FAQ
Amateur astronomy FAQs:
what kind of scope can I buy to observe and imaging the planets? Answer to this question is not easy; the best scopes to observing planets arelarge apochromatic refractor but their prize is much beyond our possibilities.In planetary observing-imagins is important the contrast of the image; this means to have less obstruction as possible; it is very very important the optical quality of the scope, so don't trust in economic chinese scopes and you should be always sure about the optical quality of the instrument. The commerciale achromatic refractors arenot indicated for this purposes; their optical quality is not so good and the chromatic aberration is to high to achieve good results A good choise is a good newtonian reflector or a Mak-Cassegrain with few obstruction; to have low obstruction in newtonian reflector, they must have big focal lenght (more than 1-1,5 meters for 20-20 cm scopes); in this way, the obstruction of 18-22% is enough little to have the same contrast than an apocromatic refractor of equal diameter. But there is a problem: these long tube newtonian scopes are heavy and they need a big (and expensive) mount, and there are not in commerce so long reflector and not with the required good optical quality; the right choise is then a good instrument, as can be a mak cassegrain (but they are expensive) or a SCT. These scopes even have a big obstruction (30 or more%) have a excellent optical quality and they are not so expensive. Before buy a scope, don't forget some considerations about your observing site. It is indeed useless to have a good planetary scope is your site has always bad seeing conditions. It is important to keep in mind this problem, always. Last but not least: good news: for observing and imaging the planets you don't need an expensive and precise mount as happens for deep-sky imaging, so you can save some money. (but not too much money; you can't put a 25 cm newtonian scope on a little EQ2 mount!)
I want to buy a big scope to observe everything and to have big big magnifications; what kind of scope I've got to buy?and how many magnifications can I reach? this question is very frequent among newbies, but this is deeply wrong. The most important aspect of a telescope is not the magnification which it can give you, but the amount of light which it is able to capture; every telescope indeed is able to offer you every magnification which is function of the telescope focal lenght and the eyepiece focal. The power of the telescope is in it diameter; greater is the diameter, fainter will be the visible objects, greater will be the resolution power. Many deep-sky objects are larger than a full moon and so they don't need high magnifications; they are instead quite faint and they need high diameter scopes. In the other hand, the planets are quite little, so you could need a large magnification; this is partially true but not too. If we discard the seeing and the optical quality, the resolution power of a telescope is given only by the diameter; so a little 50mm scope can give you a resolution power of about 2,5", a 120mm one 1", and so on. It is completely useless to magnificate an image many many times if the resolution power is low. Let's see a little example: a perfect dark adapted human eye has 180" (3 arcminutes) resolution power; If we use a 240 mm scope, we have a 0,50" resolution power. We can calculate now how many times I have to magnificate the original image to "see" all the resolution power of my scope; how many times I have to magnificate the 0,50" to reach 180"?the answer is simple!=> 180/0.50= 360, this is the magnification required to see the smallest detail visible with my 240mm scope. Since it is difficult to have a 180" human eye resolution power,the telescope resolution can be better than 0,50" if the image is well contrastated, the value of 360 is a little optimistic, so we can say that the ideal magnification is about 500, about 2 times the diameter of the telescope in mm. This is the maximum magnification useful; you can enlarge more the image but the resolution power is the same, so you will see a blurred image, and a darker too! It is then completely useless to use greater magnifications!
Ok, so I want a large scope to have greater resolution power; so what is the telescope for me? Ok, this question is right but not too much, for 2 important reasons: 1) the seeing; greater is the scope diameter, worse is the seeing; a 25 cm scope can be used at maximum resolution power only few days per year; a 40 cm scope can be used at maximum resolution power almost never! So you've gotta make a choise: observe 10-20 days per year at very high resolution, so you can buy a telescope of 30-35cm; or observe 100-200 days per year at mid resolution and buy a 15-20 cm scope. The other problem is the mount: don't undervalutate the mount of the telescope; a good mount costs more that the optical tube, so if you wanna buy a 5000$ scope, you'll need at least a 5000$ mount!It is completely useless to have a big and good scope if the mount is little and with no precision, even if you observe the planets. At last I can answer to your question: if you wanna buy a good and big scope you can consider a SCT of 20 or more cm on good equatorial mount (which can costs from 700 to 1500$).
