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Hello to all ,I read with interest a long time,and by dint of reading your “companies”
I also would like to begin the adventure of making an astronomical mirror.
Now I ask one of your own ,I have the opportunity to have 2 calciosodico discs in diameter
350mm thickness of 20mm or alternatively doubt 24mm.il thickness is 20mm on the cost are
70euro to the one and the 24mm to 4mm 110euro one more justify the cost?
I imagine that the 24mm is a measure quota mercato.ma what interests me is
the final result of the objective mirror.
thank you allCiao uraniborg. Welcome among us.
One with glass contruito mirror in strong thickness is less prone to the dates deformation by its own weight, while pointing the telescope to observe objects from the zenith to the horizon.But at the same time the glass in thick has a larger mass, and therefore it has difficulty to chase the ambient temperature.
Indeed prejudice hottest temperature ambienche that dives during almost all the nights of the year, It generates a turbulent flow in the optical path of the telescope , works , if not appropriately corrected in laminar flow, disturbs the very osservaziioni especially if high magnification planetary.
My mirror 36cm F5 has a thickness of 30mm for which a cell I realized in 18 support points. But a thickness 25 It would not change for the worse, species by checking the size and the number of triangular supports the cell with the GUI program PLOP.
I would see well enough then the disc thickness of the mirror 35cm diameter 25mm, but be careful that the tool of the same diameter could very well be economical and 20mm..perchè thickness in the machining lose its edge the same thickness that the mirror would lose the center.
The thickness comes to the aid to the fact that, As known, the machining tolerances of an accurate mirror to “minimum wage” of a quarter wavelength of the yellow-green light, I'm from 68,75 millionths of a millimeter between peak and valley of the residual asperities.
The mirror glass “thin” it is therefore more prone to warp (if not appropriately supported by a good cell) approaching quell'inifinitesimo tolerance allowed.Another important factor is the choice of the focal length of the mirror to achieve.
A focal F6 is very easy for a beginner, because the final parable almost confuses and deviates very little from the initial ball.
(See the John Dobson tutorial dialogues “narrated” in Italian, 400F6 for a present among the items here in Grattavetro).For example: a mirror with diameter 350mm focal F6 would have a depth of excavation (namely the so-called “arrow” the center mirror) only 3,64mm; And parabolizzare the initial sphere should only “Swsre” the ball itself to a value at the edge of the mirror alone 1,58 microns. (Obviously the flaring from center to edge must follow the fornula the theoretical parabola taken as reference constructive).
So in such a F6 mirror would remain a central glass thickness of over 21mm.
Even more difficult would create a focal F5, because the arrow would digging depth of 4,3mm, and parabolizzazione lead to countersink the ball to the edge of 2,73 microns, What's hard enough to perform with full tool diameter, which, however, ensures uniformity and the absence of errors excavated areas irregularly. But less than a millimeter in thickness at the center, it would not be much influential. (If I am not mistaken Mirco realized its 400mm in very thin, and he tells you his opinion).
Even more challenging (but not impossible) would achieve a focal length of F4.5, and a depth of 4,86mm arrow which, however, it would require the work with smaller diameter utensils (not recommended itself to the first carriers because of easy mistakes zone); and the parabolizzazione 3,75 microns.
It would be difficult to achieve a focal length F4, that richiederebe a 5.4mm arrow, more so with the sub tool diameter, and parabolizzazione well 5,95 microns.
With all this talk I do not know if I managed to not confuse ideas.
However, I repeat that in your place I would go thick mirror 25 and tool 20.Hello and welcome to Uraniborg Grattavetro !
I agree with Giulio, It depends on the focal you want to achieve and the cell that supports the mirror, but I would choose the 25 mm for the mirror and the tool to be 20 mm.Thanks your answers have been very precise.non I thought to be able to use two different thicknesses!Julius did not you confuso..anzi.sospettavo that the focal length would be decisive on the difficulties as well as I think my idea of configuration ottica.nella telescope
I would opt for a long focal,which I would facilitate me if I understand the construction primary objective,and also expect no secondary frequency of use,since I have not
the opportunity to move to dark skies,observe moons and planets,a planetary dobson say.
for the mechanical part,cell etc., I can handle having a small home machine shop,
lathe milling Excellencies forbid the project,It is denied with programs such as CAD and the like,non
I can only congratulate you on certain facts technical drawings from voi.quindi'd be on a 350MM F6
what do you think? thank you allI think it's the best choice, one that offers the best guarantees of a good result as a first experience, always keeping in mind that a 350 mm is not easy to work with a full-diameter tool, but I believe that the greatest difficulties will be precisely in the realization of a good fusion and adaptation of the pitch patina, but a once experienced and understood this aspect, the road leading to the dish is absolutely viable and rewarding, with the opportunity to go into the optical processing without major worries of mind, to experience and understand a technical background, various tests and checks up to treat yourself to the satisfaction of scoring a viewpoint with their own hands, as goes the famous spot, “priceless”
@uraniborg.
Just for …”you mouth-watering”..
