Choosing the Right Telescope Discussion guided by: Thomas C. Smith, Director of the Dark Ridge Observatory DRO Amateur Astronomy Series This presentation and discussion is part of an ongoing series of topics that was determined to be of interest to club members as the result of a poll conducted by several club members in September of 2006. Discussion facilitators, like myself, are not necessarily experts in the field but through member discussions it is hoped that we will all get useful information from these presentations. Of utmost importance to this discussion is that we share our collective knowledge and have fun in doing so. DRO Topics of Conversation Three important questions to keep in mind Some telescope fundamentals and terms Refractor, Reflector & Catadioptric Telescope mounts Aperture, how does this affect things Magnification, how to calculate and what is reasonable for a given telescope Power isn’t the whole question, how is the “seeing”? Is bigger always better? Scopes of all sizes and shapes The basic principals of the three major telescope types What do you want to look at? Where will you be observing from? How much money do you want to spend? Tilt and Pan (Alt-Az) or German equatorial Manual, Driven or Go-To positioning What’s in a finder scope? Everything has tradeoffs and a price tag Conclusions; what is right for you? DRO What’s Really Important Three important questions to keep in mind What do you want to look at? • This one question needs to be answered above all else and is the foundation needed for the rest of your decision Where will you be observing from? • Is it from a light-polluted downtown area or a dark rural site. Do you have a permanent home for the telescope or do you pack it up and travel to remote sites? How much money do you want to spend? DRO Some Telescope Fundamentals Aperture, how does that affect things? First, the aperture or diameter of the main optical component can be either a lens or a mirror. The telescope’s aperture determines it’s light gathering ability and it’s resolving power (fine detail). In real terms; resolution is related to the instrument diameter so with a 6” telescope you can see a lunar crater as small as about one mile across or half the size visible in a 3” telescope used under the same conditions and same magnification. The same telescopes turned towards a dim galaxy would, however, tell a completely different story… DRO Telescope Fundamentals Cont. Aperture cont. Since the surface area of a 6” telescope is four times that of a 3” telescope, that galaxy would be four times brighter in the 6” telescope. Area = PI x radius2 • so Area (3”) = 28.27 square inches and • Area (6”) = 113.1 square inches or about 4 times the area of a 3” 3” versus 6” telescope view of M100, simulated Not a true representations of what is actually visible in an eyepiece M-100 CCD Images courtesy of Dark Ridge Observatory 14” LX200GPS with SBIG ST-7XE camera (6X300sec Visual Filter @-15C) DRO Telescope Fundamentals Cont. Power isn’t everything… It may surprise you that a telescope’s aperture is not what determines its magnification or “power”. Most novice looking to buy or understand a telescope ask something like “how much does it magnify?” In fact a telescope can provide just about any magnification depending on the eyepiece that is used with it. There are two main factors that limit the useful magnification we can actually get from a given telescope, those being: • Aperture (again and used often in discussions) and • Atmospheric conditions that we observe through DRO Telescope Fundamentals Cont. Power cont… There is a finite amount of detail present in an image produced by the telescope’s main mirror or objective lens, so what is needed is to find the optimum magnification to extract that detail without spreading out the light to such an extent that we lose detail or sharpness. For this reason most observers use low power when looking at dim galaxies and nebula. So, how much power is too much? DRO Telescope Fundamentals Cont. There is a simple rule of thumb that indicates the maximum useful magnification of a telescope: • 50 times the telescopes aperture in inches or twice its aperture in millimeters. Maximum magnification = aperture (inches) x 50 or Maximum magnification = aperture (mm) x 2 • So, a high quality 4” (100mm) scope should not be pushed beyond 200x. • For proper perspective, even a small instrument of good quality will show you Saturn’s rings or the cloud belts on Jupiter as these only require about 75x. • So that 60mm department store “gem” that is quoted as capable of delivering 300x is all about hype and should be avoided. DRO Telescope Fundamentals Cont. Power cont… Calculating magnification • Since we now know the practical maximum that should be used for a given instrument, just how do we calculate the magnification itself? • Every telescope has a focal length, the distance between the objective (lens or mirror) and the point where the infocus image is formed of a very distant object. This isn’t always the same as the length of the tube as many designs have folded light paths and can mislead you. This value is normally in the documentation that comes with the telescope and quite often found printed on the telescope itself and usually lies between about 400 and 3000mm, depending on the aperture of the telescope. DRO Telescope Fundamentals Cont. Calculating magnification cont… • Eyepieces have focal lengths too, 25 or 12mm for example. • Simply divide the focal length of the telescope by that of the eyepiece to determine the magnification. telescope focal length Magnification = --------------------------eyepiece focal length • Ex. A 2000mm focal length telescope used with a 25mm focal length eyepiece will provide 2000/25 = 80 power (80x). DRO Telescope Fundamentals Cont. Why does the Moon look fuzzy? Even with the best telescope you will notice that you can see finer details on the lunar surface or planets on some nights as compared to other nights. This is due to the atmospheric turbulence that exists. It is often compounded by heat emanating from a sidewalk or roadway nearby that was heated throughout the daytime and varies significantly from night to night. Astronomers refer to this turbulence as bad “seeing”. Larger apertures allow observers to pick out faint objects and fine details on the Moon but regardless of aperture, the better the “seeing” is the more you can see. Since steady air is so important, large telescopes, those in the 10-inch-plus category, are often limited to 250 to 300x on all but the steadiest of nights. DRO Telescope Fundamentals Cont. “Seeing”, a video demonstration. Animated GIF image from Adrian Ashford, presented on “Sky Tonight” at http://www.SkyTonight.com DRO Is Bigger Always Better? One question that you might be asking yourself now is why go for an aperture bigger than 10” if the sky conditions are so limiting? Large apertures are most often chosen by observers wanting to gather as much light as possible for viewing dim galaxies, nebula and star clusters. These so called “deep sky” objects are most often viewed using much lower power than when viewing the Moon or planets so air turbulence and “seeing” isn’t such an issue. Larger apertures also lead to shorter exposure times for those interested in astrophotography especially when combined with short focal lengths. