This article is divided into four parts, in which it is fully shows the shared discussion forum from 20 February 2014 to a 9 June 2014, with the glorious completely manual realization by “Massimar”, a mirror Newton diameter 300mm focal ratio F 3,8 with excellent quality of surface roughness given by peak / valley on the glass less than one-sixteenth of the wavelength (Lambda) of 560 nanometers of yellow-green light that the human eye is more sensitive, corresponding to an optical quality of at least lambda / 8 reflected wave.
All this of course respecting the all-important Couder criterion, which states that (..in order to get the perfect canonical diffraction image giving maximum quality and contrast in the images provided by the telescope that will mount that mirror), the reflective surface of the mirror must be such that all reflected rays converge within the "diffraction notch" (also called Airy radius), which in fact, for the mirror in question, is a little disk in diameter 2,6 thousandths of a millimeter (microns) which is located at the distance of 2280mm by the mirror itself, equal to the focal length of 1140mm.
The teaching and educational importance of the discussion, for those who want to have an excellent example of how it carries out a good optical work, It is absolutely made unique by the wealth of detail in the description, both budgeted corrective actions from time to time, and that the results (good or bad) obtained, from the fixes made to finalise the processing of that superb mirror.
That didactic and instructive importance, is given by the fact that all the numerical data reported, they are the necessary that can be, from any reader, profitably typed within any program for the evaluation of Foucault tests, in order to study, visibility and inter-active experimentation with variations in the shape of the mirror surface, until its fulfillment.
Part 1 of 4
Parabolizzazione mirror 300 F 3.8 I open this thread aware that there will be few ( but fundamental ) interventions, because the subject matter is very selective and dedicated a few homebuilders fans, with the hope that it can help increase the limited number of available information on this “practice”. The mirror data: effective diameter : 297 mm Tools made with rosin ( 66 % ) and beeswax ( 33% ) and the turpentine until it reaches the desired hardness. The polishing stage, with cerium oxide started with all tool diameter 1/3 COC and continued with sub-diameter tool 125 mm ( passers ran through the center and extended up to the edge with mirror and under slow and constant operator shift around the mirror ) It is not yet complete, the opacity gradient extends towards the edge where a circular crown of 3 cm is still visibly opaque:  Click on the link to enlarge http://s26.postimg.cc/vbkmd2xax/P2200536.jpg first problems: a first Ronchi verification test showed that the surface is moving away from the spherical shape sought with a marked tendency to paraboloid:  Click on the link to enlarge http://s26.postimg.cc/88u3dwvtl/Picture_893.jpg also a careful examination of the quality of the polished surface showed numerous “micro lines” where the extension is on average 1 cm but the width is lower the size of the pits left from the last abrasive (silicon carbide 800 ) for now the questions are: 2- from whence come these microrighe, really infinitesimal visible but watching the reflection of a very bright light, on the 125 mm earlier I had not seen anything like it… |
Re: Parabolizzazione mirror 300 F 3.8 Of course you do not love to win easy: parabolizzare a F3.8………:) |
Re: Parabolizzazione mirror 300 F 3.8 the tool at the sub-diameter was a directive astrotecnico, it seems that for short focal lengths also is more appropriate for the attainment of the sphere. |
Re: Parabolizzazione mirror 300 F 3.8 Some thought “freewheel”: 1) I would say that the main fact is that the Ronchi on the general curvature indicates the presence of a central hole. Which would make me go back to the tool diameter full at least until the polish has reached the edge. At most (Having regard to the focal so short) “shortening” the length of the races (but always central) until 1/4 or 1/5, or less anyway 1/3 diameter. And along the way making sure that happens. I'd tell you to work in peace, because with polishing to the sound pitch and cerium oxide, it is very difficult that you can change the focal if a few millimeters or fraction. 2) The fact that the polishing has not yet reached the edge, It is normal as applying any polishing race, the center mirror, contrary to the periphery, It remains continuously covered by a full-diameter tool, or, however, further covered by a reduced diameter, and in all cases, more abraded. 