THE FIRST DOUBT
Is’ our first mirror, We just completed the polishing stage and we have now reached a good sphere. The question at this point blend into the head of someone about to start his first parabolizzazione is: “really just do a little’ of “zigzag” to get to the parable ?”
We try to respond “practical”(*):
generally no ! but depending on the type of mirror that we are working we could get nearer or even succeed at all even with the simple “zigzag”, ie races “W”.
The deviation of the parabola from the reference sphere, increases with decreasing focal ratio for which, if parabolizzare a small diameter f 10 It means in practice that the ball and the parable are indistinguishable, decreasing the focal ratio and growing with the diameter we will increase this difference until reaching, for very focal thrusts, always highest values among the radii of curvature of adjacent zones, which is equivalent to an increasing deepening of the center with a consequent lengthening of the working times and greater possibility of error in the fitting of the parabola.
therefore, It is necessary to previously plan what type of techniques and tools will use on the basis of optical parameters, referring in particular to a completely manual processing without the aid of machines.
(*) the theoretical and technical explanation of approach to the dish requires a dedicated discussion, to refer to other articles, in general we can say that for a “economy” processing tends to approximate the parabola making it coincide with the sphere at a point on average within the zone to 70% diameter and “digging” both the center and the periphery until it reaches the desired figure.
TELL ME WHAT “F” FAI AND I WILL SAY’ WHO ARE YOU…
1 – Mirror focal ratio F>=6
Our reflexive, pragmatic and rational led us to choose wisely , as before processing, a mirror with a diameter “conventional ” between 150 e 300 mm, with long focal ratio, greater than or equal to F6.
The parabolizzazione will be required, but it will not involve particular difficulties, Everything will take place in a fairly linear and fast, we will not ( in theory, starting from a good ball ) make corrections and the final dish you will be reached by the sole application of W rides with full diameter. Our main concern will be given to try to keep everything efficient setup ( adaptation, hard pitch, cleaning channels, etc,, )
We can therefore safely begin and continue in the way “classic”, with ran to W and checking the performance with the test. Is’ can reach the very good results even in the first experience will be an important technical and theoretical knowledge, a field test of the skills learned, in view of the next engagement.
2 – mirror with a focal ratio of F6 and F5
If our nature is more geared to the curiosity and experimentation, probably we will be working on one of these focal preferring understanding of the problem in many of its facets to the certainty of the result.
Even In this case, the processing with the full diameter may be carried out until reaching the final shape, However, the sun ran to W will not be sufficient to ensure a good correction of the parabola, We'll have to be ready, in the vicinity of the final stage to make the necessary corrections to bring any areas in tolerance, with long or short rides , W compressed or expanded, tangential or central, localized pressure with or without, in order to go and change areas of interest.
You can reach an excellent level of quality and fix also the first experience, as long as you understand and know how to best use the analysis tools, which become fundamental to detect small deviations of the parabola constructed from the theoretical.
3 – mirror with a focal ratio between F5 and F4
If we chose a middle course between these focal as before implementation, then the “spirit of adventure” do not miss, the difficulties in our, They are the ones that make it interesting and particular each path, definitely we will not be bored, we are daredevils who do not escape to the most difficult challenges, even without any guarantee of success.
Is’ probably the most popular choice with the increase of the diameter, but it is also the one that begins to complicate things further.
The full diameter with the races to W, It will be used for the initial generation of the parabola, an expert hand, You can also complete the processing without changing tool, but it will certainly be forced to intervene with many corrections during the intermediate and final stages with the real risk of “points of no return” that force you to regenerate the ball for a new attempt towards the parable.
approaching F4, the sub-diameter may be helpful, if not necessary, in the final stages of parabolizzazione, in which the accentuated curvature of the mirror limit the possibility of intervening in the individual zones with precision, through the use of very large tooling.
The accuracy in testing becomes essential to achieve even a quality “acceptable”, while the excellent result remains within the reach only of those who have experience with previous achievements, less than real lucky breaks.
4 – mirror with focal < F4
If you have chosen one of these focal as before ( or second ) realization, maybe it's time that we speak to someone, starting from your childhood ! …I did it and I can assure you that I feel much better ! however, do not despair, the good news is that you can store the tool diameter full closet and gain some’ in the working space !
End of good news, we come to the bad:
Parabolizzazione The entire process must be built with a different approach, the mirror surface must be virtually divided into zones, in six, eight or even ten circular crowns that will be processed individually and sometimes variable sequences, with various techniques, until reaching the desired depth with only the sub-diameter, which constitute a “set” of various sizes ranging from 60% and the 10% the diameter, to allow all the necessary adjustments to achieve the sought parable.
As we push toward ever shorter focal, we enter the dark territory of processing abnormalities, the easy and sudden astigmatism, local zonal errors, of the wild roughness and unexpected furrows of the small-diameter sub.
The measures with the tests become more difficult, the reticle “thick” Ronchi with over 4 lines / mm will be almost unusable, the classic Foucault apparatus must be modified and optimized to allow easy reading and ( quasi ) reliable, the number of zones to be taken into account for the measures grows exponentially, ( your Couder mask to resemble a colander ) while the required precision is the maximum possible.
sconsigliatissimo , must be avoided as a first experience, but also to those with experience, the maximum attainable quality, It is often directly proportional to the amount of “side B” owned by the operator.
We can thus summarize the considerations set out so far, in this table:
|
mirror diameter (mm) |
F ( focal ratio ) | tool diameter | processing Technique | Suitable as before processing |
Grattavetro Profile |
|
150-300 |
6-8 |
full | W | very |
Rational |
|
5-6 |
full | W + Varianti | quite |
experimenter |
|
|
4-5 |
Full + sub | W +varianti+ zone | little bit |
adventurer |
|
|
< 4 |
sub | zones | For nothing |
problematic |
Thanks to this empirical table, each novice will finally decide which focal begin the first processing and simultaneously answer the most important questions: ” but I, that are grattavetro ?”