DIY stores. (It should revise the 3mm inner hole with 3mm drill, to adapt it well to the free passage of the rod 3mm).
- A black cloth should be stitched to the right pipe diameter , whose two extreme circles have both a sewing cuff for two traits, that once made the cloth tube, It will be two circumference facing Traits. The seams are so long a little less than 180 degrees of circumference. This does not complete seam will leave at four points, of space for the release of the four tees, mutually facing, to allow the extension or the bending of the object (see sketch in the gallery).
From the same openings left by the seam will be possible to insert the rods to achieve the two bearing rings of the structure, closed by one of T.
The assembly and disassembly with flattening of the lens hood, occur only respectively for insertion or extraction of the two spacer rods that join the two circumferences straining the fabric.
Very short movie on the fast assembly and disassembly of the hood.
Some pictures that explain better than a thousand words:
