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Congratulations for the nice mechanical work of the equatorial! Robust and also beautiful to look at!
I took the liberty of downloading the pdf text of the geometry notes, which I will read carefully.
Thank you and greetings
Giulio TiberinI… curiosity remained. Somewhere, the last century, I had read that one could simply try to place an as-yet-unmetalized mirror in place in the telescope, after which the focal plane was explored with a foil, and placing a star in the center would have seen it extinguish the illuminated surface in a linear fashion (“null test”). It seems that Dobson used it like this. I would like to know if anyone has tried it, and if so, your impressions (I had tried to polish a 13cm glass. f/10 and looking into it… well it seemed to work, but once aluminized I decided that much more was needed…).
Cordial greetings.Good evening Max.
Let me give you your name, since you were one of “our” Scratchers
And. What you have described is the homologous test of Foucault's, in the variant that allows you to execute it as you described, on a source placed at infinity.
The Foucault test, on the other hand, is performed “Indoor”, illuminating the mirror with a point source located not at infinity, but at the distance of its radius of curvature, which is notoriously equal to twice the focal length.
In both cases, “cutting” the mirror reflection… If the blade is at the focal point of the source at infinity, corresponding to the center of curvature of the double focal distance of the artificial source, the complete darkening of the mirror surface is obtained, without being able to appreciate any lateral origin of the shadow.
(From this it follows that: If instead we were at a distance “intrafocale” less than the focal one, you would see the shadow of the blade come to obscure the image in accordance with the movement of the blade itself;
While if you were in position “Extrafocale” i.e. further beyond the focus (where the reflected rays cross) one would see the shadow of the blade enter to obscure the image in the opposite direction to that of introduction of the blade).John Dobson, per “light” the curvature of a mirror, frame at “star test” under high magnification a point source to see its diffraction rings. Then check by slightly pushing the eyepiece into intrafocal, that there are no rings brighter than the others in the center of the mirror; then he pulls the eyepiece slightly out of focus, and check that there are no rings that are brighter than the others towards the outer edge.
And finally he concludes that the brightest rings in the center seen in an intrafocal position diagnose a mirror with a dish that is too deep in the center; while vice versa brighter rings at the edge, seen in extra focal length, they diagnose a dish that is too flat in that area.On this very topic, see the following short excerpt from John Dobson's film
That fragment is extracted from the complete construction movie that you can find here: https://www.grattavetro.it/auto-costruzione-di-specchio-e-telescopio-o400mm-f6-con-video-tutorial-di-john-dobson-parlato-in-italiano/
obviously, in order to see the diffraction rings, the primary mirror must be acclimatised to room temperature. Otherwise
the hot mirror creates a strong turbulence of the air inside the optical path, which transforms the image of the diffraction notch into a chaotic bubbling.in conclusion, It is much better, from i pigeon method, choose the bending radius test, using a Couder mask, to “delete” slowly, area by area of the mirror determined by the circular crown on which the pairs of concentric windows of the mask open. That way you can fix it area by area, progressively bringing the mirror to perfection.
hello John.
Last Activity.
If you like you could in time and calm, If you like you could in time and calm, If you like you could in time and calm, If you like you could in time and calm, If you like you could in time and calm 5 If you like you could in time and calm.
Beautiful figure, and this already seems to me an exceptional visualization! Congratulations Massimo!
Hi Cami
Read this articleVisit this site as well:
https://stellafane.org/tm/atm/test/tester-2.html
where you can download a pdf file which is called “atm_ronchi_screen.pdf”, to be printed with a laser printer, without magnification (that is to scale 1:1), obtaining a lattice on transparent square with side 2 inch (51,8mm) composed of 100 lines per inch, which corresponds to 3,9 per millimeter.It's not the best in reticle contrast but it could be fine. Certainly it was much better than the slide da 5 and yes 10 lines to the millimeter that was in a sachet attached to the book https://www.ebay.it/itm/203445813753?chn=ps&norover=1&mkevt=1&mkrid=724-128315-5854-1&mkcid=2&itemid=203445813753&targetid=1274629810051&device=c&mktype=pla&googleloc=20527&poi=&campaignid=9556812668&mkgroupid=124191463882&rlsatarget=pla-1274629810051&abcId=1145978&merchantid=116398787&gclid=Cj0KCQjwg7KJBhDyARIsAHrAXaFx_Qu0yJnNVRE7l8rcU-UTspo4oTKX1wXJIMLLAMeqSiA04JL_BV0aAi4QEALw_wcB by Giovanni Ferioli, now only available in second-hand (obviously asking that it be complete with that slide)
Hi Alberto. Sorry for the delay, but I was on vacation for a few days.
you say:
I managed to mount protractors on my dobsonian. The supports for the hands, li ho stampati 3d, mentre i goniometri Az / Al were printed 2d on paper and pasted on cardboard. Within seconds I was able to aim for M51, glimpsing something (I'm afraid my 150mm won't allow me too much ... ) and I would say that the aiming system works with dignity, considering the time invested. To do the math on the fly, I also made an android application (an exercise more than a need) works , chosen as a reference the object currently pointed to, it tells me in real time how much to vary az and al to point to the new object.Well…here you deserve compliments. Why did you make yourself gods “digital circles” much more performing, thanks to your smart Android application which is a niche gem. Bravo! (I was left out with the programming, since that was born “to objects”)
What about the tester for the Foucault and Ronchi tests…Here too you are doing very well, because a sliding support that allows you to exchange the Fouvcault blade with the Ronchi reticle, back and forth in front of the same slit, it's a convenience, which among other things, whether your construction will have the vertical slit of the blade or lattice, allows you to test without removing the mirror from your telescope.
Just place the rubo 150F7 horizontally in front of the tester, the distance from the center of curvature of the mirror, which in your case (150F7) è a 2100mm (twice the focal length).
The slit will illuminate the mirror inside the tube for both types of tests.As for the ratings, the tester must be used with the mirror. the slit itself is a very bright vertical line even if very thin. The images of the backlighting of the mirror acquire interest. It would agree that you include them in your posts, in order to be able to evaluate them visually.
To insert them you can use that Postimages.org you have already used; Upload the image from your computer choosing not to scale it. After that, in a position below the insertion area, where you will see a list of many links applicable in different ways, choose the link “for forum preview”, to be glued into your interventions.
That type of link allows those who see a preview of your post, to enlarge it as you can see with the direct address, that is often full screen, clicking on it with the mouse.Hi Alberto
GiulioWell Alberto,
If I remember correctly the process, even here you can upload the images first uploaded to a host from images such as POSTIMAGE, which tells you to choose the image to upload that you will select on your computer, and after loading, gives you the choice between different types of links.You choose the link of “preview for forums”, click copy, and then here you will paste it where the blinking cursor is, in the window that will open by clicking on the blue square with the writing “img”.
That type of link will show the preview image, giving the reader the possibility to enlarge it with a click, and even further with a further click on a cursor by the sign +To read you again soon|
Ciao
Giulio.Hi Alberto.
Your doubts are the normal ones for all people who think about facing the making of a mirror.
In fact, each of the choices that lead to doubt, it is a prerogative dictated by the needs of one test or the other (speaking of the Ronchi and or of the Foucault), and from the resolution of the data provided.
Very low resolution in Ronchi and very high in Foucault, so it is better to start with the Ronchi but finish a mirror F5 or higher, with Foucault.Here I try to summarize the parts most targeted to your doubts, from the articles of the indicated links.
To highlight the fundamentals of the two tests Ronchi and Foucault:The objective of a Newtonian reflecting telescope must be a parabolic mirror, to bring the light rays that will make up the image of a stellar object placed at infinity into a single focal point.
It is a question of making a “revolution” parabola which is obtained by making the curvature on the mirror, walking around him around a table.At first, establishing the desired focal length (which is equal to twice the radius of curvature that the initial sphere must have) is obtained (always aided by the revolution of walking around while working) a spherical cap having "depth" (said arrow) generally a few millimeters, carved into the center of the glass, which after refining and will then be "flared" in polishing, (generally a few microns) however having parabolic progression (like the theoretical parable taken as a constructive reference), starting from the center and up to the edge.
Let's say immediately that the Ronchi and Foucault tests are done by positioning the lattice of one, or the blade from the other's knife, to the distance of the radius of curvature of the initial sphere.
And we also say that the Ronchi lattice places the center of the radius of curvature of the initial sphere…if the sphere is a good sphere…it will show the entire surface with many lines (if you are away from the fire) or a few lines (if you are close to the fire), But the lattice lines will not appear deformed, then straight,. Or vice versa curves in the presence of a "hump" or curves in the opposite direction in the presence of a hole, reversing the position from intra to extra focus and vice versa. And therefore it is understood the correction to be implemented, but it is not known how much the defect is to regulate the duration of the correction. Also in the test you can see the lines of the lattice that are distant from each other, if it fits only 0.1mm.
