EASY CONSTRUCTION of a TELESCOPE NEWTON MIRROR 200-F6
If you are a fan of DIY, investing 116 minutes of your time to see the movie that follows this presentation, You'll be able to figure out if it can be of your interest and within your reach make a newtonian telescope mirror, with a diameter of 200mm and focal F6 = 1200mm.
Building then the telescope Dobson around a mirror thus made, It is much easier than making the mirror itself, even if the realization of a mirror 200F6, even if in its normal thickness glass 15 o 20mm, remains one of the most formative easy stage for the enthusiast “GRATTAVETRO”.
A telescope with a lens diameter 200F6 has the following features:
- According to the book “The dobsonian telescope” di D.Kriege & R. Berry, page. 80, it is able to make visible over a thousand galaxies, possessing a resolving power theorist 0,68 arcseconds (Criterio di Lord Rayleigh).
- It requires a secondary mirror of about 25mm in diameter.
- It has a minimum magnification of 20x (Ø7mm with pupil),
- It has an area of net absorber 294 cm square, that is greater than the 66% by comparison with that of a much more expensive refractor with objective diameter 150mm.
The following movie , in addition to being a great practical help to the parties concerned, also makes honour to popularize merits and technical authoritativeness:
- Jean Marc Becker, who he built many years ago (movie to which, to be made available to potential future italians "grattavetro", Added translation for "narrator" Italian), and at
- Jean Texereau for his text absolutely fundamental “La construction du telescope d'amateur“, (now downloadable for free online), and full of advices.
Is’ in fact is from these two authoritative works “ancient” but always current and fundamental for me, thet I have to thank for the original technical preparation, that no additional aid or Web Forum (in those days not so common), He introduced me and brought me to the realization of my mirrors.
In the movie, on the grounds that it is a focal easily achieved, the technique is explained citing few essential details you need to understand and implement in order to successfully conclude the work.
For example, use “ancient” in manual calculations for evaluations of Foucault tests (whose technique is very well explained in the aforementioned book of Texereau), It is a special non-negligible that, as indeed it happens normally, It requires a little effort but gives security to those who have understood their sense.
In fact it allows all'astrofilo of “know”… for not to accept passively what today many computer programs are running, without troubling the neurons by those to be satisfied to take the risks that this practical approach brings with it.
At the time of the making of the movie, manual calculation is very likely that it was the only way, because the softwares analysis still belonged to the future of Foucault test.
However, because today we live exactly in that future, by those now distant days, to encourage those interested to a useful teaching meditation , I put here below, at end of article, the pdf prints of No. 6 spreadsheets (Test1; Test2; Test3; test4; Test5; test6), made to Excel, in which, for my study, I transferred the data of whole n° 6 Foucault test performed in the movie.
Is’ is surely to help knowing that in spreadsheets, the input data are entered separately only in the boxes on the gray background color, Which:
Øspecchio (box D4); Focal Length Mirror (box D5); Advance Sense of the Foucault tester (box D6); Limiti zone hx (E7-F7-G7-H7 boxes); And in the end the measures from time to time identified with Foucault tester (E12-F12-G12-H12 boxes).
Every other box is calculated, and each chart has a background differently colored, and collects data from the same background color boxes.
Of course anyone accustomed to using any other of the Foucault test evaluation program, You can enter this data into it to see its graphics.
On these n ° 6 spreadsheets is very convenient to see the mirror qualitative progression, and quantitative errors and corrections made to the 4 zones.
It is not only to revise the trend graphs wave reflected from the individual zones (see green background graphic, It generates the broken line built with data in cells of same color) dictated by the value of the famous Lambda / n, and already mapped empirically pen (…Even the “rapidograf” It is a vintage pen) from the author in the movie, but also to observe the graphics not being handled by the film, but relate to other quality criteria which absolutely must respond in a mirror making, that maybe it is good to remember with the following….
SUMMARY OF QUALITY CRITERIA TO BE RESPECTED FOR LENS REFLECTOR
1 – The best known parameter is the famous l / n. Where is the Greek letter Lambda symbol of a wavelength.
