Disk Processing

[marco95]

Hello everyone, I'm new to the forum so I apologize in advance for any bullshit I may say! first of all, however, I would like to thank everyone for their help.
I would like to ask some questions “Basic”: from what I understood from watching Dobson's video, two FLAT glasses are used to make the spherical dome, one to dig and one to dig, but Dobson presses the glass with his hands “digger” on the sides of it and not in the center, how can this glass hollow out what will become the mirror?

[Giulio TiberinI]

Hi Marco95.
Your reasoning is right and doubt is understandable…But beware that John does NOT hold the "digger" glass in his hand (that is, the tool), but it moves back and forth the glass "to be excavated" which is the mirror.

Who, with the pressure of his hands placed on a diameter of approx 70% of the total, it will wear out in its central part of the lower face, against the upper edge of the digging tool that is on the bench, gradually becoming concave the mirror and convex the tool.

You will remove your doubts by reviewing for 5 minutes the movie starting from the minute 5 and listening to the description.

[Massimo Marconi]

Haha ! if you have to apologize in advance for any bullshit said ( or thought ) I shouldn't do anything else all day than.. to apologize !

Differences aside, welcome Marco , if you also look at this article, there is a design that gives a good idea of ​​why the upper glass becomes the concave mirror and the lower one the convex tool:

How to build focal lenght and mirror's depth.

[marco95]

so if I'm not mistaken, the procedure used by Dobson and the one used in this video (https://www.youtube.com/watch?v=pQ2emcoswKY&index=3&list=PLvQ11CcwJy0d5vKch44KthskeibiItXxf) I can be considered “complementary”: one of the two can be considered more precise and performing than the other?

[Giulio TiberinI]

They are two different jobs.
John begins the mirror by making the curve and then holds the mirror in his hand and rubs the center of it on the tool edge below, applying runs that are so-called Cordali, as you will find described in articles in this blog, and as you can also see from this movie:
https://www.grattavetro.it/tutorial-video-costruzione-di-uno-specchio-parabolico-200f6/.

Instead in the movie of your link, Matteo bought the glass already ground to the definitive curvature and only needs to refine the curve with gradually finer abrasives.

A tip, however, that I must give you is the following:
Since there are several tricks and tricks to be put into practice even in the simple section of the work shown by the video of your link. And I refer for example to important instructions omitted for:

– do not chip the mirror;
– not to forget coarse craters that with fine grains can no longer be eliminated;
– not to put too much abrasive which ends up grinding itself more efficiently than how it grinds glass;
– not to put anything but drops of water so as not to take away the abrasive;
– to know how to generate the curve but also to work backwards to keep it unchanged;
I recommend you then, in addition to seeing the black and white video of which I put the link, also to read some realization diary.

This is because taking reference from the videos of a beginner who with great good will shows what he does for the first time in his life, is , like his, an equally reckless action that in many aspects potentially puts your future positive realization in danger.

The moral of all things is that in order to teach someone something it takes a minimum of authority given by the teacher's experience in his teaching subject.

[marco95]

thank you very much for helping

[marco95]

hi again everyone!
after months of research I was able to find a company that could produce an astronomical glass for me; the problem is that the maximum thickness they can build with borosilicate is 19mm, I was then offered a “quartz glass” (in this case they arrive at thicknesses of 100mm); this type of glass is workable and therefore usable as astronomical glass?

[Giulio TiberinI]

In fact, the maximum thickness that is normally used today on a large scale for normal glass constructions of glass walkable floors and building details, (but I see they also offered you the borosilicate which is more expensive) and the 20 (or 19)mm.

The purchase of sophisticated and specific optical glass such as borosilicate, or pyrex or quartz… i.e. different from normal glass, it involves a very considerable expense which makes making the mirror even less convenient. Especially if it is the first one that one tries to achieve.
A technical rule says that the thickness of the glass depends on the diameter of the mirror you want to make, and the difficulty of implementation increases exponentially with the increase in diameter and with the decrease in the focal ratio. In other words, the larger the mirror, the deeper it is in its excavation, the harder it is.

