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Today 5/8/19 made visible again all the images accompanying this discussion (I change address from postimg.cc to postimg.cc)
Today 5/8/19 I made visible again all the images accompanying this discussion (I change address from postimg.cc to postimg.cc)
This addition is due to the fact that, I noticed, that the mirror diameter 600F3,8, not finished in the document referred to in this link in the first post of this thread, He has had a following and a term with a lot of tests, it seems to me interesting to read:
After a long downtime for metabolization of several objective difficulties encountered, despite the technical work of amateur astronomers respectable, It took place the resumption of parabolizzazione until the end of the mirror, with related tests
http://www.astrosurf.com/magnitude78/telescopes/T600_site_V2/miroir_fin.html
Alas, however, it is Foucault tests ... which unfortunately for his fundamental physical principle, it is not appropriate to short focal lengths of large diameter mirrors:
A test "Caustic" probably would be less subjective (as "caustic" is A unique bright vertical bar that presents the clear absence of diffraction lines, it is easier to identify than the "flat" shadows to match and evaluate on TWO windows of the mask Couder, with Foucault test).
But mostly the "caustic test" is born to replace Foucault in achieving short and large diameter focal ratio mirrors, because the center of curvature of the areas of the short focal lengths and large diameter, It falls only optical axis for the first zone (center mirror); while for all other areas of the center of curvature it falls on the curve "caustic", which it has the shape of a trumpet pavilion, and that always goes farther and farther away from the optical axis, as much as the diameter of the mirror is greater than 300mm, then gradually moving away more and more from incorrect data supplied by Foucault, as they measure the drawdowns of the zones gradually towards the edge mirror, where tolerances are always the most restricted.
The optical axis is instead the place taken as a reference by the Foucault test, because of focal ratios equal to or greater than F5, the discrepancy between caustic curve and the optical axis is very small and negligible.
Some info on the caustic test is here: https://www.grattavetro.it/test-della-caustica/
I suspect that one large mirror and a lot “Fast”,realized according to the Foucault, always give the star test the suspicion that there is something wrong in areas beyond 70%, and that there has to content.
Apart from anything else, It is very interesting for any "grattavetro", download the Excel file titled "carnet des retouches", with at the bottom of the final link parabolizzazione.
That Excel spreadsheet describes each of the numerous tweaks implemented, specifying action, type of tool diameter sub, and processing times, in addition to presenting the results of the subsequent tests Foucault.
As always in magnitude78 things are very instructive and interesting to meditate upon.
You will see that the advantages of a traditional type dobson (I mean model obsession) They are enormous in terms of movement and openness without inpuntamenti shots; How tight collimation and a ragolazione of insensitivity to spilling a variation range of weights to change eye / accessories, given by the possibilities of mutually alienate 2 Teflon plugs of the sliding crescents.
The fact is that for a F6, which is “long”, It should be a primary cash more “high” to have greater margin of balance with the trestle along; which is counterproductive to the portion of altitude you will encounter the eyepiece , rising to about five 80. I for my 300F6″ lightened (where the impossible to balance the experienced disc brake), then commuted the low-riding for the comfortable to the eye 1,75 meters of my eye.
But in a lightweight construction it does not help much (…but without powerful technological means it is difficult) find the best compromise between increasing the height of the primary case; Ocular final height from the ground; and diameter crescents.
Increasing the diameter of the crescents increases the height of the tele, but it has the benefit of balanced given by the movement of the breech of the telescope, which should aim to achieve an increase in the weight of the primary mirror, exactly as it increases the cosine of the angle of inclination of the cosine puntamento..Questa is the rule that eliminates any additional counterweight….and no technology is not easy to calcolo..se not for experimentation and approximation.
Sorry I'm late Stefano (I'm abroad).
The solution with Solidworks is definitely the most comfortable. And having the ability and knowledge, should certainly use it.As for the diameter of the crescents, a method would be proportional to the data of the other telescopes considered from the book of Kriege.
I remember that in the book there is a table of diameter 15″ in sù..ma I am out of the house and do not remember the proportionsIf it is worthwhile to calculate the weight of the truss in aluminum tubes, you can download the catalog PDF aluminum with meter weights, This from my supplier site.
https://www.comefimetalli.it/If you can be of interest to get an idea: My Case secondary 360, full of seekers and eye, weighs a lot…or about 5 kg.
Which they require a balancing of 25kg, of which 20 are balanced by the balancing and the 5 remaining from 5 by counterweights (one of which hung behind the counter of the primary, is the field lighthouse which I converted into power supply 6 – 12 volts of my fans).It lightens the case of the secondary, whether to join the two discs of wood of its structure, using the wooden framework SAMBA 30x30mm (which is located in brico), it weighs 350 grams per meter.
Mine is heavy because I have to make recycled as spacers 4 pieces of aluminum tube Ø25×1,5mm, surplus trellis, joining them with M6 threaded rod and cap nuts.Hi Stefano.
