Acclimation of the optics of a telescope, influence on the observation and possible fixes.

Anyone using a terrestrial or astronomical telescope uses it to enlarge, that is bringing virtually an object that is at a distance, in order to observe in its smallest details.
A telescope is able to receive light rays or reflections products, coming parallel from the object in observation, to form an image of that object in its focal point.

The image formed is very small, and therefore to be observed must be enlarged with lenses that are contained in an eyepiece, and the number of such magnification is the ratio between the focal length of the lens and the eyepiece.

But the enlargement is not infinite, and in bright and pin point subjects as the stars , is limited by the phenomenon of diffraction, for which a star's point view until you reach the limit at which the diffraction decomposes image in a central point that concentrates more than 80% of the light, surrounded by alternating dark and light circles descending brightness. And more the star is bright, and over the center point will look great compared to surrounding fringes.

The magnification is limited by persons other than in point iscurimento of the image.

Done intuitively because if we look at the object at high magnification, his tiny extension will fill the field of view.
But that tiny extension will bring with him only his minimum fraction of light, than the light collected by the entire telescope lens.
From this arise two comments:
– That the greater the magnification, the greater is the darkening of the image, until it is unusable.
– Whereas, to remedy those problems you need to use a bigger lens, and by collecting much 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 popular opinion that sees the enlargements first in importance, It is now universally known species in astronomy where objects observed are feeble, that is fundamental and first in importance to the objective lens diameter.

The telescopes that provide greater performance in terms 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, that's still a small diameter for their specific nature and costs, are to be discarded. But simply that they, Although of excellent workmanship will never outperform enhance large diameters achievable as reflectors .

In fact the refractors stopped in 1897 AI 102 cm of the Yerkes Observatory in Williams Bay, Wisconsin, U.S.A.. While the reflectors are in continues escalation, and at the moment they came to 10.4 metres of opening of the Gran Telescopio Canarias.

However, there is no universal instrument and perfect for nature. And each type should 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), remain far behind in research performance in heaven, because of their small diameter.

And the fact that they return excellent pinpoint images of stars, and a positive feature is also linked to low performance still limited by small diameter.

On the other hand, 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.


Without prejudice to the foresight to 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, representing the highest possible advantage in that local context).

But for large telescopes today there are computer systems equipped with mechanical actuators that deform in real time at certain points in the secondary mirror support cell, 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 are not within reach of the amateur.


The world amateur users of large diameters of parabolic reflector of Newtonian telescopes, keep in mind that they are obviously made of an equal 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 noticeably colder than the environment in which it is preserved the telescope. Even if the room temperature was the same as that of the mirror, the outer one, species in prime evening time, and in most part of the year, descend down of several degrees per hour, to reduce its gradient of descent, just about dawn. With the result that the mass of glass, that has a much greater inertia than air; and also a much lower thermal conductivity, and it will fail to pursue.

A hot mirror then, being it of common glass, or in more expensive low-expansion glasses, creates EITHER within 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 glass, in practice it never happens, except when thermal inversion close at dawn.

It goes without saying that the use of expensive low-expansion glass, does not represent an advantage for the amateur astronomer, but only for the manufacturer of the mirror that does not have to wait for hours to perform production tests being parabolizzazione.

And as much it goes without saying that you should try to help the Elimination of turbulence, without the 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, that is causing the reboot of the image through a telescope, It actually prevents the observation of high magnification planetary details, or the separation of close double stars; 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 instrumental turbulence must be fought properly.

Except in rare cases where the gradient of falling temperature of the place of observation is very low, where reaching temperature equilibrium is possible, although not very quickly; 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, that the heat balance sought is never reached.

This statement comes from my experimental surveys of temperature data read at regular time intervals following the installation still active in my 360F5, (View slideshow 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 normal car digital thermometer.

(I used a digital car thermometer 12 euro with one probe internal to the display unit, and one outside at the top of a cable, all working with a single button cell inside the display. I not have then used, cutting away its cable, the external voltage of 12 volts for backlighting of his blue light display).

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 put at a distance to the reflecting surface, experienced and likely to create a circular slot that would act as a bottleneck to Venturi effect, that increases the speed of a very thin stream sucked .

Through this circular slot, the rear suction will draw the hot air sucked that constitutes the turbulent boundary layer seething and lying in contact with hot mirror reflective part.

With this system will get a more efficient tracking of temperature drop by 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 aspiration will be immediately visible to the eyepiece with the appearance/disappearance of diffraction rings on the star test, upon its power on/off.

The "on" position will provide an immediate "combing" of the remains of turbulence by converting them into a steady laminar flow, that will cause an immediate good vision of diffraction rings in star test through the instrument, no longer disturbed by any bubbling. Even if the primary of the telescope is not in perfect balance with the environment.

And it's still remaining in that tolerance of 4 or 5 degrees Celsius that the telescope will keep its best performances.


I say "aspiration fan" at the singular, because it is much less efficient than putting more intake fans, which add up the consumption and their vibration that bother at the highest magnification. In addition to raising invariably unwanted "bickering more turbulence crossed"that turn the gain in a clear loss.

It therefore is practically an axial fan, the type of those present for the cooling of computers. One of the best brands has remained the Germanic EBM PAPST, whose online maybe you can still download an interesting pdf Catalogue. But today those objects come from China to destabilising any European producer prices.

Their measures are different, but the most common and therefore the price of less than 10 Euro, fan diameter 80 mm and 25 mm thick; Power supply 12 volts DC, but the catalogue usually are given work equally well with halved supply voltage, or increased by 25%.

A greater thickness of the fan increases the flow at constant speed, and so is the diameter. But it is always advisable to install one.

Low speeds of rotation (1500 RPM) and low noise are preferred because even minimal audible noise is synonymous with vibrations caused to the telescope.

My chose it with power 0.5 Watts; Current consumption of just under 50 milliamper; Air flow rate of 33 cubic metres per hour. rotational speed of 1500 revolutions per minute.

In an article on this same blog, I describe the AC two speed, without the use of electronics, proceeds from the review to the lighthouse that I use sometimes to mount the telescope on the field.

Some pictures of arguments specified in this article.

Leave a Reply