3) The cause of microrighe is not so easy to spot: to their small length with respect to the applied races, suggests that they may be generated by the rotation of the tool…but alas not only: The causes can be many, by the slight ooze pitch on a peripheral point of the tool; by some plugged drainage channel in its surface grooves (which among other things must always be asymmetrical). One of the basic rules of polishing (obviously with full tool diameter) to avoid phenomena as well as the lines and the areas, and to resume a central hole, It is to make a compression at the beginning of each work session, in order to standardize the contact surface mirror-tool pitch. The cure (however experimental and unsafe) It can be the control of the channels, and cropping off the pitch direct tool support, and last chance a slight touch of beeswax on the same pitch, made with a brush lined a cotton rag (otherwise the bristles could put too much, varying the consistency of the contact). This in order to renew the contact surface, in the hope of trapping deleterious incorporated some bumps somewhere. What I write is what I would do, and it is therefore to be taken with pliers because my experience has never gone to work below the focal 5. |
Re: Parabolizzazione mirror 300 F 3.8 Julius first of all thanks… Sometimes I hear of “wheezing” all reassuring during past, which, combined with your thoughts it would assume that maybe it is better to completely redo the patina that could hide some flaw. I also wonder, if return to the full diameter with past court, forzerei the deepening of the center, which is already deep his… unless the central depression does not return a greater contact to the intermediate zone that consumed would tend to equalize the curvature with the center. ( maybe I'm making a mess ? ) The initial doubt concerned this aspect, in the sense that reached the sphere must dig the center a lot ', (according to the formula r ^ 2 / R of the draft it is 10 mm), then it is necessary to eliminate the central hole when then once eliminated must remake ? |
Re: Parabolizzazione mirror 300 F 3.8 Quote:
And, the hardness of the pitch always got to do a little. But in this case, more than one ( or with the) determinant would consider the impossibility, with a tool diameter sub, to make a “pressing” as it should, in order to standardize the pitch-mirror surfaces (and with them the abrasion resulting therefrom), before you begin each work session. Quote:
Wheezing is a sign of very strong abrasion with too little water, but usually they do not generate lines, at most could generate stairways “vibrate”, because physically they are a string of “jamming and jerky forward” tool, for every second (depending on the tonal frequency hiss). Quote:
To delete the hole (which it is a sign of too short of the central zone) with strokes 1/3 coc, the “sacred texts” suggest keep mirror above and below the tool for some dried, closely following the evolution with Ronchi….For the record indicate recovery 1 mm Wrong draw (obviously detected with Foucault) every half hour of work. Quote:
The risk you run is that, on the road the elimination of the central hole, you may encounter in a “unfortunate” :eek:condition of good or very good parable… (which is obtained when, compared to a spherical cap, the center and the edge are also slightly dug, while the area that is to 70% diameter has the same radius of the sphere)…Ma :spaf:that good parable may reach you soon enough, when you have not finished polishing the edge :spaf:. |
Re: Parabolizzazione mirror 300 F 3.8 Giulio what about, clear detailed and exhaustive as always… Ok it goes with all diameters 1/3 ( and even less ) COC until recovery of the sphere and polishing also extended to the edge. Then I suppose you will begin the excavation, in search of the parabola with Foucault support. I will post the aggioramenti, for now thanks again :ok: |
Re: Parabolizzazione mirror 300 F 3.8 Well massimar: then you've discovered, at your expense :(, the problems that can come from the use of tools subdiametro. |
Re: Parabolizzazione mirror 300 F 3.8 astrotecnico, thanks to the availability. The adaptation of the patina is good, pitch is uniformly smooth and consumed by the cerium and assumed the typical color from the first sessions in all its extension, the hardness is only slightly higher than the optimum, but there is also to be considered that work in an environment where the temperature is about 15 °, perhaps it would agree to warm up a bit’ the environment and verify the degree of hardness. |
Re: Parabolizzazione mirror 300 F 3.8 They continue to be generated the “micro lines”, independently from the tool used ( sub or full diameter ). Consider that this is a greatly enlarged, eyepiece 5 mm + zoom webcam, the “craters” you see are the remaining pits 800.  Click on the link to enlarge http://s26.postimg.cc/utd2e4ie1/Picture_903.jpg