Happy Easter!!….I am happy to reread!Since yours is a 300F6 while my biggest construction is the same as why the 300F6 has a 1800mm focal length equal to that of my 360×5, I can as an alternative, just give the measures of my calculations to see if you'll make the height from the ground and are acceptable dell'equlibratura.
Unfortunately, the mechanical moment resulting for each component element of the balancing lever dobson is very variable depending on the focal length, that is the one that determines the length of the arm of the weight forces; and the weight itself of the constructive elements, which it depends on the type of material and its thickness chosen or available.
So for the equilibrium calculation I do not know a different method from identifying a hypothetical fulcrum, do the calculation according to what, and then adjust the backward calculations by finding the equilibrium point that feels good.Beware, however, that the calculations should also include the focuser; il cercatore ottico e quello Telrad o meglio il leggero quickfinder, the cloth….(But no more scrambling too because the correction can be done with the counterweights).
Since the relationship 360mm diameter mirror of my 360F5 (calculated in proportion to the book table Kriege) mi dava 380 depth of the primary case (and because of my experience of installation and testing, into the structure of my 360F5, 300F6 of my mirror as your, first to build him the kind of light structure, without focusing problems), I would try starting with calculations assuming a depth case the primary by just under my…. that is, for example, 350 o 360mm, doing account that the fulcrum of rotation falls to the centerline of the upper part of the case.
All this if you have not built the case of the secondary…(because it would go wide as the primary case for convenience of a straight truss. And a large 350mm case would make you a bit lower and perhaps more comfortable ground clearance eyepiece).
also calculates that for example in my 360, the distance from the ground of the primary case is 130mm…(then the fulcrum of my crescents diameter 520, I know also located at 520mm from the ground).
On the basis of the depth that you choose, You should try to calculate and sum the products of the weight force of each element acting on each of the two sides of the fulcrum, multiplied by its arm, to find what is the total from both sides, and for comparison correct the position of the fulcrum by lengthening or shortening the depth of the primary case, calculating backwards which is its equivalent value to the other side of the lever, to find the balance.
(I do not know if with my chatter I managed well to confuse the ideas)
I know a little vacuum technology because I took charge of the construction of large electrical transformers in medium voltage, and transformers for high frequency induction of steel tubes welding generators. All vacuum impregnated in epoxy resins of the first and the second silicone to eliminate and blowholes in the insulating castings.
But the required vacuum was not as driven, and it was already obtainable with common to oil ring vacuum pumps.
I also know the molecular pumps that operate at “Rain”, molecular exploiting the removal by shock of the few molecules remained by the vacuum in oil, necessary to achieve very vacuums.The rotating plateau on which the mirror is installed is interesting!
I don't mean much about empty autoclaves for the storage of alumination.
You can describe how the photographed one works?A tip that comes to mind, in order to better evaluate the edge problem, is to blacken the caulk even with an indelible marker, that slightly attenuates its annoying and disturbing reflection, or to cover it with the external windows of the Couder mask well in contact with the mirror.
Another suggestion is to see with your software the Millier-La croix diagram which in practice is how the segments representing your areas are positioned, towards the “"trumpet bell"” of machining tolerances (which are loose in the center but very narrow at the edge).
Finally if you want read here an always interesting situation:
https://dobsoniani.forumfree.it/?t=68985880Bravo Fabio!
I'd say you have a nice one “mano” light and effective, because you did a good job of not easy, cautious positive progression, greatly improving the P / V ratio. So with yourself 32 nanometer error, also the bord rabattus I don't think will be of great disturbance to the star test.hello Fabio.
Your removal process is impeccable dell'alluminatura.Just to make me an idea than my work on F5: A mirror 250 F4,8 has a measurable arrow in the middle of 3,32mm against 3,18 an F5, it's a “countersink” parabolizzazione that of the edge differs from the sphere of 2,25 microns against 1,99 an F5, then 251 nanometers high. (a quarter of a micron).
I think I had to do the job, to realize the trend, I would first Test at Foucault test the actual focal length that currently has your mirror; Test which also provides the amplitude indication of the area due to the parabola, and also it indicates the amount of error in the existing nanometers.
I suppose that, not as I had no experience in the use of sub-diameter tools, and given the current state of the mirror already too deformed (if I understand it already suffers from an excess of flaring that requires you to work a fairly extensive area, not to add or modify the working depth, but to go back towards the ball…. but possibly to stop ” halfway”, that is, the parable) I would try to use a tool to full diameter.
If you were to use a sub-diameter, It would not change the procedure for the verification of working time to apply corrections in, compared to the full diameter.
In practice as first responders (guided by the technique described by Massimo), I would only two rounds of table without exerting pressure on the tool and well creamy cerium oxide. To then repeat the Foucault in order to physically see the difference in nanometers resulting from the processing mode applied in the affected area.
We also hear what approach would you recommend Massimo.
