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Thanks Giulio, we hope to get to the bottom but not so memorable, at least in a satisfactory manner.
problem Foucault, Who better than you can help me solve this doubt:
We have a very short focal, modifying the tester in slitless will surely have the advantages but not such as to allow a certain reading of the shadow zones.
As was the case at F3.8, though not predominantly, the area (7 in this case, equivalent to 9 a mirror with unobstructed 110 mm Secondary) in the mask will not have, because of high curvature, a uniform darkening, the outer part of the same window does not obscure simultaneously with the inside.
This aspect will certainly be more accentuated in this F2.6, the question is: how to behave ?
In other words, It is trying to assess the darkening simultaneously in the intermediate area of the windows ?Okay that we should try to get closer to a correct surface of lambda / 4 and then proceed with the caustic test, but the risk of not being able to interpret foucault is real…
well Massimo. In fact one F2,6 mirror I think with Foucault give a hard time, It is seen that it gives already a “normal” F5. Surely the evaluation of the central areas of the windows is already what is recommended in practice to do so very critical, to evaluate with Foucault.
In these days, wanting to better understand, the operation of the caustic test, which should be ideal for very short focal lengths, I used as a guinea pig my 250F5, after changing (very boldly) my tester.
The boldness is due to the construction of a new sled orthogonal added above the trolley with the Foucault knife.
The new slide has the great advantage of being cost-free, and mechanically reliable for its lightweight nature, scorrevolissima and assolutamene free games, and allows a 35mm stroke (so is the large-writable surface of a CD-ROM), ie as many mm is required to move the laser diode which must “to write” With millisecond precision microscopic tracks on a CD-ROM.In fact, the quality of the slide is adequate, I disassembled having it taken from a CD-ROM driver from one of the three picked up free of charge to the ecological technology of the landfill waste of my country.
The trolley is pushed by a micrometer and is equipped with the apex column with the only ocular reticle with which possess (It is a former microscope eyepiece), but which it has a focal length of 31mm well (provides only 8 magnifications) when it would serve to magnify 28 times, with focal recommended 9mm.The ocular door post is provided with new fissure laser cut and True quality “scientific” from 10 micron integral with the carriage. (years ago I had made to give some parts for cracks 10 microns from the quantomero maintenance technician with whom did the chemical spectrometric alisi of steel coils raw material for the manufacture of welded tubes which were my “world” for over thirty years)
With this Amba-Aradam I played a little with the 250F5 in the Couder mask 7 zones, and with a caustic pseudo from 9 zones.The Couder mask shows the eyepiece in place intentionally out of focus position, as many fissures are the windows.
Fissures that with the spestamento longiudinale trolley tester, They are approaching each other to overlap all merging into one perfect image in the focus of the parabola. (The catch lies in the fact that I have tested a finite mirror and very good, ie with all the overlapping images sole fire).I have not tried then to “to measure” the lateral displacements of each window taken individually as it should do with the caustic test. But I realized that even with this test will be difficult to identify the slit perfect focus reflected from that which is the important central area, whose unfortunate misjudgment would extend to all measures (…as in Foucault).
At that point we go again that (at least with my ocuolare providing 8 against enlargements 28 Recommended) a shift in any central area, It does not make you appreciate difference (unless you make a line on the LED head, or vice versa …else much better (I thought) would be inserted between the LED and the slit a lattice cannibalized by a microscope by 8 EUR plastic, which presents a millimeter divided into tenths, taking care to make a way that at least a dash of such groomer is visible from the eyepiece through the slit (also only transversely, that is, to divide the net fendditura into two sections), and thus facilitate the unique identification of the focus of the LED head or the hyphen, through the vision eyepiece..
So there is to try and meditate, but I think in the end he will be in charge, as always we are working on it.
And, the trials were planned and surely will not wait, then all you are experiencing now with the caustic in good stead, so well put in quote my river of explanations / support requests, how long ago did Foucault
Also I am organizing the tester for sodium and will use an old mini-lathe , removing the parts “superfluous”
It remains the system for the movement that is precise as well as stable, at least after over twenty years I will start to use it 
Looking at the glass half full, The positive side is that I have a central location to be evaluated, moreover my measurements start from the edge, the periphery is by far the most important part in this scheme, In fact, over the obstruction of the secondary we have to consider that the hood of well 135 mm , which in this configuration it is essential. It means that the measures and correction “guaranteed and certified” It must be performed on the surface of actual use, ie almost “half mirror” up to the edge, at least we eliminate the problem of the infamous evaluation of the first central area
A few photos of the experiments made on the approach (..from afar ..) to the caustic test without pretension precision, but just to “I realized when I realized”
what does not work, or what I did not understand, and what surprises all.
tester views 1
tester views 2
tester views 3
Setup generale
A Pseudo Couder 6 zones
B Pseudo Couder 6 zones
zones 4 in eame
zones 4 split because out of focus
zones 4 unique because it focuses (..not in the picture)
A-Mask 9 zones
B-mask 9 zones Add the
filmatino of how moving the cart longitudinal images of 6 fenditire dates from 6 zones, flow from posiz.extrafocale, to a single image to the mirror fire, and more than slightly intrafocale (It spoiled by the fact that the camera mounted on a tripod apart, He did not follow the removal of the carriage from the lens).Very beautifull ! the tester I want it too
. Giulio where you take them those spacers ?