Please note that a Dobsonian 350F6 classical Newtonian type would have roughly the shape and size seguentied…typically for many parts identical to those of my telescope; and another (except tralicio in diameter aluminum tubes 25×1,5mm) very similar to those seen in the image of my greatest tool (and as better stability and sealing “rocky” collimation):
https://s18.postimg.cc/92bxw1nxl/Dob360_F5_Govone_CN.jpgMaximum Dimensions:
the position of the telescope primary mirror aimed at the zenith) on 19 cm from the ground; My focal length = 1830 mm; its (350×6)=2100mm
Diameter (mine and yours) inside the secondary cell 380mm , and external 480mm
Focuser INTES with the eyepieces field diaphragm to a 297mm radius from the optical axis.Height of the center of the eye of the land of my, canvases pointing at the height = 1740mm
(If your F6 would 270 mm higher, but do not use a stool at the zenith, a focal point could be made between the 5 and of the 6, or tilt the secondary and lower the telescope, as I did in my 300F6 https://www.grattavetro.it/dobson-light-300f6-fase-2-trasformazione-in-low-riding/ ).
The telescope of this type is designed according to the so-called classic homonyms Dobsonian style American Obsession, made with proportions of the book Dobsonian Telescope (A summary of which some’ too terse, but interesting for a practical idea is here: http://www.astrofilirozzano.it/Documenti/Articoli/Lez.Dobson.pdf ).
Note that if you have friends who use CAD, and I write in the newsroom, I have no problem to send the CAD drawing scale DWG or DXF format 1:1 my telescope , from where it can freely entire any variation or modification.
I do not know where you are and you have the sky available, but a tool like this for me was definitive as soon as 2003, after the first (and finally)… poor… refractor 80F5.
I do not own the machine-tool, except for a small hole high column China 50 cm. so my building has great room for improvement, but it is the poplar plywood that is king
Thank you always esaurienti.Massimo is a life that I would realize “my” telescope
but before you the information and my knowledge were really scarce. I saw
Dobson in the video that you have recommended me the tool of the pitch is smaller lens
It is feasible for a beginner?a curiosity to the bench system using Dob to work the mirror is different from those using ex-ballasted bins,what do you think?
Giulio I saw your Dob is a fantastic achievement species tilted focuser is brilliant
I observe from Lombardy in a sub urban setting near a park I also used by other astronomers then planets and moon are my targhet.
I would be happy to receive your cad
health to allThe smaller tool but not too much, It is a viable option because it still ensures the extension of the processing to the entire surface, which it is therefore unlikely to be non-uniform area of introducing defects.
But I would say that John Dobson has used that tool as a fallback “economy” because he had already from previous smaller mirrors processing.
The bench is an alternative to stem, and I think it's just a matter of taste. But the dirty floor, It's mine “half the sky” he would not have accepted even in the laundry.
In fact, for example, I prefer to turn step by step around the stem, without having to remember, or forget) to rotate occasionally even the glass which at the moment is on the bench. which instead with my journey is as if constantly rotated.
I then usually not rotate from time to time but continuously in the opposite direction, glass in my hand right now…Also because fewer things I remember and I work better proceed, which also becomes a pleasant exercise for my “arms” by old.
I'll send the drawing as soon as I enter your e-mail address for private mail to redazione@grattavetro.it
… it is a life that I would realize "my" telescope
Uraniborg, Yours is a great motivation, The fully I share because it was the same as it did start to scratch even windows me…
And, says Giulio, slightly smaller than the tool performs the same functions of the full diameter, If you descend below the 75% ( more or less ) diameter are equivalent… to be honest some small difference, but easily fixable.
In any case I think the experience with the full diameter should be done at least once, because even if you can use other tools, the full diameter and its action with the various techniques remains the reference, the “standard” on which you can process and compare all subsequent variants for the tool, in order to evaluate and then make informed choices about the use of reduced diameters depending on the situations.
hello I am fought on the purchase of glass disks,because I would have the opportunity to have
a blank optical glass by former observatory thickness 50mm diametro350mm but since it is
It has been worked(focal unknown) I was wondering if you could “reset” or pianare for zero.scusate from the rough explanation
regardsI would say yes.
I would ask that measure the arrow to know myself a regular (who is not an F2 that would make me die of work).
With elbow grease and carborundum 80 I would put it face down on a flat plate by wiping 10 15mm or up to a flattening sufficient to obtain as residue an arrow depth equal to that which serves to you, to which then you begin racing 1/3 c.o.c. to correct the curvature to make your initial ballWell!
If so, with a focal length of 1900mm (that is, with the reflection of the sun at the best mirror center fire at this distance) the mirror should have a focal ratio F5,42. And then an arrow to deep center 4mm.
As for making a F6 to you just deep 3,64mm.I think you might get a tool in dental plaster cast on the actual curvature of the mirror, and of equal diameter, incorporating into casting a series of M10 nuts do you see here from the minute 6′ e 34″:
Keep in mind, however, that Gordon has to use his type machine “Fixed post”, whose tool to work should be approximately proportioned 30% smaller than the diameter mirror, (because with that machine the tool works in neutral, ie only driven by the differential rotation induced by the two diameters and tool specchiio)
For this reason he has to use a wooden ring to attack the retaining strip to the lower circumference of its tool.Instead you to produce your tool you wrap the containment strip directly around the diameter mirror.
How minvece drowning dice (cha've seen little to abrade) you can not put the nuts paste but after casting a series of stoneware tiles or pieces of glass on the curved face, as he did recently Mirco, and then you would have two alternatives:
1) apply racing 1 / 3d c.o.c. with an abrasive carborundum order to return towards the sphere of those 36 hundredths of a millimeter you too, and finally take your parabolizzazione.
2) L'alternative 2 but do not really like while being equally effective, It would be to pave those 36 cents on a planar flat glass sheet, as said previously, and after casting the tool to generate the new ball and then parabolizzare.
We feel the thought of Maximus and Mirco
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