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The aluminum coating must be removed before the mirror refigure, then finished work must be done again.
Franco, beautiful your enthusiasm and it is critical for these things but… believe me, parabolizzare an F4 is a long thing laborious and complicated.
Think of it without having practice with Foucault and Ronchi tests, addition to the necessary experience with the form of construction techniques with full utensils and sub-diameter, It is like trying to cook a wedding lunch of fish per hundred guests without ever having had a hand in the kitchen for even a fried egg
I say this not to discourage you, but be realistic, Then whatever you decide to do, we will be here anyway to give you a hand
Hi Massimo
it was just to chat, I sleep little and do not watch television.
I decided to make the telescope, I left for a 250 / f6, but then it is considering that more than one I can not help, I decided for a 300 / f5.
I already have experience; more than 40 years ago a friend got me the grinding of a 200mm.
We went to take blanks from the parts of Fiesole, where the workers of Galileo had a small workshop. Blanks and abrasive. During roughing my friend gave forfeit, I went on but after a while my girlfriend asked me very seriously if it meant passre time to turn around the table. I gave blanks and abrasive to a boy who can not remember Pistoia.Franco
…I went on but after a while my girlfriend asked me very seriously if it meant passre time to turn around the table.
AH,ah
the legitimate and understandable observation of your girlfriend, I would have responded by building a rotating floor !A 300 F5 is a good choice, difficult enough, but within the reach even for those in the first embodiment. Satisfaction guaranteed
Very true, but it will start to come away from the outside because I started parabolizzazione.
Eventually I hypothesize, and I ask you to confirm, which will take the form of a parable; it is possible that the center of the mirror hang in the aluminizing, and in this case I wonder;
will bother to various tests?
Inpedirà redo the light?hello Frank.
The last mirror that I finished was the 300 F6 my telescope with disc brake described here. In that case I made the canonical tool in plaster and I started working cerium and pitch over the aluminizing, which it was of multilayer type high reflectance made to me by Zaot.
The aluminizing did not give any trouble though I resisted I would say a lot, and that is for a few hours well in center. But rest assured that the polishing or refiguration does not suffer delay or damage from removal of aluminizing.
To remove it in my previous cases had left the mirror one night with the concavity of the reflecting surface full of ferric chloride which had at home, and forming part of the kit for the realization of electronic printed circuits (It serves to eliminate the corrosion of copper to the outside of the tracks protected by the ink of a special marker).
My ferric chloride was pretty exhausted (why it is recycled continuously until it takes too long to corrode copper). Definitely again it would be enough less time.
Ciao,
I use hydrogen peroxide and hydrochloric acid to etch the copper, fast and disposing well.
Having a 200 / f4 to bring it to a f5 or f6 action should be taken on the medium / mirror outer crown.
It manages to do this with ceria?
You used chalk by dentists to build the tool! What's special about this chalk?
Muccioli in his videos he used a tile in gress, it is regrettable?Franco
do as you like, but you might well scratch sull'alluminatura because you have to work hard to shorten the mirror arrow F4.
The arrow (ie the depth to the center mirror) 200F4 of a sphere is to 3.12mm
Indeed, recalling that the curvature radius R is twice the focal length; and that a 200F4 has a focal 800mm and R = 1600mm ; and Y is the mirror radius 100mm;
the arrow is = R-(square root of (R^2-Y^2)) = 3.12mm
While that of a focal F5 is 2.5mm and a 2.08mm F6
So to go back to a depth of 2.5mm to 0.62mm you should be removed around the edge
While the arrow to bring an F6 should eliminate glass 1.04mm.Both processes should not do it with pitch and cerium oxide because it would take you a very long time, as you can see here: http://www.alcoat.net/webtele/im-11/the-pit.gif
For the tool, there are a number of ideas, but the ideal would be to be able to pour one on one rudimentary mold that has a curve close to that of the desired depth arrow.
It uses the gypsum dentist because it is a lot more mechanically resistant than normal gypsum and also less hygroscopic, but I've seen that you can also use glue tiles Keracoll.
the stoneware tile is not objectionable but risky because it is necessary that it can withstand a sufficient thickness and without breaking until the end of processing.