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Re: Parabolizzazione mirror 300 F 3.8 Sure you do not have a few grains of carborundum, survivor of the earlier stages of processing, that slips between mirror and tool? Maybe the plan which supports the tools is not completely clean. ……. |
Re: Parabolizzazione mirror 300 F 3.8 and Deabis, it's possible… although I was careful to cleaning when switching from abrasives to pitch, it is not impossible. In fact, a few grains of abrasive embedded in the pitch would leave a lower track of the excavation size compared to those which would if interposed between the panes. the thing that puzzles me is that the lines are clean cut, continuous and regular, carborundum, do not sembra. I will try with a new support, let's see what happens… |
Re: Parabolizzazione mirror 300 F 3.8 I am inclined to a few grains of 800 finished in the pitch. also I would think that precisely net grooves being continuous and it corroborates this, because the damn bean does not give sbricioarsi craterizzando the mirror, as it would if it had not the pitch of the support that holds the latch is located where mpantanato. Is’ a beautiful nuisance. |
Re: Parabolizzazione mirror 300 F 3.8 It Giuliot, a beautiful nuisance, Also because I have no idea how to delete it without redoing pitch … Anyhow, after a while’ It should stop doing damage, albeit more slowly should be consumed them ( the grain or the bloody beans ). |
Re: Parabolizzazione mirror 300 F 3.8 Hmm …I'm afraid that if they are abrasive grains or those that remain stuck them to do damage “random”. A possible elimination would colandogli above other pitch, but then you do not agree reuse it as you usually. |
Re: Parabolizzazione mirror 300 F 3.8 something is missing … at this point there should also be the good news ?!? :confused: Jokes aside , think colandoci above other pitch be able to merge with the upper layer that is smooth and “paved” by cerium ? |
Re: Parabolizzazione mirror 300 F 3.8 For this reason, the hot pitch sticks well on anything. But the work to be done would be very close to that to redo from scratch coating. It rifai, Also consider doing an autopsy at the workplace and to your clothing or aprons that eventually uses, that would be even change those. A friend of a French forum had similar Rogne in step dall'abrasivo the pitch, and after cleaning, he later discovered that the abrasive had it in his hair that had fallen earlier…. :D |
Re: Parabolizzazione mirror 300 F 3.8 :D incredible … I try with the additional casting, at least you'll save on the fact of having to detach from the glass, which is another great hassle.. first I try to verify which of the two is affected by contamination, the probability that they are both is low (excluding the usual Murphy ), then regenerates the top layer, |
Re: Parabolizzazione mirror 300 F 3.8 massimar the rows are to 99 % caused by an abrasive grain embedded in the pitch. Is’ something unfortunately known to me. The effect of a granule embedded in the pitch is to a lining type cutters ….. Just like your lines ! |
Re: Parabolizzazione mirror 300 F 3.8 I confirm the diagnosis made by all of you: abrasive between the pitch. |
Re: Parabolizzazione mirror 300 F 3.8 Add two questions, I am taking advantage of the fact that I will have to redo the pitch. 2- reading another thread, Giuliot spoke cerium oxide from the pink color, What I found more sources, while mine is white ( than white can not ). There are perhaps several supplies of cerium oxide, if there is no difference ? |
Re: Parabolizzazione mirror 300 F 3.8 Even mine is white |
Re: Parabolizzazione mirror 300 F 3.8 Quote:
Using the rosin (alias colofonia), which it is a lot harder than the resin (much better) pitch black, It becomes mandatory melt beeswax in addition to significant weight proportions, in order to lower the softening temperature of around 50-60 ° C (pure turpentine can add… but minimum quantity type thimble). When I use the slang term (and improper) “dura”, I mean refers to room temperature, as rosin softens at temperatures ranging around (if not over 100 ° C) absolutely useless for optical processing, for which it is required that the pitch is quite pliable, but not too much, so that at a temperature not much higher than the ambient temperature (increased by the friction of processing) may incorporate ceria, and deform with a PASSIVE behavior, Following yieldingly and favoring the mirror deformation processing. A pitch too hard is likely to take on a totally undesirable behavior ASSETS. As for the “hardness”, I would say that it is useful (and can act as a guide) the following figure, where you see the different gradations….It is noted that no optical pitch has a higher softening temperature than 90 ° C. I, since the work of scratching predominantly winter glasses, use and recycle the #55 which it is the most “soft”..but in summer it would be too.
