Observation 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, from a time when the observer is a beginner to when he has become a advanced observer. A recent trend is that some authors are asserting that, for a given aperture, telescopes which are built by amateurs (which most of the time are of the Newton type) are most of the time as good as commercial ones (and even better when the construction is especially accurate) as one may find telescopes trials which question the quality of some telescopes and mounts of some famed, or known brands -the sole restricting aspect of that question is that the construction of a telescope is a heavy duty, especially when one is not a advanced bricoleur. The purchase of a astronomical telescope beeing a important financial expense, the better, when available, is to get to one of those observation sessions which are often organized by clubs of amateur astronomy. Such a opportunity indeed will allow you to observe through miscellaneous telescopes, of varied brands, which will help you to figure out what you can expect in terms of visual images or of what kind of telescope you would like to own. Notwithstanding discussions one can have with participating amateur astronomers. To read telescopes trials and comparisons as published online or in dedicated magazines is also of use, albeit with a lesser benefit. Amateur astronomy at last, in terms of telescopes, is subject to fashion. Such or such type of telescope, which is fashionable at a given time may not be anymore a dozen years later, for example. The best statement about chosing a telescope is maybe that following one, found on the Internet: 'the best telescope is the one you will use the most!'

A small refractor is often is a good beginner's telescope! site 'Amateur Astronomy'

Basic Advanced Criteria

From Beginners to Advanced Amateurs

Basic Advanced Criteria

Although a flurry of parameters affect an astronomical instrument -of them the price ;-) the basics are that the larger the aperture and the sturdier the mount and tripod, the better. This leads to the fact that the magnification is somehow a secondary consideration only. The further parameters are mainly based about the field of observation you will practice. At last some other considerations may be of interest. You will note also that astronomy clubs members during parties usually allow to look through their telescopes, giving you some idea of what is seeable and what you might want for your personal needs

Aperture and Sturdiness of the Support A Tool for Each Field of Observation

Portability, Informatics, Accessories

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. As, when you will take the telescope from your home to the outside, you will need to wait that it reaches the ambiant temperature. They need a specific maintenance too, called 'collimation' which consists into adjusting the alignment of the telescope's optical systems between them. 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. Let's add too that Newtons, according to some, look like the most polyvalent telescopes because -like hinted to by their proponents- the SCTs or other different types are mostly telescopes which are dedicated to the observation of such or such celestial object. As such, SCTs are disabled somehow as far as the observation of deep sky objects or large objects is concerned. SCTs further are subject to collect dew upon their correcting lens

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, Accessories

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. Moon is already providing with numerous exploration possibilities, of the regional type however. As far as deep-sky objects are concerned, such initiation telescopes are a good step already and fine, sights of the most obvious and known objects like the Andromeda Galaxy, the Orion Nebula or some open and globular clusters are allowed. These instruments are allowing large steps too in the world of the binary stars. Such refractors usually come with a altazimutal mount. Somewhat a notch beyond in terms of capacities -and, also, in terms of price- you might get interested too into a 100-mm (4' ½) Newtonian reflector, with a equatorial mount -motorizable or not. Such telescopes however have the inconveniency of being reflectors, and, as such, to need a thermal time of adaptation before observing, and a collimation by interval. With such a telescope, Moon, is now becoming a rich field of exploration as you will already accede to well-delimited areas. Access to planets, on the other hand, is remaining limited as such a instrument is only improving what is allowed above. The number and details of deep-sky objects are improving. A 4' Newtonian thus is allowing to the 10.4th magnitude in semi-urban night sky conditions. 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 usuful 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. In terms of accessories further, a recent trend in Europe, for example, is accessories boxes ready to use. Just do not get interested as it is better to buy quality accessories as you need really those. In terms of the quality/price relationship, keep reasonable however: do not buy pricy accessories to use with a beginner's instrument, for example (maybe that is above all true for optical accessories, as, personally, I however, for example, opted for a astrowebcam the price of which is the one of my -quality- 60-mm refractor and I really get a advantage)

Of note that the possibilities of observation even with a small aperture telescope -like a 60-mm one, for example- are important. You will need time indeed, from a same observation location, to exhaust all the possible variables, like the miscellaneous celestial objects you will observe, the varied conditions of planets (opposition, phases, etc.), varied phases of the Moon, the accessibility of all such objects, the state of the sky (turbulence, etc.). Thus, even when you will buy a beginner's telescope, you will to be well aware that it will allow to a important time of observation, of the order of some years maybe. Such a factor thus may play a role when buying one's first telescope as one will need time, should one observe on a scientific basis, before considering to pass to a more important telescope!

A eypiece-holder barrel's dimension of 1.25 inch easily allows to accessories, like a star diagonal! site 'Amateur Astronomy'

Middle Range

What is often considered a good choice in that rande is a telescope of the type of 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" Newtonian reflectors may add some performance compared to the ETXs. It is possible, on a other hand, that, as far Maksutov-Cassegrains are concerned, the question arises, in terms of luminosity, to opt, in the same range of observational capacities, for a quality refractor

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". Of note, generally speaking, is that some brands of Schmidt-Cassegrain may prove disappointing enough because they may lack luminosity. That can be made up for if you consider to use such a telescope in terms of astrophotography only. That question of the telescope's luminosity thus asks the one to opt for a quality refractor. That, in turn, brings to the question of how such a telescope costs and, above all, of the overall dimensions of a large aperture refractor!

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. 7x50s, 11x80s may be a good choice. Many larger binoculars specially built for astronomical purposes, with a large aperture at 100mm for example and movable oculars, may be affected with chromatic aberration at a magnification above 40x