of: Giulio TiberinI
The reasons are differents and subjective, and one answer "tout court" cannot be given if not entering a little in the topic. At our days (year 2015) , the simple economical affordability was greatly reduced for DIY of telescopes having current and popular aperture, ie up to the diameter 16 "= 400mm, especially after the arrival on our market of finished mirrors made in China that are now GOOD quality than that which characterized the first arrivals of a few years ago. Continues instead to be convenient, for the excellent quality obtainable, making a parabolic mirror diameter 300 mm with normal "windows" glass (calcium sodium) 20mm thickness normally available on the market “poor” not of the expensive astronomical blank glass, but normal glass calciosodico, that is, “Windows”.
Of course it remains convenient albeit much more challenging, DIY objectives. mirrors diameters of over 400 mm, but obviously proportionally greater thickness to 20 mm quoted. increased thickness of up to 30mm, however, only, beyond which the blank in thickness and cost appropriate, It remains the only choice.
(NOTE: Normal glass, namely calcium sodium, it is easier to find custom cut to size round, at the so-called “industrial Glassware”, that is, those who build glass furniture for furnishing and exhibition stands, as well as in glass walkable plans for the building, at an average cost around one hundred euro for two 300mm diameter discs, one of which will act as a tool and one mirror);
INDEED:
Today (7/2014) a complete tube dobsonian telescope Meade 300F5 costs 800 euro, or vice versa 1000 euro if I type portable collapsible,
The assembly of constructing a telescope only the structure and acquiring a mirror GSO 300-f5 + a secondary 70mm + the focuser, It would amount already 715 euro, leaving a margin for the structure comparable to 85 euro making up the difference with Meade 300F5, the purchase of which therefore remain the most convenient choice.
BUT THERE’ S THE GOOD AND THE EXCELLENT
There remains the fact that is not positive that a good number of great telescope:
– It has a heavy structure because it is made with economicissimo chipboard, although “melamine nobilited” outwardly with a pleasant and very thin surface layer of melamine “at sight”. And that is why for weight and encumbrance, an awkward structure to manage against the plywood used in Do It Yourself, that although poplar is a composite material, that is characterized by layers ranked with realtive to 90 ° crossed fibers between the layer and the adjacent layer, and therefore light and sturdy by nature; While the particle Board is a messy pile of crumbs of wood held together by a heavy plastic glue, being a material that despite his good appearance, It is by no means robust nor “composite”and therefore subject to deformation, simply because of the humidity encountered in time.
– also those GOOD parabolic mirrors manufactured on a large scale, quickly and unceremoniously because for the production of standard time is money, They suffer from surface roughness which decrease the contrast compared to EXCELLENT mirrors, polished by hand in a time much longer than the industrial production times, that However, other costs will determine.
This fact that the manual is better than the machining, debunks the myth that the "machining made" is better and more accurate than "handmade".
Indeed, for Optics is absolutely true the opposite, and the “Handmade” is absolutely better than the “made in large series machine” (at least to the exclusion of plane mirrors). And the “machine-made” does it just save time and money to the producer.
A FEW WORDS ON THE PURCHASE OF "NOBLE" RAW GLASS TO WORK :
Buying a blank glass (raw glass) , for the realization of a mirror from which recommend a thickness of 300mm 1/10 diameter, ie 30mm, It is made very difficult because of the thick rough measure, after the invasion of the Chinese with their finished products, It has become the almost unobtainable, and only in rare industrial glassworks. Then, if he be found, it presents prices "European" very disproportionate compared to today's price of the same type of end product (single glazing) Chinese. … But if one future day I arrived here round rough glasses ...”with "almond-shaped eyes"” …greatly change the discourse in the name of our convenience.
THEN SAVE THE CONVENIENCE, USING LESS NOBLE “VETRACCIO" (....windows glass)..
Convenience, however, is all right! For those who buy two glass discs (one for the tool and one for the mirror) 20mm in thickness, works , (barring rare exceptions to the above 30 mm), is the maximum most widely available on the thick glass of the international market "Float” normal calciosodico (which as mentioned above is the window glasses and walked on floors or bathroom furniture). In this case a pair of 300mm diameter glass will cost one hundred euro (but also less).