This policy is also known as Lord Rayleigh criterion, and it establishes a’light wave It reflected should not be damaged by glass surface errors greater than one eighth of the wavelength of yellow-green light (…that Lambda = 550 nanometers, or 0,55 microns) which the human eye is more sensitive, considering the fact that errors of that magnitude would already be observed with the telescope observing.
The mirror reflection fact, light wave inflicts a double error corruption present on the glass, as during the reflection the wave becomes first damaged during the incidence phase, and a second time emerging from the reflection.
And that's a flaw on the glass of the size 1/8 Lambda (also said Lambda / 8) , transmits twice to the wave that value, and the damage becomes (1/8 + 1/8)= 1/4 Lambda, expressed Lambda/4 , which it is the minimum acceptable quality…Ma…
But the only lambda / n greater than 4 not enough to ensure the quality of a mirror, if it not also satisfied the next criterion.
2 – This “second” quality criterion, It's not really second, but complementary to the first well.
This is the so-called Criteria of Couder, which states that all the rays reflected by the entire surface of the mirror must converge to the focal distance, within the characteristic diffraction diameter notch of that mirror, and determined by its focal ratio, according to the known formula:
ØAiry= 2,44*Lambda * (F/ D)
That, with the values expressed in the bracketed units:
ØAiry[microns]= 2,44*Lambda[microns] * (Length. focus [mm] / mirror diameter [mm])
Please remember that a highly enlarged bright star spot is perceived as such only until it reaches a limit beyond which appears his fitness that is determined by the diffraction, and which presents itself as a bright central diskette (which it is the Airy disk ) surrounded by alternating light and dark rings always less bright.
Policy that on the practical side, in the case of the mirror in this movie, It establishes that all the rays reflected from it, They must bring their light within that central diskette, that for the mirror with a focal ratio F6 has a diameter of 8,05 microns, and It is located at a (focal) distance of 1200 mm from the Center mirror.
If the beams reflected from one or more zones do not converge within the central disk, It happens that their light is scattered out of it, going to increase the brightness of the rings surrounding the central spot, thus enlarging the image point in a great spot, ie less “point” and with Dim light because most widespread, and with this causing a loss of contrast in the telescope.
The SPREADSHEETS
In the spreadsheet, the graph that expresses the Couder criterion It is in orange background (built with data in cells with the same color), where the reflections of 4 zones connected by a broken line Green, must be within the 100% its area, ie without reaching neither exceed the upper and lower edges of the graph.
Framed the fundamental criteria, on spreadsheets other interesting graphics are visible which express the degree of quality achieved.
For example there is a graph which expresses the position of zones in relation to the "trumpet" shape of the machining tolerance limits of a parabole (chart yellow background, built with data in cells with the same color). In which the absolute utopian perfection would be represented by the sequence of broken # 4 representing the 4 zones, aligned to form a straight line exactly on the central axis of the “"trumpet bell"”.
And finally with the chart in relation to the thickness in nanometer of glass still be deleted to equal the perfection of the shape of the theoretical ideal parable, taken as a constructive reference (graph on light blue background, built with data in cells with the same color).
These Spartans spreadsheets that I found useful to me (…because I let you work in progress to enter my estimates datas “previsions” to simulate if what I imagine would happen is positive or negative for the work), though equivalent to other programs for the analysis of the test data Foucault, are just my translation in Italian and sometimes widening to a different number of zone, of a work by Pierre Strock, that in the original language You reach nice and clear instructions, downloadable who in formato Star Office; oppure Open Office; oppure Windows Excel.
A realization of a telescope mirror today no longer has huge economic benefits, also in normal using two glass discs, that for this mirror would be valuable in thickness of 15 or vice versa 20 mm (perfectly possible embodiment because the excavation of “arrow” It would stop at 2,5 mm).
With DIY is predictable in fact a saving perhaps only until 30% on how to buy a Chinese-made commercial telescope, in the face of a little hard work but source of enrichment and satisfaction, as well as being in part a substitute for gym for biceps, for the times of year when outdoor sport is anything but pleasant.
One who, Once well understand the job , He decided to try, can also count on our help in our Forum , formed not by specialists, but surely by passionate precursors.
| Test1 | Test2 | Test3 | test4 | Test5 | test6 |