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Good morning,
I see that nobody has posted here, so I preferred to put here so as not to pollute other threads.I wanted to tell you about a related topic (wrongly) surface roughness of the optics, where many have written little knowing and little knowing. We tried to impose the thought that light diffused (correctly) reduces contrast in some celestial objects.
In reality I have carried out careful tests in this regard.
There are two orders of problems:
1) the surface roughness determined with the S / D parameters (80-60, 60/40,40/20 ETC) which can actually reduce the contrast by creating a scatter of light (in a fairly marginal way I would say, since I also found that this spreading between a Chinese perspective (80/60 and a well polished optic 40/20 it is very level due to seeing,and only when the latter is more than excellent can you see the difference);2) the blackening of the tube and the various internal pieces.
If I have to say it, I consider blackening to be much more important than roughness (obviously without falling into hyperbolas….). Currently I have experimented with a matt varnish at almost all wavelengths, so much that later 15 minutes of exposure under a black sky the spikes around the stars are not yet observed, this in collaboration with a willing and competent client.
I then did tests with a cassegrain from 250 treated with this opacification and one with the classic velvet. There is no history on the nose the contrast increases at least by 20%.
therefore, assuming that a well-polished perspective is a plus (in fact with Norman we entered into an agreement to get off 60/40 until 40/20), those who are aware of the fundamental importance of blackening the path of light ?
Know that some builders (Feather Touch to make a high-sounding name) have the inside of the focuses and extensions, bright anodized……..in fact, some ghosts sometimes fly……
Maximum
Wow ! interesting !
Two questions :
1- I would like to know more about these parameters for measuring surface roughness (are perhaps these ? : http://www.innovativeorganics.com/uploadedFiles/SGinnovativeorganics/Documents/Reference_Materials/Scratch-DigOptical-Specs.pdf )I assume they are industrial standards or at least have precise regulatory and production references.
If this were the case, it would be interesting to understand which standards industrial optical productions are subject to , more than anything else to understand how it is possible to quantify the value of surface deformations. In fact, at an amateur level, roughness is evaluated “eye” , mainly by examining the figure obtained with Foucault's blade. This does not mean that it is not possible to achieve a good quality of gloss or even super gloss, but only that it is not possible to quantify its value.2-I understand that this painting is the result of experimentation, then the study of a product aimed at a specific use, I don't think we are talking about the matte paint can bought at the brico…
Hi Massimo,
the link you report is correct. Industrially, there are feedback systems both with special sticks, both with bench measuring methods. In fact, do not think that the production of industrial optics (not astronomical, but maybe for the laser sector where super polishing or ultra polishing is the norm) do it with the artisanal methods that we amateurs are used to seeing. Normally microscopes are used that scan surfaces and photograph and measure micro-grooves or deformations.The big problem is that , for example, with cnc systems for machining optical surfaces, it hardly drops below the level of commercial polishing ie’ in virt’ the need not to commit too much a machine to the high hourly cost (not by manpower but by own machine). Vice versa, to tailor the reasoning, the same machines produce surfaces for laser optics with S / D 10-5. Just know that a surface with this parameter, flat and with a diameter of 50 mm costs over 4000 euro.
For the above, and not wanting to write a boring pistol, I always remain a little hesitant when certain numbers are presented to me. From experience I learned that a good test must be done on the sky with perfect seeing, so for us amateurs, otherwise they are good speeches but very academic…….the mirror never works in a neutral environment.
The paint is the one used in the professional field, very difficult to apply, where you must have good expertise and good level instrumentation (a very slight pressure difference in the spray gun ruins the work), respecting all safety standards (i think mice in range 100 die……
). At this point someone will say: but how much does this soup cost? half a kg. 500 euro. And once the surplus is opened it is thrown away in a few weeks. Is’ therefore it is clear that it is not within the amateurs' reach both for the cost but also for the rest (also difficult to have it), and I could say that in a tube from 280 mm one meter long affects about 180 euro. 
Maximum
The topic is very interesting, and the fact that, at professional levels of high quality optical constructions (…dedicated to a use, (I think), purely photographic?), the need to contrast every spurious photon coming from any surface of the optical path becomes very important and prevalent.