That price although cheaper than special glass blank, but still high in relation to the fact that it is only two discs 300mm diameter, price may lie elsewhere under, since it contains more of the recognition of a higher cost that the supplier at its discretion, do you pay to skip his processing series work, to realize for You a small supply of only two pieces, (but in my case they were cut and beveled (caulked) perfectly).
MOREOVER Normal glass is more convenient for the amateur astronomer for the following reason:
The number one enemy for astronomical observation is air turbulence; turbulence that can be of two types: generated by the environment, or vice versa generated by the hot primary mirror inside the telescope.
On ambient air turbulence (for English speakers “seeing”), C’ are little to do, and it is universally known that can be limited by choosing a grassy observing site, and especially with no roofs that convey hot air columns into the sky, dissipating the heat accumulated during the day, and making it impossible to observe stars through those salient currents of hot air.
The turbulence inside the telescope tube is instead normality for all telescopes, as the primary mirror is always warmer than the ambient temperature of the observation site. And even never you can follow “close” fast descent gradient of the night temperature, escaping from that of the mirror one, end not ever equaling them, or at most with appropriate artificial ventilation, only mentenendo tracking at a distance within a few degrees centigrade from that environment.
That minimum temperature difference can be made sufficiently irrelevant and not very damaging observation, If applying suction instead of ventilation for the “turbulent boundary layer” born from the contact air-hot mirror, eliminating artificially the turbulent disorder with its transformation into a smooth laminar flow.
(In this regard it might be useful to see the article Acclimatization of the optics )
The glass “boron-silicate” (of which Pyrex brand is the most widespread), or other equivalent or best type having low expansion coefficient, not its shape changes with temperature, but when hot, it generated within the telescope the same turbulence that would create a mirror of the same mass, in cheap normal glass.
Then with a hot mirror inside the optical tube we see indistinctly bad, either through an expensive mirror Zerodur or Pyrex, and either through an inexpensive ordinary glass.
This shows that the economic advantage of using glass with low expansion coefficient is not for the amateur do-visualist. But that advantage instead is absolutely important for the manufacturer of the mirror, for which the processing time is money, and therefore with a low coefficient of expansion glass may make qualitative optical measurements (that require a minimum accuracy of 68.75 millionths of a millimeter, namely the famous Lambda / 4) immediately as soon as they have finished processing for frictional abrasion that always warms the mirror, because that borosilicate glass is 2.6 times less thermally deformable than normal glass, that would call for eight hours of acclimation to perform those same measurements and optics corrections.
An amateur astronomer for whom time is money, Save in buying glass, buying normal glass, knowing that, however, he will have to wait eight hours “by setting and acclimatation for the mirror” following each retouching of the surface of the mirror session, before it can run quality tests of his work.
HOW IS THE CONVENIENCE ?
In this case, one dobson telescope gives 300 mm F5 could be built at a cost of 500 euro: (100 for 2 mirror glasses and tool) + 100 (abrasives and mirror construction equipment) + 99 (secondary mirror with its support) + 50 EUR plywood + 100 EUR screws and bolts and aluminum tubes). But that savings 300 EUR requires much more effort than simply purchasing the complete telescope, commitment but a harbinger of enrichment of knowledge and satisfaction of great human and moral value, as well as technical.
EPILOGUE: GENERAL OR THE CHOICE "QUIET".
On average, the most common and most tranquil,, which focuses on the modern reluctance to face commitments, rather than an open-confidence in their ability, It is to move towards the purchase of a complete Chinese Telescope, having good objective diameter (for example 300mm), use making good use to see if the astronomical hobby is kept alive in their own interests. Then, like (PERHAPS) it happens naturally, over time, and with it the inevitable increase of the interest and the requirements in terms of optical performance, It becomes stimulating the desire to move from performance to those EXCELLENT GOOD re-appearing in hand and super-polishing its mirror, or creating another bigger….(strong work perhaps motivated by the desire to quit smoking, or want to get out of a deeply regret engaging in something that deeply involves physically and mentally, and therefore distract).
MAKE A DIFFERENCEIN ASTRONOMY IS STILL ONLY THE OBJECTIVE DIAMETER
The move towards a lens diameter of about 300 mm for a home telescope is justified by the fact that in astronomy objects to be observed are far away and very dim or low light, therefore necessary that the telescopes have large diameter lenses, in order to capture more light as possible of those objects, Because is with that greater or lesser amount of light that is rebuilt the image more or less bright and detailed to observe.
