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Well, so I will proceed. The pressure, if present, It must be exercised with the thumbs in the right central area? Using palms and fingers to just go and come Mirror. It may have been a contributing cause excessive pressure on the edges?
Mah… hard to say, the only certainty is that shown by the Ronchi namely, a more machined area compared to the other with the races 1 / 3D, This indicates a non-uniform contact. The contributing factors may be all and contrary to all, why I think it is important to get to know and optimize your setup prior to the application of specific techniques.
Finding the cause at this time prevents reaching the ball is more instructive than a parabolizzazione performed without any problem… but I want to repeat, the work was done very well up to this point, we are talking about very minor defects that constitute the norm in optical processing manuals, it would have been unusual and unforeseeable circumstances otherwise, ie reach the ball without any problem of processing.
Same thing will happen to the parable, therefore each grattavetro sooner or later realize that the essence of the process is precisely this, find the causes of manufacturing defects and act accordingly each time finding the right fit, It is seen that each processing is a story in itself, different from previous.
Hi guys.
Stefano I see that your work is going well despite no dramatic delay in the extension of the spheroid to the edge of the mirror.I agree with the suggestions of Massimo, leggendoti and I think the only suggestion that I add (if you do not already follow), regards the avoidance of any pressure that is not the weight of the sun hands arranged in hours 3 and hours 9, the diameter “neutral” that is approximately 70% the glass in your hand.
This is because even a slight pressure (worse if the central), of races 1 / 3D (who already work just), It tends to favor the lowering when it is exercised, more than facilitate the unification of different radii of curvature of these mirrors and tool (obtained even if only visually. when it disappears the famous air bubble trapped in the two glasses).
The exercise to exclude the influence of the weight of the hands at work, It is preparatory to the fact that you will have to totally exclude it being parabolizzazione, in that work sliding the glass for only its edge
Another thing that probably already do is to stretch a little later ran back the 1 / 3D, but without exceeding the 1 / 2D.
I think that the step that separates the central sphere from the edge of both minimal and not visible with the classic central air bubble, otherwise it would be definitely recoverable with pitch, but should return to abrasion with glass glass.
Unfortunately, although the extent not visible as it might be at the end of polishing one micron, it may still be a flaw thousand times more difficult to eliminate, for the great amount of time necessary for the pitch and oxide, which they are able to remove only a few nanometers per work session.
I admit preferring the Foucault test, I never “did the eye” to judge from the poor even approximately Ronchi tests that entity, and not being able to know if there are many nanometers error, or if there are few, I find it hard to choose the method to be recommended to the step disappear, and also especially to estimate the duration of the application of that method to expect a good result.John Dobson, to reach the edge with fine grit (then still without the use of pitch), It suggests wetting the mirror peripheral zone with fingers dipped in warm water, so as to “to lift” for the local expansion glass, and speed up the removal of localized….It's easy to use the same principle of hot water in suburbs mirror you can also help with the use of pitch. If you feel you could experience it.
See for about a minute back the movie at the following link, from minutes 21 e 17 seconds, video https://www.grattavetro.it/auto-costruzione-di-specchio-e-telescopio-o400mm-f6-con-video-tutorial-di-john-dobson-parlato-in-italiano/
I had seen that video Giulio, according to me, however, it becomes difficult to uniformly heat the corona. My little experience does not allow me to say too much though, so feeling, watching some mega visible defects in Ronchi Test of other scratchers that are still accommodated in the polishing stage, I would hope that the deviation in my case is as recoverable. I'll have a clearer picture of the situation with the new tool, putting into practice all received advice and experience made so far. If even then the direction will not change, then I will assess, after moments of despair, other roads
If the grating is used around 10 lines / mm, then the defect is really very small and recoverable in the short term, two or three sessions at most ( once you have found the cause ). I think the difference between the sector “indicted” and the sphere does not exceed lambda.
No Massimo, to be honest the pictures I made with 125 lpi then around the 5 lines / mm because with the 200lpi not see well the separation between the lines. But tonight I finish to open new channels tool, then hot pressing, cold ripressatura after a while and again. Fingers crossed!
P.S. The other day they gave me an unused and dusty Skywatcher 150/1200 mounted on a non-motorized EQ3, I disassembled the mirrors were covered with a thick layer of dust, I put them in a container with distilled water, neutral detergent and floating cotton to graze the surface and came perfect! reassembled and collimated last night I managed to see Andromeda (Well, to see a whitish halo with a brighter core), the Pleiades and the moon. Be a 150 so poor with only one eyepiece (10mm) still he managed to give some small satisfaction, with 300mm and ocular park worthy I think that we'll spend the hours!
And. a 300 It is truly a sight!
You have done well anyway to equip yourself with a centoconquantino! Why “Recon” in sruttura, become a dobsoncino “by backpack”, who happen to be able to use on occasions of walks where you can go only on foot…..And usually it's exclusive and very dark places.
where you already with the naked eye you can see M31.Here I begin to fear… I think it goes worse. Yet new tool, pressing 2 hours and then 25kg of ripressatura 10 minutes each 45 minutes of work. No pressure. Spend 1 / slightly abundant 3D (1 or 2 cm) An hour and a half of work, and I find myself so.
I do not know how to get on.Stop! Maybe I see the light at the end of the tunnel… I have reversed a half-hour games, Tool required above and below the mirror and it would seem to go your way. Alternerò Mot and Tot for a while and see how it goes.
Very well. Bravo Stefano. We see eye to the surface you are aligning.
In fact, the basic rule of so-called normal racing (that the family 1/3 D c..c. or so) He says just that with the mirror above only deepens the center to the 70% the diameter; while exchanging position and putting the mirror under, the only deepens the 70% diameter onwards towards the edge.
Besides, that's how we ride to reach the ball or keep it constant. While the piosizione privilege one over the other serves to correct the excesses to the edge or center, to return to the uniformity of the sphere.
Go comfortable that what is only when you overdo it with the length of the races, when there is also the risk of swinging one glass that is on the bench pushing what you have in hand.
Usually that risk you really run only when you're in parabolizzazione, and at the end of the mirror maybe you're forced to lower the raised edge that you prudently kept (…all we do so for prcauzione). that it is just a matter of the table with the ceria to break down a little on edge.
You'll see that when spheroid obtained and finished polishing, you'll start the parabolizzazione, there is very high surface (a hundred or more naometri) at the center and at the edge, with the lowest point that is in that famous 70% the diameter.
One thing that happens, It is to see a board countered TDE in the star test, or greater as light emanating from the edge mirror, which in effect is given, (or amplified) by the presence of the normal of the mirror chamfer.
The mirror chamfer is a disorder for this strength during processing, and also in commercial mirrors can make you diagnose a ribatuto board that there is. (Suffice blacken the mirror chamfer to the disorder cessaare, but during processing you have to learn to live with it).
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