We went from "polenta flour" mixed with matte black paint, quote (I remember well, and it made me smile!), and used at the beginning of the technique, by a well-known Italian optics manufacturer for the internal coating of the refractors; to much more sophisticated and efficient paints, as well as lasting.
The roughness of the surface is in fact a problem that is easy to solve only for those who create the optics themselves for a purely visual use.
For all other buyers of cheap industrial lenses, where time is money, the problem becomes very visible in the train of a residual of diffused light and poor contrast, only when you receive it (I quote an example thread on Dobsonians), his own mirror returning from a reconfiguration by a good craftsman, than usually, and for economic reasons, tackles the problem by acting manually not on the whole surface, but only locally where it is needed, with sub-diameter tools.
therefore, diaphragmizing the area untouched by the polished one, turns out that the contrast of the diffraction mark, in the latter, dramatically improves in favor of the central light spot.Yes, but be careful. If I produce an excellent optics in XX hours and to make it much smoother I have to work it XXX hours and maybe I neglect everything else, I can say that the extra X hours are wasted?
My hint refers to optics for non astronomical use, but not photographic (signal transmission), but I mention for example that a very qualified company to which the problem is subjected with manual specifications brings the cost of a piece xx to 2000 euro a 18000 euro.
If I can improve CONSIDERALLY enough to see it visually in the sky and not at the counter, with a slightly high cost but affordable for everyone, I think this is a way to go. Today the times of the manufacturer of optics (which are now on half a hand in decline) they can no longer be those of 10 years ago, unfortunately the tongs of the tax authorities and the competitiveness of the market no longer allow us to deal with certain issues in a romantic way…….
If one works for oneself, then can’ also devote to surface finishing 100 or 1000 hour, both have zero cost. But I don't think we can expect this from anyone who builds without the right economic price.
Maximum
Hi Massimo, interesting discussion this. Just in the project I am carrying out on the construction of the primary, I am evaluating the extent of any slight surface roughness before declaring the mirror finished. What I have read so far will certainly be useful for evaluating the extent of “defect” which before was not entirely clear to me since I did not fully understand the real consequences of roughness. In addition, now I understand how important it is to make a good blackening of the interior of the telescope which otherwise could make me believe that I have a bad mirror when it is not so.
Wow 500 euros for half a kilo of paint !!!
It must significantly increase the overall quality of the telescope to justify such an expense I assume…If I'm not mistaken, the interior of the Hubble telescope has been coated with a state of carbon nanotubes to blacken the surface. Apparently there is nothing blacker and more anti-reflective than this at the moment…
Yes, right, I remembered it well.
This is the blackest material ever created: vantablack
It only reflects it 0.035% of the incident light at 700nm. Is’ so black even if there are folds on its surface the human eye cannot see them and appears to him as a simple two-dimensional figure. If you think about it, at the level of vision, it's the closest thing to a black hole we've ever created…
This would be ideal for our telescopes, pity that the price is exorbitant
Better to think of some cheaper alternative… However the blackening of the tube is more important than I thought.
Just to give a real example here is the difference between the use of the Z306 black paint (one of the best created by NASA specifically for these purposes) and a carbon nanotube coating:You can see very well from the photo, as well as the different reflectivity profile of the two covers, also the marked improvement in the contrast in the two images. I didn't think it could affect that much.
Obviously at our levels we cannot think of obtaining similar results, but reporting everything in proportion it is clear the importance of minimizing the effects of diffused light.
So on an amateur or self-builder level what should be used? ideas?The telescope will perform at its best as the weakest point of its design.
It follows that if you decide to make a telescope in a certain way, each component will have to maintain that type of technical quality. To explain me better: useless to build a super polished super optic, a super cell etc.. and then put a focus from 70 euro. Many performances of the tube will be lost precisely in the off-axis of the focus.
If you notice the difference between the paint and the nanotubes it is very wide, think a little’ when they intertwine with paints that have not even been studied for these uses what is lost in terms of final performance.
The flocked in nanotubes (which is not even difficult to do) and a little’ exuberant for the amateur astronomer (we will never have the conditions they have in professional tools). Furthermore, its application requires a lot of expertise already in the design phase (typical angles for example).
The economic alternatives are economic according to the meter of each, there are intermediate routes that cost a bit’ but they are practicable and clearly visible during use.
Maximum
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