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Observation Theory
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How to Choose an Instrument?The choice of an astronomical instrument is usually a jigsaw for most amateurs as the parameters are wide upon which the decision has to be made. One may consider that, on one hand, some basic -although advanced- criteria are useful to know and that, on the other hand, another way of taking the question is to consider it like a progression from beginners to advanced amateurs
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| Aperture and Sturdiness of the Support A Tool for Each Field of Observation | Portability, Informatics |
Aperture and Sturdiness of the Support
Any astronomical telescope is a light-gathering tool. The uncontested rule is that the more the aperture -that is the diameter- of the instrument, the more the light gathered and the brighter the objects observed, and the larger the resolving power (the resolution, the ability to see details). The larger the aperture, the more light collected and the brighter and better the image!
Any astronomical telescope, on the other hand, does not come on its own. It needs a support. The uncontested rule there is that the sturdier, the better. A support is always made of two parts: the mount -which serves to point the telescope and to track the apparent motion of the object observed, and the support strictly. Both parts are concerned with the sturdiness. The purpose of the ensemble of the support is to provide stability to the observation. Any vibrations have to be the lesser possible on one hand, and to be dampered the quickiest possible on the other hand. Outside the question of the type of mount, most usual supports found on the market are aluminum tripods. Advanced amateurs often are tempted to settle their observation site on a pier
The magnification may be seen like a secondary factor only of sort. The magnification is performed by an eyepiece magnifying the image produced by the instrument's primary light-gathering tool. Due to the turbulence in the atmosphere and, generally, to the atmospheric and weather conditions, magnification -even with the larger amateur instruments- is limited to 250-300x most of the year. Some rare nights only allow to push the larger instruments to their best
These first points bring a first conclusion. Your bank account permitting, the larger the instrument, the sturdier the support, the better. It's such a telescope only which will allow you the top studies in each of the observational fields -from the Moon and planetary studies to the deep sky
A Tool for Each Field of Observation
Planetary studies need a minimum magnification (a power of 100x) as deep-sky, faint, objects need a luminous vision. Hence each field of observation requires a specific tool. It's usually stated that long focal refractors are the best tools at planetary astronomy as large reflectors are better at deep-sky objects like nebulae and galaxies. The focal of an instrument, on the other hand, is the length at which the optics produces the first, non-magnified, image of the object. The shorter the focal length for a given diameter, the more luminous images the instrument yields. Such instruments are thus fine for deep-sky objects where the field's luminosity is imperative. The longer the focal the higher the magnification, but the dimmer the image. Hence long-focal instruments are used for planetary observation where it's the magnification which is important. To summarize, long-focal refractors are to be used for planetary astronomy. Short-focal reflectors are to be used for deep-sky objects. Each category of instruments, further, has a series of advantages and inconveniences. Reflectors, generally, work with open tubes and with several optical devices -most of the time a primary and a secondary mirror. Hence they are sensitive to temperature (like when you take the instrument from your home to the outside; you need to wait that the telescope reaches the ambient temperature); and they need maintenance (to maintain the collimation (alignment) of the primary and secondary mirrors). They are too more sensitive to light pollution. Refractors, as far as they are concerned, have closed tubes. They are about not concerned by the temperature question. As they have one optical device only (mainly a single -or composite- lens) they do not need any collimation. Refractors however need tall supports (bringing back the question of vibrations), and they are expensive
Once this said, a recent trend however has been to develop short-focal refractors, which are considered good for deep-sky and good for planetary objects at the same time. Such instruments however are using -as far as the planetary observation is concerned- such optical accessories called 'barlows' towards which the amateur astronomy community always was defiant. As another consideration is that a one type of instrument, the Schmidt-Cassegrain telescopes (SCTs) are considered good multi-purpose telescopes. Such telescopes are reflectors with a correcting lens closing the tube and a secondary mirror sending the light to the bottom of the instrument. Hence they have the advantage of the refractors as far as temperature accomodation, for example, is concerned. They need to be collimated however. It's a fact that the amateur community has long been interested in these all-around SCTs which, although reaching a certain price, are good compromises, and popular for astrophotography
The second consideration is about astrophotography, whatever you center of interest is. The question there, is the mount. The mount is this part of the support, atop the tripod -or the pier, which serves to point the telescope and to follow the apparent motion of the object observed. Due to Earth's rotation, the celestial sphere apparently rotates from East to West while you are observing. The tracking part of a mount's job is to compensate for this Earth's rotation. As far as the objects you are to image are concerned, deep sky objects need long exposure times, planets mostly need accurate and sturdy tracking with exposure times up to several seconds (as a high power is needed, hence a loss of light). Moon needs short exposure times only. A mount is not essential in that latter case. The best choice for a mount was long a "German equatorial mount", that is a mount an axis of which is pointed to the celestial pole. Only one motion is thus needed to counteract the Earth' rotation. The telescope is rotating at the same rate, but at the opposite Earth's, along the Earth's poles axis. Most usual German equatorials -except at beginner's instruments- either come motorized, either can be added with motors. The most recent trend for mounts is altazimuthal, motorized mounts. Altazimuthal are mostly fork-mounts, one axis moving East-West, another up-down. Such modern mounts are mostly associated with SCTs and allow a precise, automated, tracking for astrophotography. An additional tool, a field de-rotator is mandatory when long-exposure times are needed to maintain the field similarly framed along the exposure. for more about astrophotography see "Astrophotography"
Portability, Informatics
The portability of a telescope is its aptitude to be easily transported and mounted. Even when building a mini-observatory dome in your backyard, you might most of the time take the main instrument on and back, letting the mount in place. Hence portability is a factor. Think about the weight of the instrument and whether you will be able to transport it on your own, or need some help. How easy is it to fit the ensemble telescope-mount-tripod together? Or how is the mount power-supplied (batteries, car-adapter, or a field-portable battery?)