http://s30.postimg.cc/j7ncoogo1/Pece_Gugolz.jpg A final caveat is to heat the pot of pitch on an electric fornettello a metal plate. Never use a naked flame or electric heaters discoveries. The vapors of the pitch are highly flammable, and if you are on fire high flames more than two meters. Regarding the oxide Cerium, I too have a very white and pure pinkish / beige. Then I also zirconium oxide, He also whitest, which it is more abrasive cerium, but I like to just use it because it is… “ungrateful”….In the sense that the cerium is grateful when it does a good job, emitting a froth between the pitch of the paintings…What that does the zirconium, that as “noble” probably, believes not having to bend down to talk to a laborer grattavetri:rolleyes: I have dwelt again as an Evangelist :spaf: |
Re: Parabolizzazione mirror 300 F 3.8 Absolutely you have not dwelt, all the information with such intellectual generosity expose, They are always precious resources for people like me who are trying to understand and learn. Do not forget that, apart astrotecnico, which all know and appreciate the preparation and availability , there is only you and ( perhaps ) very few others can provide elements of knowledge and insight into this topic, although we are in the forum which boasts the excellence of Italian amateur astronomers, so what to say except thank you and again “dwelt” how much do you want !!! :) |
Re: Parabolizzazione mirror 300 F 3.8 He has…I noticed that the image of the pitch there is a misprint about the (my) degree pitch # 55…which, as it is written, :eek: which ones would have a higher melting softening point.:spaf: |
Re: Parabolizzazione mirror 300 F 3.8 For what concerns the pitch is obviously better for that specific optical type the Gulgoz, because it is a product made especially for this application. Of course, it should be chosen in gradation suitable in accordance with the temperature of utlizzo and then, reflexively, also the season, unless it operates in an air conditioned environment, as generally it tends to do professionally. |
Re: Parabolizzazione mirror 300 F 3.8 @ Astrotecnico I found Gulgoz the US site GotGrit.com , (that surely you will know ), the site is reliable ? |
Re: Parabolizzazione mirror 300 F 3.8 ….Mah… I discovered this supplier, but it is new and unproven reliability: http://www.optiques-astro moindrot.c…_de_verre.html It would not hurt because there's half a customs… And and, I would say the #55 |
Re: Parabolizzazione mirror 300 F 3.8 Completed polishing, He disappeared microrighe and reached the ball. in any case this is the situation at the Ronchi test:  Click on the link to enlarge http://s26.postimg.cc/4s0ear2mx/ron_101.jpg it seems to me that there is a first problem to the edge where it is forming a kind of “collinetta”, because I see that the tendency curvature close to the edge is first increasing and then immediately decreases . I do not know if my interpretation is right… I also wondered if I should start from now the measurements with Foucault, although they are still far from the parable request. p.s. the “stain” dark hundred at the top of the test on the visible image is the cradle latch. |
Re: Parabolizzazione mirror 300 F 3.8 Quote:
Patina pitch: I'd do the patina. But I would do it my way, with my usual squares of 20x20x10mm pitch, to be bonded one by one (heating the base of each square with the flame of a candle, and then on the tool poremendiolo) with the usual caveat of asymmetrically positioning the first picture (putting the tool center that is roughly coincident with the center of the area “A picture of DIAL” …I do not know if I spego in understandable w ay). Collinetta: I would say nothing to worry about, however,. Foucault: In practice this operation is done by executing a Foucault test bare mirror (ie without anteporvi the Couder mask), and measuring how much is the draw difference between the focus of the mirror center, and the focus of the peripheral crown. If the ball was perfect, the mirror would become completely gray percent from the edge evenly and sitantaneo, Your draft would be zero. The amount of draw for a given parabola is calculated with the following formula: draft[mm]= [r]aggio ^ 2 mirror / [R]aggio of curvature (which it is twice the focal length). Your 300F3.8 mirror has a mirror radius [r]= 150 and a curvature radius [R]= 300*3.8*2=2280mm Your maximum draft is therefore (150^2)/2280 = 22500/2280 = 9.86mm |
