Anyone using a terrestrial or astronomical telescope uses it to enlarge, ie virtually bring an object that is at a great distance, to observe it in minute detail as possible to their own observation instrument.
A telescope is capable of receiving the parallel light rays that originate from the object under observation is, to form an image of the object in its focal point.
The formed image is very small, and, therefore, to be observed it must be enlarged with lenses contained in an eyepiece, and the number of such magnification is given by the ratio between the focal length of the lens and eyepiece.
But enlargement is not infinite, and in bright and pin point subjects as the stars , It is limited by diffraction, for which a star is punctiform view as long as they do not arrive at the limit in which the diffraction divides the image in the so-called diffraction notch, It constituted by a central point which concentrates more than 80% of the light, surrounded by the alternation of light and dark circles of decreasing brightness. And the more the star is bright, and the more the central point will appear larger than the surrounding fringes.
The magnification limit in which the point image is decomposed in the diffraction notch, It is very low and easy to achieve in small lens diameter telescopes. But the diffraction notch becomes much smaller than the diameter and the resolving power grow, thus requiring to be seen, a gradually higher magnification.
The vision of the diffraction notch in the larger-diameter telescopes is therefore conditioned by the absence of turbulence (commonly called and graduated as “Seeing”) both of the upper atmosphere, that within the optical path of the telescope, if the latter participates in the worsening of seeing that most degrades the observation at high or very high magnification. thus pushing Observation which becomes impossible when you can not see the diffraction notch because of the chaotic seething star light.
The internal turbulence to the optical tube is correctable, however, because it is caused by the difference in temperature between the glass lens and the observation environment, and then curing the acclimatization of the optical, it makes it possible to push the magnification up to make the diffraction notch visible, and with the vision to have the confidence to raggiugere maximum sensitivity to the resolving power of the telescope, and better visibility of the smallest details.
The magnification of subject other than in pin point is limited from darkening of the image, and it is much less disturbed by bad seeing.
Done intuitive because if we look at the object at high magnification, its small extension will fill the field of view.
But that tiny extension will bring with him only his minimum fraction of light, with respect to the light picked up by the entire lens of the telescope.
From this fact results in two observations:
– That the greater the magnification, the greater is the darkening of the image, up to be unusable.
– Whereas, to remedy those problems you need to use a bigger lens, that collecting more light, will increase the fraction of it reflected from the small extension of the field of view, allowing a stronger magnification.
This explains why, Contrary to the conventional wisdom that sees the first magnification in importance, is now universally known species in astronomy where the observed objects are faint, which it is fundamental and first in importance the lens DIAMETER.
The telescopes that provide greater performance in the sense of quantity as well as quality, of observable details, are the big reflectors, that for the amateur astronomers are mostly of Newtonian type, mounted in simple and economical Dobson frame.
This is not to say that the refracting telescopes, which they are still small diameter for their specific nature and costs, are unwrapped. But simply that they, Although of excellent workmanship will never outperform enhance large diameters achievable as reflectors .
In fact, the refractors have stopped in 1897 to 102 cm del Yerkes Observatory di Williams Bay, Wisconsin, U.S.A.. While the reflectors are in continues escalation, and at the moment they came to 10.4 metri di opening of the Gran Telescopio Canarias.
However, there is no universal instrument and perfect for nature. And each type must be handled with appropriate care.
The refractors of the best apochromatic species correct from allhromatic aberration (but do not take naturally place in all reflection telescopes), They remain far behind in research performance in the skies, because of their small diameter.
And the fact that they return excellent pinpoint images of stars, is a positive feature also linked to low performance still limited by the small diameter and small resolving power.
Conversely, the further you go in the largest reflectors and most important is the control of turbulence present in our atmosphere, both in the length of it that separates the unit from celestial object under observation, and both within the same optics of the telescope.
AS TO THE ATMOSPHERIC TURBULENCE (the aforementioned "Seeing"),
Subject to the foresight not get to observe on a terrace, an object prospectively low over the turbulence gived from the same terrace and from the tiles housetops, There's no way to fight it as a amateur stargazer, (except to choose a grassy place in open field or country, which represents the maximum possible advantage in that local context).
But for the large telescopes we exist today of computerized systems equipped with mechanical actuators which deform in real time some points of the support cell of the secondary mirror, in order to create a temporary correction/optics compensation and in opposite sign to the Visual deformation caused by turbulence. It is so-called adaptive optics systems. And they are not within reach of the amateur.
REGARDING THE TURBULENCE IN INSTEAD OF TELESCOPES:
The world amateur users of large diameter parabolic mirror Newtonian telescopes, keep in mind that they are obviously consist of an equal 'large mass of glass, which is at the temperature of the storage environment of the telescope itself, for times that last until shortly before being exposed to the night sky for the observation.
The environment under the night sky is dynamic and mostly significantly colder than the environment in which is stored the telescope. Even if the ambient temperature was the same as that of the mirror, the outer, species in prime evening time, and in most part of the year, descend down of several degrees per hour, to reduce its descent gradient, only toward dawn. With the result that the mass of the glass will not be able to pursue, because of its much larger thermal inertia than that of air, and a much lower thermal conductivity.
A hot mirror so, being it of common glass, or in more expensive low-expansion glasses, creates INDIFFERENTLY inside the reflecting telescope, the same column of warm air rising upward that creates turbulence throughout the optical path of the telescope.
