- This topic has 57 replies, 4 voices, and was last updated 7 years, 4 months ago by
.
-
Posts
-
..From the top of my ignorance of extra Newton architecture, I was wondering just what the devil of useful function could perform an elliptical secondary mirror and therefore with two fires. But I still really have a hard time understanding how those reflections work:
That is, if the sequential reflection takes place from Infinity to -> Primary -> to the 1st secOndary focus -> to the 2nd secondary focus -> to the Eyepiece, or otherwise.
@ Lelio: Curiosity, the biplane “Olando / French” 1930s with an American engine represents another passion of yours?
In theory it is quite simple, much more than in practice
I try to explain it:
in this image there is the property of the ellipse that I said before: the rays from one of the fires are reflected on the other fire, for any position of X .
in the following image this property is applied for the Gregorian telescope:
1- the rays coming from infinity are reflected by the parabolic mirror in F1, focus of the parable.
2- F1 is also the fire of the ellipse, therefore the prolongation of the rays reflected by the parabola, beyond its fire, they will hit the elliptical surface.
3- being rays coming from an ellipse focus will be reflected on the other focus F2 beyond the primary mirror. ( the shape of the ellipse is calculated mathematically as a function of the focal length of the parabola, position of the focus beyond the primary and imposing the coincidence of the first focus for a given primary secondary distance )
Wait to launch yourself on this project, I still have many doubts.
With the evolution of focal courts in recent years, coma correctors and wide field eyepieces has become difficult to make a choice.
For me, I have a technical background left in the 70s′ it is even more difficult.
Definitely the 20-inch mirror′ it will be under F4, a nice ultrashort and light dob for visual observation and wide field as far as possible obviously, I hope to reach 1 field degree I would be satisfied.
The 380 at the beginning I took it to take my hand on a still human diameter, and I would not like to make a small replica but a different instrument and as Gregorian teases me because it is abandoned and relatively simple to make, I am equally attracted to Mel Bartels' projects.
If I have to make a choice based on my experience, there is no history, newton point.
Instead, what matters most to me is the possibility of using the glass for large diameters combined with the foam glass to give the mirrors stability and non-deformability.
There are several studies and attempts both hot and cold and this is the option I want to try. Glass-foam sandwich with cold glue, then tests will validate the results or not.
@ Giulio, yes model aircraft is the other toy.Thanks Massimo. Clearer than that….The design of the ellipse and its two foci is what I was missing.
As for model aircraft, in 1960 I stopped to buy the construction drawing of a yachtsman called Scout. Drawing purchased at Aeropiccola di Torno (that maybe I still have somewhere today). Never made for the difficulties of distance from Turin and economic (I had 12 years) to purchase the necessary balsa wood. But somewhere I still have a Supertigre “diesel” from maybe 1.5cc that a neighbor gave me whose son no longer practiced modeling, seeing myself always active in building sleds, trolleys with steering bearing on broom handles and other things, on the workbench that was under the porch of the house where I lived.
Imagine Giulio…
the application of the properties of the ellipse in acoustics is also interesting: many large and important theaters ( the original Teatro Regio of your Turin was an example, before it was rebuilt after the devastating fire of 1936 ) they were designed and built just like that, placing the stage in one of the fires of the elliptical room to facilitate listening to the most distant spectators, who could clearly hear the actors as the front row viewers.sorry for the digression…
A question, Ronchi and Foucault tests for normal mirrors but for short focal lengths?
Astrotechnician recommends the Bath interferometer, on youtube I found this: https://www.youtube.com/watch?v=wINnmX15qrw
But it is enough to test a 20′ F 3,5?P.S.
I have not yet finished studying the texts that I have found here, thanks Giulio.Yours is a nice and interesting question that was asked some time ago also in the American Cloudy night Forum, where I intervened creating with only three posts 9, 13 e 35, some “gravitational wave” of disturbance, as you can read by scrolling the discussion from the following link.
http://www.cloudynights.com/topic/538048-parabolizing-and-testing-low-focal-ratio-mirrors/
However, Ronchi always says something about the entire surface of the mirror being worked on. But we know that it cannot tell you how big a certain flaw is. And this is disturbing as much as the focal length of the mirror is shorter, why corrections on parables a “bowl” very deep because of the short focal length, they are very difficult to connect for the better, if you do not have an idea of the quantity of the defect to calibrate the duration of the correction to be applied to the bench.
Another matter is that of the Foucault test which is good for bringing the processing very close to the detection of a surface shape in Lambda / 4, but then a crossroads appears:
– Being satisfied with the level of quality obtained which, perhaps, for the visual could also be satisfactory, but it can never be excelled because the Foucault test establishes that the radius of curvature of an area does not change for the whole width of a pair of windows that identify a circular crown of the mirror, while in reality the deeper the parabola the faster the variation of the shadows, so you will have to have a large number of windows in order to better evaluate your mirror, but at the same time in any of them as you go to the edge, you will no longer be sure to see the “flat grey colour” close by 360 °, but you will have the impression that you no longer understand anything because it always seems to you that the spot color can also come from right or left.– In this case, to obtain a qualitatively better optic, starting from about lambda / 4 you must continue using a test like l’Hartmann (complicated), or l’Simplified Hartmann by Mirco (that you find described in this blog); or the test of caustic, also described here, and used by Massimo for his 300mm of very short focal length, but which requires a very precise and micrometric tester trolley also on the orthogonal axis that in the Foucault tester carries the knife blade.
