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Non riesco a comprendere perchè le focali corte siano ambite anche in visuale… Mettiamo che io abbia uno specchio da 300mm e sia indeciso sulla focale. Tralasciando il discorso della comodità o meno dell’altezza a cui si troverà l’oculare, non è sempre meglio una focale più lunga? Visto che in una corta ci sono problemi di coma, di ostruzione superiore, di minor contrasto e di difficoltà di lavorazione, quali sono i vantaggi della focale corta oltre all’ingombro, altezza oculare e ingrandimento minimo minore? La luminosità è la solita, rather, avendo meno ostruzione dovrebbe averne un briciolo di più la focale lunga… dove sbaglio?
Non sbagli, anzi secondo me hai pienamente ragione
, pregi e difetti di una focale corta li hai ottimamente elencati , oltre quelli non ne vedo altri… ma credo che ci siano diverse “scuole di pensiero” a riguardo, vediamo chi la pensa diversamente 
The reasoning is all correct. The fact remains that all the characteristics deriving from the choice of the focal length take on a different weight according to the needs and preferences of each, and therefore the choice of the focal length is a highly subjective compromise.
In my case, for the first dobson, my personal compromise for construction was for me for one “definitive” 360F5 tipo obsession, because I considered important some things like the contained and pre-calculated weight of the parts of my project in view of a transportability / movability to the limit of my possibilities also future. That is, a telescope that also excluded the use of stool to reach the eyepiece; which matched the smaller magnification , useful for large fields that I like very much, without renouncing the planetarium favored by “big” opening.But then I also built the briefcase 250f5 and the 300f6 and finally the backpack 130f7….So the 360f5 didn't turn out to be definitive at all. ….At most, it is today that I am in condition “definitive” having available an optimal range of choice for all my stationary and travel observing needs by any means, from the hike in the refuges,to holidays in distant countries.
Considering that it was precisely for astronomical use that in 2000 I bought my first one “camper” wolksvagen westfalia jocker for holidays “hit and run” and others more extensive, useful for overnight stays in mountain and dark places without the need for reservationsOk, therefore it can be said that, removing the observer's needs, in visual, a longer focal length instrument would be objectively preferable but subjectively it may not be for x factors. To simplify, if one does not need very open fields or particular geometries to be respected, there is no point in aiming for fast focal lengths.
It is clear that for large diameters it becomes an obligation to stay low… making a dob from 60cm f7 would become geometrically a bit problematic
However, I noticed that a value changes by changing the focal length, all other conditions being equal. While the angular resolution remains the same, the calculation of the linear resolution gives a higher value with the lower focal length. Why? These are things that can be found visually?I would say more than doing a dobson “long”, the use of such a tool in the field is critical, because it needs stairs to access the eyepiece, or configurations with tertiary mirror, accepting the major obstruction.
As for the visibility of that linear limit, I have many doubts that it is visible until the magnification is pushed beyond a reasonable limit that does not generate the darkening of the image due to framing with the eyepiece of a microscopic reflective surface (…. and then dark).
Here, an OT thing, studying the obstruction due to the secondary and its support, I noticed that the main problem lies not so much in the reduction of light as in the refraction of the same, especially on the spider that generates those crosses of light on the image. I was thinking, but if in place of the spider or other squats, to support the secondary, I used a glass disc with excellent transparency, excellent leveling, a few mm thick and the use of a lens hood to avoid light sources coming out of the optics field, what problems could I run into?
To eliminate those diffraction artifacts it is sufficient to put a non-straight secondary support, that is, as a metal ring connected at one end of the diameter to the secondary case, and at the other extreme to support the secondary.
However that disturbance is overestimated and very limited, as described in an article in an American book (that I have at home) and that I can then send you.
Thank you, I would gladly read it… I have actually seen images of different forms of support that minimize this effect. Anyway, the glass experiment could work or in your opinion could give problems?
It would certainly work with a so-called “optical window”.
You could not use a common plate, neither thin nor thick due to problems of non-plane-parallelism of the two faces,
in addition to localized stress, and then you would find yourself having to make the optical window , which, like all optical plans, is very difficult to make, and very expensive to purchase if extended.in practice the installation of a transparent plate support for the secondary, which effectively preventing the mild diffraction disturbances of the classic support crosspiece, but by the very fact of being traversed by all the light of the telescope's optical path, would create you a sure free disturbance as an unwanted deterioration of the optical quality of the primary, it would be too counterproductive and senseless a risk.
rather for a long focal but completely unobstructed Newtonian reflector it would be interesting to build a SchiefSpiegler…. German double word meaning something like angled mirror.
see here:
In the end then I took a 300mm blank and 25 thick borofloat glass 33, so I'll have to make the tools… My doubt remains precisely the focal length. I was now sure of making an f5 but maybe an f6 wouldn't be a bad choice, it would allow me more enlargements if I decided to do even a little’ of planetarium. And on the deep, nothing would change me. Undecided.I agree with you. Moreover, an f6 is easier to parabolize , furthermore if any excessive length would bother you, it could be reduced by about fifteen cm with the stratagem of “low reading” like I did on my 300f6
This seems to me a good solution to take advantage of a greater focal length! From what you write there don't seem to be any optical or mechanical problems. So I wonder, why this solution is not used commercially?
The reasons are many, two of which in the first place:
1) telescope manufacturers follow the wave of the generic mass market that provides larger numbers of buyers than those in some sectors “niche”.
2) manufacturers of cheap telescpi are not amateurs using the products they sell, and it shows in the fact that they often supply accessories for “heard” , that is, of which they have not understood the real useful function , but which they implement equally in their production as “solutions” that in the end they are not up to their function (see the case of the fans on the primary).
However, there is a tendency for amateur astronomers “insiders” which shows cutting-edge solutions. And I'm certainly not referring to my work, but to the work of the authors of an American book on the construction of large diameter telescopes ma “lightened”, in which one of the problems beyond balancing and transportability, it is also the possible correction of a focal length “uncomfortable” without going into the major obstruction of inserting a large secondary mirror and a tertiary to obtain a “fuoco Nasmyth” low and comfortable.
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