So the golden rule and the advice for a person who embarks on the path of realization for the first time would be to choose a diameter of 300mm which also represents the maximum limit for the use of thickness 19 or 20mm of the glass, which can thus be of the calcium sodium type, ie normal, and can be purchased from industrial glass factories at a price of approx 100 euro for two discs (one for the tool and one for the mirror).
The quality of borosilicate glass is only suitable for mirror manufacturers because they do not have to wait hours after processing to let the mirror cool and then recover the shape before carrying out the optical correction checks..

But from the amateur's point of view, a mirror “hot” because it is kept at home and taken outside in a telescope, it will create an identical turbiolence that will make vision equally bad until it reaches thermal equilibrium with the environment, whether it is made of normal glass or of very sophisticated and expensive glass.

You should look for savings in an industrial glass factory (that is, those that work those thicknesses for construction).

[marco95]

Thanks for the reply, however, I asked the question because I contacted a Chinese company that cut astronomical quartz / borosilicate glass for rock bottom prices, in the end the price “facade” it was only symbolic, it actually turned out that they cost a lot more than what they showed; however my goal was a 350mm with a thickness of 30mm, but from what you told me for a 300mm you can go down to 20mm thick, even if this is an extreme case because it would leave no room for corrections: so I ask this question, a 300mm with 25mm thickness is enough? astronomy has always fascinated me and I am entering this world as a pure ignorant simply because I want to have the opportunity to observe the closest nebulae / clusters. for example, a 300mm is enough “thick” to allow me to see nebulae like that of Orion? because I have a 150mm shmidt-cassegrain and honestly I could not even see andromeda

[Giulio TiberinI]

Hello Marco.
My 360F5 is 30mm thick, and a 300mm with 20mm thickness is an exceptional telescope that does not have the workability limits you imagine, for the reason I tell you later, and which in many cases can be considered almost definitive, but this depends on your thinking.

For me, the definitive was the 360F5mm as a compromise between size and portability and my aspirations, But everyone knows his stuff and has his aspirations.

I tell you this because the Orion Nebula and the Andromeda Galaxy are already visible with my 80F5 refractor., with which the globular M13 is also seen but only as a round and milky fuzzy speck. While with a 300mm you would see a spectacular M13, like “a christmas tree” illuminated by a thousand stars / fairy lights seen from above.

So your doubt of visibility suggests me to advise you to check the amazing amount of visible deep sky objects with a diameter of 300mm by going out sometimes with a group of amateur astronomers from the club closest to your home..
(But where are you from? You are far from Asti?)

An anecdote that I often tell, refers to the vision of M97 (the well-known owl nebula), which with the refractor 80F5 was a “hollow eyes” that I had to “flush out” with a UHC interference filter to be able to detach it from the greyness of the sky background….Col 300, the first time I looked for it even without UHC filter, when I passed over him, he introduced himself to me with the delicacy of “a punch in the eye”…In the sense that as it passes, you instinctively take a step back by removing your eye from the eyepiece, so much is evident.

Returning to the technique: A thickness of 19mm for a 300mm diameter mirror does not have the workability limits you imagine, it just needs a cell a 9 support points, because the thinner the mirror, the more potentially it could flex a few nanometers while tilting the telescope…and being the machining tolerance for a lambda / 4 quality mirror 68,75 nanometers high. (millionths of a millimeter), the cell has the purpose of keeping it well supported to prevent it from flexing and going out of quality tolerance.

A few numbers: For example, to make a 300F5 you have to dig a sphere that has a 3.75mm deep arrow in the center. So if you have a thickness of 19mm and subtract 3.75mm there is 15mm of residual thickness left.
Subsequently, the polished sphere you will need then “svasarla” in perfectly parabolic shape, which means removing again a layer of glass which has its maximum thickness at the edge, with 2,34 microns.

The formula for calculating the arrow to be carved into the glass
Arrow = mirror radius squared divided by 2R

Where 2R is double the radius of curvature of the sphere, which in turn is equal to double the focal length.

So a 300F5 has a focal length of 300×5= 1500mm and a radius of curvature of the sphere of 2×1500=3000mm, so the arrow for a 300F5 is: arrow = (150×150)/2×3000) = 3,75mm

[marco95]

Thank You very much

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