And this effect is durable until the temperature of the mirror is cooled reaching equilibrium with the temperature of the external environment. Thing that in the normality of the dynamic trend of nighttime temperatures, in comparison with inertia and thermal conductivity of the glass, in practice it may not be, and it never happens, except at the time of the nearby thermal inversion dawn.
It goes without saying that the use of expensive glass with low expansion coefficient, It represents a marginal advantage to the amateur, compared to the very high that the manufacturer of the mirror, in that it should not wait hours of acclimatization in order to perform reliable production quality tests being manufactured.
And as much it goes without saying that you should try to help the Elimination of turbulence, in the absence of certainty of being able to reach thermal equilibrium mirror – versus environment.
The turbulence in the optical path is well observable and discernible than the atmospheric because identified in sight with the so-called "Star test", presenting the seethe of stellar points in intrafocal vision.
If you don't correct the turbulence, which it is the cause of the bubbling of the image to the telescope, it actually prevents the observation of planetary particular at high magnification, or the separation of stars near double; While in deep sky it disturbs less, but prevents the best pin pointing of the stars, always in a manner compatible with their magnitude.
Therefore, the instrumental turbulence must be combated adequately.
Without prejudice to the seasonal rare cases in which the falling gradient of the ambient temperature of the place of observation is very low, and subject to temporary reversals, in which thermal equilibrium is possible; In all other cases, the temperature fall in prime evening time, and his night decreasing gradient will continues, and causes, in confiding solely in atmospheric convection-only, the thermal equilibrium is never reached searched but only approximated (mainly in small mirrors, thanks to their lower mass and inertia).
This statement is the result of my experimental measurements of temperature data at regular time intervals, after the installation is still active in my 360F5, (see gallery of photos at end of article), of two temperature probes, one of these liberated from its self-adhesive shell, and placed in contact of the rear primary mirror, and the second probe measuring the room temperature, using a regular digital thermometer car.
(I have a digital thermometer used to drive from 12 euro with one probe internal to the display unit, and one outside at the top of a cable, all it is working for years, with a unique internal coin stack to display. the tension I then used the 12 Volt scheduled for the blue backlighting of its face, cutting away the corresponding cable.
The, solution of the thermal problem can come by confining constructively the primary mirror, in a posteriorly closed cavity by the presence of a electric fan, While around the front edge of the mirror is installed one diaphragm having diameter equal to that of the primary, and placed at a distance from the reflecting surface experimented, and such as to create a circular slit, for which the bottleneck (thanks to Bernoulli and Venturi), It increases the flow velocity aspirated from the rear depression produced by the aspirator, removing efficiency with the turbulence from the optical path.
Through this circular slot, the rear suction suck the hot air, inching closer to thin for cooling, the boundary layer lying in contact with the reflective part of the hot mirror.
With this system, you will get a more efficient tracking of the ambient temperature fall from a part of the mass of the primary mirror, as long as its temperature will approach to’ at a distance of only few degrees from that of the environment, While failing to perfect match it.
At that point, the effect of the suction will become immediately visible at the eyepiece with the appearance of the diffraction rings in the star test, upon its ignition; and the disappearance at shutdown.
The ignition will in fact to an immediate "combing" of turbulence remains present in front of the mirror, converting them into a constant and orderly laminar radial flow, which will result in an immediate good vision through it, not bothered by any bubbling, despite the primary of the telescope is not in perfect temperature equilibrium with the environment.
And it is still remaining in the margin of tolerance 4 or 5 degrees centigrade that the telescope will keep its best performance, witnessed by the visibility of the eyepiece siffrazione notch.
THEN COME TO THE EXTRACTION FAN
I say "aspiration fan" at the singular, because it is much less efficient to put more intake fans, which add up the fuel consumption and their vibrations that bother the highest magnifications. In addition to generating invariably unwanted "bickering more cross turmoil"That turn the gain into a loss.
The vacuum is then practically an axial fan, the type of those present for the cooling of computers. One of the best brands remained the German EBM PAPST, including on-line, perhaps you can still download an interesting catalog pdf. But today those objects arriving from China at affordable prices destabilizing any European manufacturer.
Their dimensions are different, but the most common and therefore the price of less than 10 euro, It has a square structure 80x80x25mm, which contains an impeller diameter 80 mm e 25 mm thick; Supply 12 It was CC, but usually the catalog are given work equally well with supply voltages halved, or increased by 25%.
A greater thickness of the fan increases the flow at constant speed, and so also the diameter. But it is always advisable to install one, although sometimes with thick mirrors, It may also be useful to experiment with a front that will sweep the reflective surface in a diametrical fashion.
The low speed of rotation (1500 rounds) They are preferred, avoiding buying rotating fans on ball bearings, preferring to their place of those impellers sliding bushing (“bushing” made of plastic material), because the intrinsic noise of the bearings is synonymous with even minimal vibration also caused the telescope.
My vacuum cleaner I have chosen with power 2.5 watt; Current consumption of just under 200 milliampere; Air flow of 33 cubic meters per hour. speed of rotation of 1500 rpm.
In this article on this same blog, I describe the 6 or 12 V DC feeder for two speed 750 and 1500 RPM, using electronic, proceeds from the amendment to the lighthouse that I use sometimes for mounting the telescope on the field.
Some pictures of the arguments cited in this article.