The Foucault test could also be carried out at the same point as the Ronchi, on some pairs of windows open on different circular crowns. (The measurement made on the diameter of a surface made by "revolution" is by definition considered equal to any other diameter of the same surface).
The only difference is that the Foucault test provides graphs with the slope of the workable areas, and the amount in nanometers of the error, which facilitate the estimation of the severity of the correction to be applied, with a resolution of approx 600 a thousand times more sensitive than the Ronchi.
But the threshold of the minimum quality of a mirror because it can be considered limited only by the diffraction of light, and of 68,75 millionths of a millimeter between peak and valley of the maximum residual roughness.
Here is therefore a non-exhaustive reason, but consistent with possibly carrying out the tests in conditions just outside the diffraction limit, using as a light source NOT a naked Led (source too large and roughly distant from the diffraction) but not even a pinhole (very difficult to make with a diameter of 2 or 5 hundredths of a millimeter), that also so small would reduce the brightness of the LED too much, But making an easy crack, which has the same physical behavior as the pinhole because it is wide 2 cents, but it is much brighter because it is "high" 4mm in front of the LED.
And to make a wide slit 2 or 5 hundredths of a millimeter, just fix with double-sided tape, two razor blades with the cutting edges facing each other and separated by a spacer consisting of a small piece of magnetic tape from a videotape, which has precisely that thickness.
And the plate is then easy to put on and remove from in front of the led, to trace its luminous dot reflected from the mirror, and aligning the tester, bring it to the Ronchi lattice, or on Foucault's knife.The width of the slit is not mandatory, as well as the low sensitivity of the Ronchi, because a certain bad behavior can also be sensed with the Ronchi test, showing errors across the entire surface, even if sometimes imperceptible due to the low resolution compared to Foucault.
The ideal is to use both of them but finish the mirror with Foucault, (Up to focal ratio F5.
Hi Cami
To bring a 200F6 to an F5 focal ratio you would have to redo the entire mirror figure ... which is a great job that frankly you have to judge if it is convenient to do. (For example, I have never refigured my dobson 130F7 from an invicta backpack weighing 3kg, to bring it to a more easily balanced F5)And to reshape the mirror you would unfortunately have to redo the tool to deepen your current parable of 42 cents mm, taking it from your current 2.08mm F6 arrow to a new depth of an F5 which is 2.5mm.
As a tool you could make a hard plaster "for molds" like a thick dentist 3 or 4 cm (purchasable in supply stores for dental technicians) casting chalk on your current mirror (after you have protected the surface with a polyethylene sheet or parchment paper, and surrounded its circumference with a cardboard tape to form a sufficiently high gypsum containment dam).
Then on the curved and dry surface of the new plaster utensil you should glue with epoxy (which in the meantime waterproofs it) about 15x15mm stoneware tiles; spaced asymmetrically but never in contact; or glue irregular pieces of glass of similar size in the same way, or even drowning hexagonal nuts in the plaster cast (with the hole facing the mirror) for M8 or M10 steel bolts (for key from 13 o da 17mm) arranging them spaced on the baking paper, and then pouring the chalk on it, making sure that the holes in the nuts are filled, and not to move. But to avoid unwanted movement it is best to secure them in place to the parchment paper with a small drop of hot glue, just so they can't pile up in the plaster.This is the only way because I don't think there is a focal reducer to put on the eyepiece that reduces it by such a small ratio = 0,83 how is the relationship 1000/1200 (where 1000mm is the focal length of the F5 and 1200 that of the F6.
Focal reducers are positive doublets that work in reverse of barlows, which are negative doublets that increase the focal length.
There are therefore reducers for larger jumps, and at affordable prices only starting with a halving of the focal length (i.e. 0.5x)My question is ... why would you want to change the focal length of your 200F6 ?
Ah…On cutting glass I forgot to mention that it is better to work while wearing glasses, and for cutting discs (by those who do not do it for a living) it is best to do this approaching for easier straight cuts; First by cutting a square with a side equal to the desired diameter, and then it is possible to continue in two ways; to remove the edges according to straight cuts, or vice versa (but I see it more difficult) groped to do it by engraving the curved sectors with a compass.
The important thing is to wet the tip of the glass cutter with petroleum or svitol or similar, which would favor the cut by minimizing the size of the micro splinters, and maintain a continuous cutting pressure and speed which is neither small nor high, but such as to make you feel, with just one step, and by ear the noise “zzzzzzzz” I keep making the tip of the glass cutter when it cuts well.