Re: Parabolizzazione mirror 300 F 3.8 Him….:in: Always staying on the subject, ma to put you on the taste of Foucault, I show the figure of the following excel sheet, with some comments:  Click on the link to enlarge http://s18.postimg.cc/3oli04wx5/1_6.jpg In the sheet I faked “end of work” Your Mirror. (clicking on it to enlarge, with perhaps some additional magnification with CTR +), you'll see that: You would have gotten a mirror with a lambda 7,1 (box F27), whose numerical value expresses (see green background graphic) the ratio between the wavelength of 560 nm (green light; box H5) and the distance in nanometers between the two limit parabolas, red and green, which comprise at their broken extension that identifies the slopes of 5 areas of your satellite dish. I would have got the rays reflected from your mirror converge (see chart on an orange background, “transverse aberration to fire reltiva ar radius Airy”) s all in your mirror of the diffraction notch (diameter 2.6 microns) to obtain a contrast crazy, All this while the graphics (the insatiable) a light blue background, It tells you where are the points where you should still remove the glass, and the ordinate (But we have to change the scale of the graph) He tells you that the glass thickness in nanometers should take to improve even. Lastly, the horn chart on yellow background, shows you the bad habit of your zone than the tolerances of the mirror, which are loose on the left (the center mirror, ie in the area 1) and instead become very narrow extreme right, to the edge mirror. With a mirror so (and it is only a matter of time), would in telescope with an upper long great contrast, the average of the commercial mirrors. And all this is also my wish…obviously. P.S: This paper I like it because I understand well (having previously started doing hand counts as the book Texereau) . Ma, “Credit where credit”, He invented the famous Pierre Strock. (I have only studied well and realizzandone translated versions with different zone numbers, until 8). p.ps: the total draw of this hypothesis would mirror (I13-E13)= 8.3mm, which is in agreement even if conservatively a little less than the value that gives the theoretical formula indicated in my previous post in 9.86mm |
Re: Parabolizzazione mirror 300 F 3.8 hello Giulio, the detachment of the patina is not partial but total:  Click on the link to enlarge http://s26.postimg.cc/7osivora1/P4040583.jpg to make the patina using your method into squares, We should use the Gulgoz, I do not think that with the rosin achieve good results, I have at least made an attempt… with bad results. This weekend is dedicated to parabolizzazione Foucault , I think it is the best thing. By the way, okay the mask 5 zones ( Carlin) or you have to increase them ? In fact they are already beyond the sphere ( I had reached a few days ago ), there is a beginning of already visible parable from Ronchi, although still far from the final form. :spaf:I saw now your next action… Now I read it. |
Re: Parabolizzazione mirror 300 F 3.8 – If the pitch disk is full, the problem would only stick it well. But I do not know the method advise. Astrotecnico I can suggest a viable solution. – I believe that 5 areas for a 300 goes well, is not the last buffer zone , which it is among the most important, It becomes small and hard to read, because disturbed by the reflection of the joint bevel bastard, in addition to the diffraction fringes. -Better if you are already at the parable…you have less work to do. |