There is one thing that I do not back and that I would understand:
we are with the source of the bending radius, or on the stove with the infinite source, then in this setting the mirror areas ( if parabolic ), which have radii of curvature different, ranging in focus at different distances . If it it is clear that the opposite windows of a same area share the same radius of curvature and then the fire, I do not understand why in the video all six areas converge into a single fire as in a spherical, What is escaping me ?The tester is very patchy, and it is still to be seen if for its mechanical deflections could be used with the minimum accuracy required of 10 microns (I highly doubt).
What spacers? Those under carriage are 4 screws / washers / nuts.
The mirror is my 250F5, I take me off guard and I do not know. But who would venture a bill I think is the apparent overlap of zones, and an account the exact focus of the caustic component of each area, that from my terrible movie is in no way identified.
However, it is a good question !!
“spacers”
a bad misnomer I definitely used to indicate comparators grads ( They are so called ? ) for measures. I also think that it is the apparent overlap of zones and non-fire, by virtue of the fact that the caustic of a F5 is not extensive.
I was a little’ perplexed by the video, they seem to go perfectly in focus, and then I wondered where was the caustic.About this is interesting to note that caustic equivalent spherical mirror ( with infinite source and hence parallel rays) It is calculated simply with the application of the fundamental laws of reflection, just a normal CAD for precise measurements of the focus of each area.
( on the contrary, the particularity of the geometrical reflection in a sphere with source in the center of curvature is that each incident beam is perpendicular to the surface, and then returns a “perfect focus” )
It would therefore be interesting as occurs, comparing the obtained values of the two caustics, with spherical mirror on artificial star ( or calculated with CAD ) and mirror parabolizzato with source in the radius of curvature, which (I think) They should vanish in the longitudinal values on the optical axis, just because we go with parabolizzazione eliminate “original sin” of the spherical aberration which is known to converge the reflected rays from infinity not at one point but in a caustic reflection.Cncordo with your reasoning.
As regards the two Micrometers centesimal are cannibalzzazione by dismantling the screw in normal Palmer electronic clutch of Mytutoyo, that plant is often harmed and were replaced for ISO9002 quality reasons, My other “hat” professional.
..Azz! Him. It is a tool that “gets better” the vision of things. But do not overdo it!
(I am also Astigiano adoption…not so abstemious!This caustic test I like more and more…
I also was using a similar tool, only mine was red color and fruity aroma I imagine that the visual aid is particularly in “doubling” the images of the slit…
(Now I understand why many have recommended that further tests at a later date )
Ciao, very nice your tester Giulio, It looks beautiful and precise sound…
And this caustic test seems very interesting and promising.
In fact I have also seen it in some tests I did (although not for the caustic) that is not so simple to find the exact point where the slit is brought into focus, but I assume that as in all things with a bit of the operation training should be easier.
In this regard I found online this:http://bobmay.astronomy.net/DAFTtest/dafttest.htm
Although we are talking about a Foucault test version to try to improve the reading zone, I think we can extend the same principle for the caustic test. Essentially using the same principle that is used with the Batinov mask when you want to find the exact focus of the telescope to photograph…I do not know if it can be useful…
It is the principle of perfect focus is similar to what is applied by using the simple Hartmann mask with three holes at 120 °; or that more complicated Batinov.
The purpose of both is to create a number of diffraction rays (spykes), that when they are born they are a sign of a more precise overlay of the windows of which masks have.
The fact speech is rather interesting.
Many years ago, mask it built a Hartmann, three holes at 120 ° for my oldest “ottantino”, carving a plastic lid barley soluble.
By following certain instructions had practiced the three triangular holes precisely to give birth spykes those that do not arise if the holes are round.
But I believe that there is a difference between the focus of an object at infinity and the focus of the enlarged slit looking from the center of curvature of a circular crown of reflective surface.
This activity requires a greater accuracy than Im “a naso”, hardly arising from judging the focus of a dark crevice in strong backlight, and with a very small depth of field (found in my experiment).
my experiment in the slit is professional and has a width 10 microns.
The eye that I used has a focal length of 31mm and not 9 as required by the test description of the caustic on the book by Lecleire.
The magnification of my 31mm is 8x, whereas with a 9mm eyepiece would jump to about 28.Reason why, with my 8x further magnified by the camera display, I see that a microscopic dash that passes through the slit would be readily identifiable and perhaps well focusable in her pretty evenly illuminated field.
For that probably in the future would like to try in some way to put it in front of the LEDs directly behind the slit, a few bits of microfilm (which now do not even know where to find, if not cannibalizing a microscope reticle), in such a way that at least a dash imprinted on it and the light passing through the slit, can be focused in objectively perfect way.
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