Astronomy FAQs
why do I see in the sky stars with different colors?what does it means the different color of the stars? The stars have different colors, from red (for example Antares or Aldebaran) to blue (like Betelgeuse or Sirius). That difference in colors is due to a "superficial" temperature of the stars. We can consider with good approximation the stars as a black body. A black body is an object that absorbs every incident radiation (and then it is black) and re-emit the radiation absorbed as a funcion of its temperature. It is very difficult to have a perfect black body, but in the other hand we can consider most of the objects like black bodies with a good approximation. This means that every body above the 0°K emits radiations as function of its temperature. The wavelenght of maximum emission is given by a simple formula: λ=0.29/T (cm) where T is the temperature of the body (°K). Using this formula we can extimate the emission of a human body (310°K); it is easy to show that the emission is in the IR light, and indeed the thermal visor of army are able to see persons in the night using their black body emission. In the same way, we can understand why an hot metal emits a red radiation. If we heat to 1000°K a metal, the black body emission shift to lower wavelenghts and it is visible to human eye as deep red radiation. The stars work in the same way. A red stars look red because their superficial temperature is about 3000°K and the peack of emission (λ=0.29/T) fall just in the red band of the spectrum. A blue stars, has a superficial temperature of about 30-35000°K and our Sun has a temperature of 5770°K. This is called the color temperature and it is the temperature of a perfect black body which emits that radiaton; since the stars are not perfect black bodies the real temperature can differ by some (few) degrees. So, the stars are of different colors because they have different temperatures, while their composition is the same.
what is the composition of the stars? The stars are giant roughly spherically balls of hot gas; the composition of the stars is about the same; the main element is the Hidrogenum (H), about 74% of the mass; there is also a large amount of Helium (He), 24% of the star's mass, and the 2% are composed by the metals (for metals in astronomy we mean every element heavier than Helium, so we have: Hidrogenum, Helium and metals); the main metals present are Oxigen (O) Carbonium (C) and Litium (Li); Every element is in the atomic form, and it is mainly ionized, that is, without one or more electrons. No molecules are present in the stars because the highest temperatures.
why do the stars emit so much radiation and for how much time a star can live? The radiation of every stars is due to atomic fusion of light elements (especially Hidrogenum) in the center, into heavier elements; this process is higly esoergonic, so a big amount of energy is released. The sun, a medium star, release an energy every seconds which is billions and billions of time greater than the energy released by the powerfull bulb in its whole life. This radiation and the pressure of the gas keep the sun (and every stars) in a equilibrium state against its gravitazional force which tends to compress the gas. Without these processes a stars like our Sun will collapse on itself in less than 20 minutes! The emission of energy and so the fusion is stable until there is enough Hidrogenum at the center; when the Hidrogenum get over, the star start to collapse until new reactions start, like the Helium fusion. But, sooner or later, the helium get over too and the stars start to collapse again; new reactions can start with the increase in temperature, but the life of the star is getting over. If it mass is not greater than sun's the star collapse and become a white dwarfs with a planetary nebula around; if the stars in bigger than sun, it end of life is more more...explosive: we have a supernova event; an huge explosion visible even at huge distance; after this explosion the nucleus of the star become aneutros star o a black hole. For better explanation go to the stellar evolution page.
how is possible to measure the rotation period of the solar system bodies? Is not easy answer fully to this question; the most easy technique is observative; you observe the planet and the features, and you record when the same feature cross at the same point; this technique depends on the smallest details visible and on their proper motion. Another and accurate technique is the measure of the doppler shift of the light reflected by the body; you need also the diameter of the planet to know the rotation period; from the doppler shift you can have the radial velocity, derive then the linear rotation velocity, and then the rotation period. If the body is unresolved, the things are more complicated; the only way is to analize the light curve of the body and trying to see any periodic signal. This technique is used to measure the period of some little asteroids and KBO objects, but the measure is affected by a large error. Many satellites orbiting close to big planets such the moon and all the galileian's, have the same rotation and revolution period around the planet, because the strongest tidal forces.