Primary 400 / F 3.2

[Massimo Marconi]

An amateur astronomer friend, an excellent self-builder ( which was also a good grattavetro ) had in a closet from approx 15 years a mirror still to be aluminated, it wasn't hers, it had simply been left there, as a reminder of a work on a 400 F4 di “vetraccio” 19 mm, made completely by hand ( including glass cutting ) which unfortunately had not had a good result and which had stopped in the final phase.
The blank was poorly finished, the processing stopped at almost 1 cm from the edge and asymmetrically, scratches on the surface and figure ( I have the portable ronchi-phone ) it was pretty bad.
Knowing me, he will have understood that just seeing a mirror abandoned and forgotten by everyone was a cause of deep sorrow for me and he said: “you keep it”
therefore, I brought home the blank stray, I cleaned it thoroughly and gave it a good one “kennel” where to settle down with the others “puppies”
I like the idea of ​​giving one “second chance” to things, and I would like this piece of glass to finally see the light together with some observer who is satisfied with it , as a kind of symbolic “revenge” in the name of everything that is discarded and abandoned because it is deemed inappropriate, not of value… but the value is not a default property of things, but it is the fruit of the passion and work of those who build them.

Then, I started working it to extend the curve to the entire surface ( now has an effective diameter of 396 mm ) and digging to add depth, enough to bring it to the focal length of 1300 mm, so that you can use the entire set of tools already adapted for the 600 a meniscus that has this focal length, to carry out the two jobs simultaneously and with the same processing and analysis tools already set.

Now I'm at the grit sanding 400, the radius of curvature looks correct, next week I plan to start polishing it and hopefully get a nice parabolic out… so let's see how these short and thin focal lengths go also in Newton configuration.

In the photo the mirror while working with the 120 and with the edge not yet finished.

[Giulio TiberinI]

Well.
Philosophizing, I have always been a possibility by my nature, especially in technical matters, which have always been my job.
I remember that in the factory, for a general and historical assumption, it was believed that neither we nor the competition, a certain product could have been made….until we succeeded by opening up a new market.

I therefore believe that an enjoyable use of a large aperture and short focal length telescope cannot be excluded a priori., only because a high Strehl cannot be achieved with manual processing.

it is therefore important to be able to verify “in economics” what limits would arise in visual use; both caused -for the “alti” magnifications- the quality of the surface machined with sub-diameter tools; and both -for low magnification- caused by the curvature of the field given by the considerable depth of the arrow.

As a possibilist “with your feet on the ground”, I could certainly be wrong, but in conclusion I always have in mind the fact that it happened to me, that with a large aperture it is easier to see otherwise invisible objects…. such as happened with the NGC 2359 “Thor’s elmo”, which makes a huge difference regardless of magnification, if you are looking for it with a 300mm or a 600mm.

[Massimo Marconi]

It is clear that a f3.2 Newton will be used with a coma reducer, even a mirror “perfect mirror” with the focal one it has a very small corrected field.
Giulio, because you say that a high Strehl cannot be achieved with manual processing ?

[Giulio TiberinI]

I'm not saying you can't, but as we know it is very difficult to achieve that precision of the parabolic curve over the entire surface with tools that are not full diameter, especially on large mirror diameters. This is done with professional machinery on large and very large mirrors.

But then… in my possibilist view of my amateur level, I wouldn't even be interested in reaching a very laborious high strehl.
Or at least on the scale of interest, I believe the feasibility of the opening prevails, because I think that the quality of the workmanship would be instructive to check the progress during processing (as I did out of my curiosity), placing the non-aluminized mirror (as it is in the parabolic phase) inside the structure of his telescope, to see on the Moon and Jupiter, with an eye on the progress of work.

In that case one would perhaps understand quite well how many enlargements the state of the work in progress can tolerate, because the elevated strehl could be a more not strictly indispensable….I do not know if (as a possibilist) made myself clear

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