Personally, I had a single Foucault test experience to test on the mirror diameter 24″ (600 mm) F 3,8 of a friend who lives near my house, using a Couder mask a 8 zones, and I saw that they were too few, and that it was very difficult from a distance of 4,56 meters to understand where the center of curvature of an area is, even averaging 4 readings (two uphill center-> edge, and two downhill Bordo-> centro).
I know only about the interferometric tests in theory and I leave the floor to Massimo and Mirco, ma “a naso” I believe that like Ronchi they cannot give you the quantities of glass to be eliminated for corrections, as you would get for example from caustic.
But the need I felt for a greater number of windows of the Couder mask to evaluate a greater number of areas of the mirror, it also brings up the criterion of optimal visibility at a distance (without web-cam) of the smallest most peripheral area of the mask, indicated by Texereau in the famous 15mm a 3 meters, which therefore correspond to 5 per thousand of the radius of curvature of the mirror.
So for a mirror that has a radius 4560 mm as had the 600 F 3,8, the optimal width of the last pair of windows had to be his 5 per thousand, ie 22.8mm. which, however, would correspond to a Couder mask from 7 zones…that would have been even worse than my a 8 zones, while one 10 areas would have been better, despite having a width of the tenth zone of only 15mm for Texereau too narrow, how can you calculate it from this excel sheet of mine that I obtained from the calculation sheet of the Foucault test by Pierre Strock
https://www.grattavetro.it/wp-content/uploads/2016/11/Calcolo-maschera-di-Couder-secondo-Strock.ods
So many words to say that I myself would not know how to advise or advise Foucault (which is also recommended up to the threshold of the famous lambda / 4), that the whole world seems to use even for focal courts. But that we glass scratchers being by nature experimenters and possibilists, (and knowing what you would meet, but also knowing how to remedy it), we should really experiment in practice to see what comes out of it on a mirror that can always be corrected later.
Lelio, the problem of focal courts is quite complex if one wants to achieve high degrees of precision, both as a construction and as a measure of the parabola. We have written articles about it, in two words I can tell you that :
– The Ronchi test is to be used with a reticle not exceeding 4 linee/mm .
– the Foucault Test can be used in the version “slitless” and minimizing the source / blade distance, and I keep in mind that uniform shadows will never be seen inside the mask windows, but always a more or less accentuated gray gradient.
– Interferometric tests are difficult to read and require a setup “ultra-precise” a small misalignment of the system shows deformations on the fringes that are greater than the precision one tries to achieve.I recommend the caustic test, but this is also quite long, laborious and requires a setup made with extreme precision.
Ah, I hadn't seen Giulio's answer, exhaustive as always…
There is also to say that an F. 3,5 needs a coma corrector, the field “quasi” correct ( that is, within which the coma is tolerable and not annoying ) is less than half a degree.
Is’ it is also true that seeing will almost always limit the perception of aberrations as the image of the star will almost never be punctual but will be amplified by the turbulence in its apparent diameter well beyond 0,005 theoretical mm of the Airy disk ( for an F3.5 ).
Very comprehensive as always Massimo and Giulio.
In fact, while relying on proven and safe theories, the same is not true of the tests where the turbulence of the air also affects the few meters that divide the mirror from the tester. I also have doubts about the efficiency of the use of dedicated cameras and software because they need a practically perfect system both mechanically and processing, and the possibility of subjective interpretations is increased with short focal lengths and large diameters. It may be useful to repeat the test and look for the statistical average to get closer to reality.
So far the very little experience accumulated has supported me in dubious readings, no objective quantification to remove glass, but the hand and in the end only the errors or the successes directed the processing. You have probably always been satisfied with being under the canons 1/4 lambda to evaluate the quality and correctness of our work, some seeing that their tests were well defined and clear thought that they had reached who knows what perfection, but probably it was simply due to the use of the ronchi with copper wire in the air and a super-shiny optical surface.The windows have arrived!
https://goo.gl/photos/AZCGFTJ1LGKYGcsVAThe three rubberized bearings of the rotating table have arrived 700 mm and I modified a plumbing die for motorization a 58 revolutions per minute for traction. I am well on track with changes to the workbench, early photos.
Belli i vetri!
The start of operations is approaching!A good idea is to use an electric supply chain (Ridgid type or similar), as a rotary table motor, both for the mechanical power it provides, than for the speed already well reduced. The only drawback that these collector motors carry is the noise, which instead with induction motors is almost non-existent.
- You must be logged in to reply to this topic.