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Hello Carlo,
Benvenuto sul forum anche da parte mia!
Ottima scelta quella di iniziare con un 250 f5
Ottimo rapporto tra probabilità di successo e soddisfazioni osservative date dal diametro scelto.
Speriamo che ti arrivino in fretta i vetri, così si iniziaaaaa
Hello and see you soon
MircoInterestingly Soo, thanks Giulio
I have to read it calmly and compare their results with the simulations I'm doing too
Ciao
MircoNo no you absolutely must use a tool from 41 cm, especially in the polishing phase and until a good sphere is achieved.
Even the initial part of the parabolization I suggest you do it with the full diameter.
Sub diameters require considerable experience and practice, in order to manage them.
I don't remember anymore, what focal length has the mirror?
Ciao
Mircohi Luca,
Ah ok now you can more easily explain that weird flaw to 3/4 ray. The’ tool must be full diameter, not so smaller than the mirror.
You absolutely must also make the tool of 41 cm, otherwise the mirror surface is not machined uniformly!
Then You will have to work with racing 1/3 COC (or ran of 13.6 cm) equivalent to a part-by-part overdraft 6.8 cm.Yes, right, I agree with Giulio, be very careful about the temperature that the water-plaster mixture reaches, during the chemical reaction that leads to its hardening. Unfortunately the effect of a sudden rise in temperature, I experienced it during one of my creations, and it wasn't pleasant at all:
https://www.grattavetro.it/forums/topic/primario-420mm-foca/page/2/#post-9845
On that occasion I had put a sheet of aluminized film (that from food) on the mirror surface, but obviously the aluminum being a good conductor has effectively transferred the heat from the plaster to the mirror, breaking it.
To minimize the chances of breakage, put something in it that is not a good conductor, type of parchment paper, or better 4-5-6-8 states of parchment paper. Or it can be as good as you thought you would, making yourself a concrete cast and using that.
Ciao
See you soon
Mircohi Luca,
:”I must also say that the tests presented previously were not so reliable since I took the image projected on the wall at a distance of 1-1,5 meters from the reticle.” aaaaaa here now I understand why the test images were strangely oval
From the last image, the edge seems much more riveted than it seemed in the previous images. What kind of rides are you using? Use an overflow of 1/6 of the diameter part by part or use an overflow of 1/3 of the diameter part by part? I ask you because it seems you are using too long strokes.
The advice is always valid to proceed first with the new realization of the support and see how it works, then we will make some other considerations.
hi Luca
good job hi Luca,
The plaster I used had kindly given me as a gift, my dentist's lab technician, and I can't tell you what brand it was.
However, in the village near mine there is a dealer of ceramic plaster, which also sells plaster for dental technicians. The type they sell to them is the: PRESTIA DUR n ° 1 of the Siniat, a lot from 25 Kg costs more or less 10-15 €, as an order of magnitude.
I have already recommended it to other friends as well, who then told me they found themselves very well. So if I have to recommend one, I'd tell you that. Then I think other types may be fine too.
Hello and see you soon
MircoI will try as soon as possible to do some simulations on the deformations, let's see how much they come.
A solution to test the mirror without having to hold it vertically, that I also tried some time ago, was to put myself to perform the various tests on the flight of stairs (I was on the landing eh), in this way the mirror remained in his cell and a little inclined, part of its weight like that, it was discharged into the supports below and not on the side edge.
Or you need to make yourself a dobson with twice as long pylons, so perform the tests with the mirror perfectly horizontal, from the eyepiece barrel Look when I made my record with dental plaster I had thought the same thing: “if a speck comes off, the whole mirror streaks me”, so I completely coated it with epoxy resin. Maybe it is also okay to completely cover it with vinavil, or in short, something that makes an adherent film that prevents any grain from peeling off.
Oh yeah, if the lines rotate from intra to extra it is probably astigmatism and you would be forced to return at least to the grain 600. But be careful because it could be fictitious astigmatism introduced by an excessive separation between the source and the returning light beam. Keep this distance as limited as possible max 15 / 20mm. If despite the precautions the lines still rotate, then it will be necessary to consider returning to the trouble.
But let's proceed step by step:
1) remake the most rigid tool and then you will evaluate / evaluate how it works.
2) run some new ronchi tests with the set-up nice aligned.
Hello and see you soon
MircoHi Massimo,
oooh very nice minimalist support
You can already see that it starts to reflect well, steps, flower pot etc.
The only point that makes me think is the meniscus positioned vertically in that way, which is great for a normal blank, but for such a thin meniscus mmmmm I don't know, I don't want it to flex too much. What do you think about it?
You really have a nice cat to peel eh? I can't tell you dome sphericization can be done by hand with a sub diameter. I did it and it's okay, working on the machine, but by hand I just don't know how to do it.
Ciao
Mircohi Luca,
I'm not sure I understand how you are going to make the new tool. If you poured tile glue over the mirror to trace the curvature, OK, the important thing is that you make it of an adequate thickness, 25/30 mm in my opinion. I've never used tile glue, then I recommend you have to let us know if it works well or not As for the ronchi, if the lines always remain inclined by the same amount and on the same side, passing from intra to extra, It's not a problem. It is probably due to the fact that you have not placed the grid perfectly vertical, so check both this and the alignment between the beam of light and the lines of the reticle.
The thing changes instead if you see them rotate, or change inclination from intra to extra.
Ciao
See you soon
Mirco Here I am hello Luca
Oooo now that there are pictures of the ronchi you can understand a lot more things.
I fully agree with what Massimo said, both on the shape of the mirror (no riveted edge, but raised hillock, and hole in the central area), is on the inadequate stiffness of your wooden tool, and on how to proceed.
As for the support, in my experience, the absolute best I found to be a disc made with dental plaster. Otherwise, a glass disc of adequate thickness or a multilayer wooden disc from 25/30 mm thick though. Is’ true that I had also used a marble disc (I had it at home for free), it worked great, but it really weighed a lot and I was not at all well when it was time to work with the tool on it, so I wouldn't recommend it.You see the lines of the ronchi rotate from inclined on the right to inclined on the left, or viceversa, passing from intra to extra focal? Compared to the light source (crack or point) at what distance from it the returning light beam passes? it passes by or over the source?
Anyway good, I also agree with Massimo's compliments, even my first ronchi was soooo worse than this!
Ah other thing, what kind of pitch you are using and what temperature is in the room where you work?
Another possible explanation, also for the signs you find in the mirror, it may be too hard pitch.
Ciao
MircoEhilaaaa great Massimo
I follow with interest
Of course it's a nice beast!
I can't wait to see the first ronchi
Good continuationhi Luca,
The tool looks well done, bravo the squares all seem equally affected by the processing. However, I confirm what was said in the previous post, I would try to work with all the components at the same temperature.
As for cerium oxide, no it is not normal for him to behave like this, yours probably got contaminated with some unwanted particles, you do very well to sift it before each process (I recommend the sieve must have a very fine mesh).
I have never had the need, but I know that some let the water / cerium mixture settle for a few hours, so that larger and heavier particles fall to the bottom of the container. I know somewhere in the forum or in the articles, Giulio had mentioned it, now I don't remember exactly where, I try to search. If I find the post, I'll bring you the link.
Hello and good scratches
Mirco