If it doesn't make that noise, or if you go over some sector, it is easy for the cut to be jagged, or don't come at all.
Any residual glass imperfection exceeding the disc is only aesthetically ugly, but it can be filed with a whetstone (wet so as not to breathe dust) maneuvered in plane with respect to the plane of the edge and along it.
There is also something to be said about this grinding, that in order not to give rise to chips that affect the reflecting surface, you get a good help using a Dremel type diamond grinder with passes without stopping to overheat, to realize a safety groove along the two edges of the disc.
This is because making that groove by maneuvering a whetstone along the thickness, that is, orthogonal to the cut, it's easy “to lift” chips that, however, cannot reach the reflective surface because they are interrupted by the groove that circumscribes them within the edge.
That caulking is also essential that it remains present throughout the processing of the mirror.All this chatter of mine to say that it is better to do some cutting tests to gain practical awareness of what glass is “a misunderstood easy traitor”…and makes unexpected jokes!!
Ciao Skylab
I agree with the indications that Massimo gave you.
In fact, the multilayer blanks of glass glued with plastic glues have never yet been explored for optical use, due to fears of sealing over time and deformation induced on the glass by the shrinkage of the glue during drying, given the very narrow tolerance of 68 millionths of a millimeter allowed between peak and valley of the maximum defect of the final parabolic curvature, to achieve the famous lambda / 4 quality.It would only be a matter of courageous experimentation, because there is a non-calculable risk of working unnecessarily.
The tube of liquid silicone RS glue you mention in your post, it is a rarity because it is liquid (and therefore it flows positively while keeping the thickness to a minimum); although (as we read on its documents on the same site RS) an acetic silicone, i.e. from the family of common syringe sealants, which, like this, hardens slowly without adding catalyst, absorbing environmental humidity, like that Saratoga you put in the picture.
As for the glass: However, I believe that in the great difficulty of finding a 25mm thick blank at a non-jewelry price, it would be better to orient yourself to the easiest to find 19 mm, rather than paste 4 dishes from 6 mm, for the reason that the more glue there is between the glass, the greater the uncertainty of stability over time, which is precisely the risk already indicated.
As for the calculation of the arrow, you can do it knowing that a mirror diameter 250mm is easy to make if it has a high focal ratio, for example F7, or the latter would have an LF focal length of (250*7) = 1750mm and an RC Curvature Radius of the initial sphere of (2*LF)= 3500mm which would provide a height of the eyepiece from the ground of approximately 1550 mm, and a glass digging arrow of
Arrow = (Mirror radius squared / 2*RC) What for a 250 F7 is 2.2mm
a bit’ the 2.6mm arrow with the eyepiece height from the ground of about 1350mm is less easy for an F6)
And the maximum difficulty for a beginner is an F5 which would have the arrow of 3.12….The processing must not scare, or immediately stimulate to invent new ways , so as not to immediately transform it into an experimentation of a road, complicating the already not simple old one
I say this because for example the processing time of my 250F5 (whose diary is described here in Grattavetro in a series of 4 articles) which excluded the roughing made to me by machine by Reginato, was of 77000 He ran forward back (that last 1 second), then 21 hours of scratching the glass reaching approx 60 total hours of tests included, which could have risen to about 150 if I had to make the initial sphere by hand.I noticed that Waite created grooves in the tool in the plaster between the nuts. Are needed?
Hello Franco
I can't tell you, MA dubiterei… that the machine can be of some help.
I certainly could be wrong, but honestly I would think not, both in hand and machine work.The reason it seems to me is that you could use any object in blank, good if flat and in diameter like, or near that of the mirror, which is the only requirement for reasons of uniformity of the machined surface.
While the grooves are absent during surface refinement, whether they are small or large, I don't see how they can help or penalize the work of the abrasive, because the metal nuts are not worn very much, at most the abrasive would already tend to abrade the intermediate plaster a little, however maintaining the surface substantially complementary to that of the mirror being worked.
Obviously such a tool with metal nuts embedded in plaster, it must not be changed during construction so as not to ruin everything. And once used for a mirror, can be reused for another, but starting from roughing, so that thanks to the coarse abrasives he will be able to readjust to the progression of the new job.
All this does not apply in polishing, where the pitch coating absolutely needs nice wide grooves around the 5 or more mm, to allow it to yield and expand without obstructing the channels and piling up by varying the curvature of the contact surface, ruining everything instead of improving.