The amateur telescope industry has now entered the computer-era. Hence you will have to consider what computer-based performances your telescope support. One aspect is that hand-held controlers are moving and aiming the telescope at wish, or towards objects coordinates of which are stored in their database. On the other hand, most recent softwares are allowing the operation of the telescope from a desktop or laptop computer, incorporating astrophotography functions at a basic level. Inquire about all this before your purchase
From Beginners to Advanced Amateurs
Another way to put the question of chosing an amateur instrument is to see it from the beginners-advanced amateurs point of view. That is that most amateurs are following a step by step path in their interest to amateur astronomy. They start as beginners, looking around, trying several fields of observation. Then, they move further into the hobby. Hence the knowledge of the capacities of the instruments is useful. A section is dedicated to the specific case of the binoculars
| Beginners Middle Range | Advanced Amateurs The Case of Binoculars |
Beginners
Good 60 mm and 70 mm refractors are fine initiation instruments. They are already allowing a lot and are easy to set up and transport. Most of lunar features may be identified, like craters, mountain ranges, and rilles. Mercury and Venus phases are seen. Jupiter clouds belts are seen as are the four Galilean satellites and their motion around the planet. Saturn is a fine view as Titan only is seen as far the ringed planet's moons are concerned. Even Uranus and Neptune already are resolved into finite-sized discs. The less well seen object in a small instrument is Mars. Polar ice caps and dark surface markings may be seen but any serious observation seems a challenge. As far as deep-sky objects are concerned, such initiation telescopes are a good step already to get improved, and fine, sights of the most obvious and known objects like the Andromeda Galaxy, the Orion Nebula or some open and globular clusters. These instruments are allowing large steps too in the world of the binary stars
The other main good idea for a beginner's instrument is to purchase one you will be able to expand so it may bring you a little further into the observation activity. It's nearly mandatory, on one hand, to acquire a telescope the eypiece-holder barrel dimension of which will allow further standard accessories. The most basic standard is a focuser able to handle 1.25" eyepieces. It's such a diameter which will allow you, for example, to move to your first steps in astrophotography or to use additional optical tools like a "star diagonal" -which is an accessory sending the light at a 90° angle of the axis for an easier observation's posture. The 1.25" standard is usueful too should you want to use your astronomical telescope as a terrestrial instrument. In this case, you will use a "diagonal erect image prism" which will set up the image back, as astronomy telescopes are reverting the image in various ways. Equiped this way, you should make a good start
Middle Range
A good value in this range seems to be something like the Meade ETX series. Such telescopes are Maksutov-Cassegrain telescopes, well-equipped for the price. Such instruments are the first step into detailed views of the Moon, Venus, and Jupiter. The Cassini Division is seen at Saturn's ring, the shadow of the ring, or cloud belts. Mars however remains under the par with such instruments. Further deep sky objects are at reach like M57, the "Ring Nebula" in the constellation Lyra, or the structural shape of the Andromeda Galaxy (M31). On the other hand, the ETXs are allowing first serious photographic work. Some 8 or 6" reflectors may add some performance to the ETXs
Advanced Amateurs
A 8" Schmidt-Cassegrain is the entry of the high-end amateur work. A 10 or 11" Schmidt-Cassegrain is the instrument which will provide you with a lifetime of observation. With a 8" the Moon becomes an inexhaustible subject of study. Venus and Mercury are allowed to further details. Mars begins really at that aperture, as Jupiter is further seen and the Galilean satellites resolved into small discs. Saturn is further seen as Uranus and Neptune are allowed in more details. Pluto is not seen below a 10". Deep-sky objects becomes infinite. Large nebulae or galaxies are seen in splendid details, like the dust lanes at M31. 10" to 12" telescopes are going a step further than the 8"
The Case of Binoculars
Binoculars are another case. They may be considered like an initiation instrument and a "safety tool" for advanced amateurs at the same time. When used like an initiation instrument, binoculars are mostly good at getting the sense of the sky and of the constellations. That is binoculars allow to sweep the sky and to get the first deep-sky objects at reach. Unluckily they are about null as far as the planets are concerned
The purchase of a pair of binoculars, on the other hand, is always useful, as any serious amateurs astronomers will always have one at reach. Either to sweep the sky in search of fine general vistas, or for a general and preliminary work, like having a first view of a field, or to quickly verify an astronomical event like the position of a comet. Further, binoculars are excellent, and easier-to-use, tools in specific cases like Moon eclipses. Should you purchase binoculars for the purpose of having another telescopic tool of easy and useful use, the best will surely be to choose ones which are specifically deviced for an astronomical use, that is, mostly, a wide, and luminous, field of view