Re: Parabolizzazione mirror 300 F 3.8 And, I had the opportunity to see and appreciate your spreadsheet for analysis of Foucault, useful and comprehensive ! I'm surprised the difference of draft values with respect to formula, 1.3 mm are many ! p.s. the image you posted , it magnifies , It only remains preview :( |
Re: Parabolizzazione mirror 300 F 3.8 I read that you do not magnify the image. Is’ strange !! In my PC it is enlarged by clicking upward, and even after, with some CTRL +. Quote:
No ferratissimo in theory, and I go ” ear”, but I think that value of 1,3mm of spherical aberration, corresponds to the length of the optical axis line that contains the focus of the central and peripheral ones rays, It acts as a bit’ from grace. Practically, the central point of the dash (then it is up to 0.65mm from one extreme), corresponds to perfect focus. But I think I might as well be easily mistaken !! |
Re: Parabolizzazione mirror 300 F 3.8 now it is open magnified, mah… I did not understand in your spreadsheet, with which method for the radius of the central zone ( Texerau, Carlin, Couder ) It must calculate the mask. |
Re: Parabolizzazione mirror 300 F 3.8 The calculation of the couder mask is only suggested in the box at the bottom right of the sheet. So much so that the data (…those or other) They must be entered manually in the E8 I8-sheet boxes. I do not think there is a great difference in philosophy to make that calculation because for any measure Hmedio of any window, there will still be a unique aberration value calculated with the formula of the parabola Hm ^ 2 / R. The only care is to make sure that the windows become progressively narrower as it goes toward the outer perimeter mirror, where there are tighter tolerances because the flares curve faster, and then with large windows it would run the risk of not being able to see with certainty the uniform dark tint chje characterizes the focal point of that area. |
Re: Parabolizzazione mirror 300 F 3.8 Update: Make about 10 sessions 15 me. with sub-diameter tool, alternating switch to W and past COC to deepen the center. the focal length is further reduced after the last measurement made at the beginning of polishing, now we are 1120 mm for a focal ratio 3.77 ! As you can see the edge is to be placed, but being a raised edge should not be particularly challenging. Ronchi Test:  Click on the link to enlarge http://s26.postimg.cc/ysu60ngux/Picture_939.jpg Readings Al Foucault test with Couder mask 5 zones zone 1: 0.95 Report test Foucault:  Click on the link to enlarge http://s26.postimg.cc/5n0b66a6x/image.jpg  Click on the link to enlarge http://s26.postimg.cc/6dxmpp1y1/image.jpg
I think there is to work with the peripheral zone, ( board including ) that seems to be the one with more problems. |
Re: Parabolizzazione mirror 300 F 3.8 A touch-up area 3-4, e zona 5 and lambda values you squirts a dream….(You have already finished fine polishing?…I ask this because the risk is to hit one of those astronomical lambda with the mirror still not perfectly polished):spaf: From the Millier La-croix graph I see in the mirror center it is no longer absolutely touching, and yet we still have “drag into” the “trumpet of tolerance”, the zone 3-4 and 5, which at the moment do not reflect their beams into the diffraction mark Airy. This “back in the ranks” of the zones 3-4 e 5 to provide a dazzling contrast. |
Re: Parabolizzazione mirror 300 F 3.8 It Giulio, polishing is complete. I also compared the results with your spreadsheet, lead to the same conclusions , even if he says that there is still a lot’ glass to be removed. ( I had to change the values of the outer radii of the areas, slightly, They differed from those recommended ) |