DEMAND AND LAWFUL ': BUT THEN THE MAGNIFICATIONS DON'T COUNTS?
This is the common thought of the layman, that misconception that is decisive the number of magnifications which can provide a telescope, as if any instrument could furnish in endless amount. But this is not true because a telescope can provide magnification only proportional to light his objective aim is able to pick up and concentrate on reconstructing the image at his focus. And the uptake of light is greater than the higher is the lens AREA (lens or mirror that is). So the higher magnification useful are the prerogative of large diameter telescope.
In fact, zoom in a little dim object means go to observe an optical surface tiny, whose brightness is a tiny fraction of the total light collected, and is soon to provide an image too dark and details not noticeable because it mixed and confused.
REFRACTORS SPOTLIGHT AND OBSERVATION AND PLANETARY
So here, with regard to the diameter of the objectives, the higher magnification and the details are easier to get for very bright objects, like planets and the Moon, but for deep sky objects, such as distant galaxies, they become generally only possible with lens diameters belonging to telescopes REFLECTORS, that with a single large optical surface consists of a simple parabolic mirror, are ables to capture much more light to form the image, how much light can capture diameters on which stop the refracting telescopes, famously formed by Lenses.
That increased amount of light captured by a mirror of large diameter reflector (learning to handle the turbulences), is able to show an infinity of details very fine also on bright planetary surfaces, that they are seen in many other major dimensions given by other major enlargements.
The amateur REFRACTORS stop mostly at diameter 150 mm, for their high cost that in that case arrives already beyond the thirty thousand euro. Cost due to manufacture of the four, or six optical surfaces, belonging respectively to their two lenses (for achromatic), or three lenses (for apochromatic).
(Alternatively: For example, a commercial Italian dobson reflector diameter 711mm F3.1, , it costs three thousand euros less than a 150 mm diameter apochromatic, having a single machined surface. However, thise surface presents a captation area 22,5 times larger, making visible 20 thousand galaxies…. against nearly a thousand visible through a lens diameter of 150mm).
In the objectives “apochromatic” are completely corrected the natural chromatic aberration flaw, that belongs exclusively to the REFRACTORS, whose lenses, (like prisms), decompose white light into its component colors, that are focused on different points of the optical axis, in function of wavelength of each different color, mixing and canceling the finer details of the vision.
While their smaller dimensions and weights, is an advantage for easier engine required for use in astrophotography.
The undeniable advantage of REFRACTORS lies in the dimensions and weights contained by the limits of their own always relatively small diameter, that while the heavy lack of a low resolution and useful magnification against very large reflecting openings and performance, present with this an obvious less sensitivity to turbulence, and do not require the user contented by their modest performances, to learn how to manage the necessary control to achieve very different major satisfactions obtainable with big reflecting telescopes.
Reflecting telescopes that by their nature are exempt from lack of chromaticism that afflicts the refraction. And if large (especially if they are placed in frames type DOBSON), become for their size and in spite, telescopes voted for the most part only to visual observation.
OBSERVATION OF THE DEEP SKY
If, however, it did not apply a strong magnifying, because of feeble brightness of a distant galaxy, this would still be observable “solo”; or vice versa “better”, with a large diameter telescope, Thanks to much greater light quantity from it focused. While in telescopes even slightly smaller, far that object would remain invisible, because not "detachable" from uniform “background” nero (or…Alas ... gray background due to light pollution) of the sky.
For example: A telescope that possesses a diameter lens 15 CM has a reflecting surface area of 177 square centimeters:; While a telescope diameter 40 cm has a diameter which is 2.6 times greater than the first, but its surface is 7 times greater than the first, and also its performance in “remove” from the bottom faint objects sky, they will be 7 times more.
This suggests that supplied useful magnifications, but also the amount of deep sky objects that are made visible by an instrument, are prerogative of large diameters of objectives, that they are usually easily Constructible with a simple Newton mirror in a mount type Dobson.
An indicative comparison between telescopes objective diameter increasing, is listed in the following table.
In conclusion: As with all decisions requiring a commitment to new experiences, the choice to act in a sense (purchase) or another (manufacturing DIY) it is for each of us, which remains the best advisor of himself….Not for nothing, believing and honoring their own choices, in our Society is recognized as “style” of a person.