Self-construction of mirror and telescope Ø400mm F6 with Tutorial videos by John Dobson (spoken in Italian)

John Dobson (1915-2014), is Astronomical known worldwide for his work “missionary” towards the popularization of astronomical observation of the sky, taken as an irresistible urge to spread (in "working classes") simplest existing telescope construction, all wood and cardboard, that being one of his magnificent project, that bears the name of DOBSONIAN TELESCOPE

The tutorial below is footage of 1992 lasting 84 minutes, succeeded an older VHS, to which   I took the liberty of adding an audio track with the narration of a stewardess “in speech synthesis ", that translates, and in some cases sums up the sense of the explanations “off the cuff” by John.

The purpose, for a document of similar importance, is to make useful insight from interested Italians, that are excluded from important knowledge because of our language “niche”, Since the end of the Roman Empire is spoken only here  “4 cats”, belonging to the trifle of a 140th in world population.

John was very familiar with optics and its rigorous scientific rules, and even a few nanometer tolerances necessary for the realization of a goal to mirror Newton “perfect mirror”.

But his brilliance and success lie in having studied and introduced smart and simple construction techniques reduced to bone, in order to achieve that goal and diffusion of scientific construction of a telescope, not perfect but   good at least for Visual observation, without having to write a single formula, and still talking about easily, and I would say "agricultural"(*), with solutions that do sometimes make "a leap in the Chair" to those accustomed to other technologies, or optical test, or precautionary fears.

It's simple but incisive for example:

  • When (makes you do a jump on   chair because) it drive two nails on the bench using the crude primary mirror as a hammer.
  • When he describes the magnitude of optical quality tolerances without citing microns or nanometers, but simply saying that they are expressed by a number of   "about one thousandth (of thickness) of “Saran Wrap "… (…that are out of my mind to understand what could be the Saran Wrap, to find out later that it is American polyethylene film, equivalent to the Italian "Domopac" ).
  • When indicates a quantity of abrasive for use in carborundum grit 60 in each rough grinding wet, approximately equal to "4 times the salt for the scrambled eggs"
  • When the indicates the arrow depth of excavation on the concave side of the mirror how about equal to the thickness of a "nickel ", that is, a one cent coin.
  • When using hot water to spread the polishing abrasive …( and this is a show of ingenuity and intelligence that's also interesting the most technological Grattavetro (alias glass scratchers))… used in the correction phase of the mirror to "raise" the glass for local heat expansion, reducing the risk of incurring the flaw # 1 fearsome and injurious "Turned Down Edge" on the mirror, for the considerable loss that it generates in the contrast of the image by dispersing some of the light trapped.
  • When does not use nor the Ronchi test nor the Foucault, and executes the optical tests "by eye", in plain view, or by only using the star test with the "artificial Star" provided by the reflection of the Sun on a high voltage power line insulator.

Today the “evolution of the species” and simplification of portability have plagued even the dobsonian telescope-making “first manner”, that have passed since the original form to John tube, Lattice-type shape Obsession, and the latest lightweight form  .   All though with some technological solutions’ more sophisticated.

But the simplicity of the original design by John remained synonymous with “Summary”; and   the summary is “beauty” of thought.

And this beauty is found in all solutions that combine simplicity with efficiency. And therefore also in all those expressions that contain the highest meaning in a few simple strokes.

In this respect is very beautiful a chemical or physical formulas…Or a poem by Eugenio Montale. Because both simply sealed letters contain huge concepts.

And the beauty of the design thinking of John was to prove that even a seemingly difficult thing "you can do…in a simple way "; and that one thing from raw appearance actually works fine.

But stop to the prittle-prattle:  watching this tutorial brings benefit to all concerned because:

  • A Grattavetro "novice" (without neglecting the handling problems of the big telescope) can understand that it might to build as well do it alone, a big telescope diameter: 400 mm F6, scratching a glass diameter: 400 mm and 25 mm often buying the glass of a demolition of a ship porthole;
  • While a more technological Grattavetro, that have been down that road a few times, can definitely learn profitably to use warm water. But it can also realize that there is a parallel opportunity to arrive at a positive milestone for Visual use only, Although working so much less technical, without making use of demanding measurements that guide you in your work to reach a very challenging maximum, but with the simple method of John, Although the optical measurement is rudimentary, It is possible that the final quality is not discernible degradation by a naked eye beginner, While still offering stunning images maybe up to 500 magnifications. Air turbulence by permitting.

I indicate here the minutes where the tube meets the various chapters:

  • Title within the movie  —————————–Start time
  •   ROD BREAK with COARSE GRINDING – 5' 30”
  • FINE GRINDING – 19' 38”
  • CASTING OF PITCH ON THE TOOL – 22' 59”
  • POLISHING – 33' 12”
  • PURCHASE OF THE TELESCOPE TUBE AND FRAME CONSTRUCTION – 43' 44”
  • CONSTRUCTION OF THE TAIL GATE “TAIL GATE” (I.e. the MIRROR CELL) – 51' 47”
  • EYEPIECE HOLDER AND SPIDER VANE SUPPORT SECONDARY MIRROR (and also the STAR TEST for correcting optical curve) – 1h 04 ' 49”
  • FIGURING AND CURVE CORRECTION – 1h 21 ' 15”
  • METALLIZATION OF THE MIRROR – 1h 23 ' 13”
  • FINAL STAR PARTY – 1h 25 ' 51”
  • END 1:28’07”

“TELESCOPE BUILDING” by John Dobson. Length 84′ – With NARRATOR ITALIANA  

(*) I define “agricultural measurements” those such as a   farmer who became a machinist of rolling mill, I outlined the faults stamped on the laminate without need of the metric system, or as large as a "grain of wheat", and if more, "corn"; or oval bruises as big as a "WaSP" or a "Bumblebee", or a roller mill (because bearing break) "ran like a potato."  All universal and everyone's intuitive…Almost like the metric system.

Leave a Reply