And I think the stoneware tile is too thin and fragile. So correresti the risk of having to redo the tool with fine abrasive grits for polishing and parabolizzazione, ie in the most critical stages of processing where it is necessary that the tool is exactly with the precise curvature of the mirror. What impossible if you have to get a new tool on the way.The thickness of the plaster I had done 4cm precaution.
Is’ also a good idea to a tool realized drowning hex nuts:
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcR84on8Mix9y8WuLG2N8JrD2RjhAMaFT_asCsgIqKeKIfJ_LUqyNot to put much time you have to work with abrasives, and the quickest route always passes through the grits coarser, with type sequence 120-240-360-500-600 e 800 premium cerium, (or jumping some steps but lengthening the time).
Mel Bartel appears to have used a waterworks flange as tool:
http://www.bbastrodesigns.com/JoyOfMirrorMaking/RoughGrinding.htmlBut in your case, however, the flange (or a pulley) would that it were not a smaller diameter than that of the mirror because you do not dig but pave.
Ciao,
if to pave the edge I need to use abrasives, then it's not worth the effort. We abandon the idea of China and back to 300 / f5
Thanks for the attentionFranco
From a highly respected account, It would take about 1040 work hours (*) with pitch and cerium oxide, only to eliminate one of 1,04mm glass thickness equal to that which constitutes the difference between the arrow of the spherical mirror 200F4 and the arrow of a 200F6.
By comparison calculates that to achieve my 250F5 starting from two glasses already blanks machine to the radius of curvature (and arrow) almost final, I worked 27 hours in all. It calculates that if I had to hew hand curve with grits 60 or 80 I might have doubled the work coming around to 60 hour.
(*) : (The picture I posted yesterday, you indicate that to clear with pitch and a crater left dall'abrasivo oxide grain 120, they want us 100 work hours:
Because of a crater 120 great about 100 ie microns 1/10 in mm…To remove 1,04mm need a time ten times, and here is where would the more than one thousand hours).ciao,
60 hour, 1 Now in the morning and in the evening they do a month + 15 days for measures, prove, mistakes. 2 months to alluminare, 1 week to build the structure. In the spring I have the new telescope
Go!Franco
Ciao, I found 3 companies that cut me the blank.
1) the glass is Lospessore 19 mm.
2) The glass color is greenish or clear. Given that goes aluminized think it should be well greenish. I'm wrong?Franco
Hello Franco, the greenish glass is fine, It is the common float glass….
There is also the clear (transparent) which is obtained thanks to a particular process that is able to almost completely remove the iron oxide which is present in the float greenish (and that is precisely responsible for the color). I only do it for a matter of design or design but for our purposes is fine greenish (it also costs less)…I agree with Mirco. Viva green vetraccio from bottles!!.
…. Also because at least all'astrofilo visualist, there is no point in having a more expensive telescope mirror Pyrex or Zerodur or other borosilicate,
as also that precious material, while not deforming, It heats normally taking the ambient temperature, and then creating in the optical path of the telescope turbulence same disturbing the formation of the image that would create a mirror in excellent vetraccio from bottles. And the turbulence makes you look like crap even if the mirror is unnecessarily resilient type ..( and maybe you blame the poor seeing).This is to say that the convenience to realize mirrors borosilicate is exclusively manufacturer of mirrors for whom time is money, and that does not wait eight hours of cooling after grinding, to run the product quality testing (admitted that further execution!!), but it can do immediately and brutally possible final corrections (admitted that the face!), gabellando then that the thermal dimensional stability of the borosilicate glasses of Pyrex family, It serves really alla'astrofilo….. that unnecessarily pays even more that mirror.
“Sun spiegame tut from bin”? …that is: “I explained to well? (….it says here in Piedmont
)and in any case the borosilicate and its derivatives have become an obvious choice for diameters over 40 cm, because the more often vetraccio 19 mm unfortunately did not produce any more, but also a 400 F4 would need more thick, not to mention my primary 300 F2.6 that the center has become thin and short of raw material… seems my bank account !
Ciao,
this morning I'll get the glasses
I need to order abrasives, I recommended grain and quantity?Franco
there is all written here in relation to the diameter of the mirror in processing, take a look ;-), My advice is still to take for the moment a pack 1 kg for each grain ( the available packaging minimum ), if you're lucky and good could also be sufficient, otherwise ( if you're lucky, you're definitely good
) you will need an addition of 80
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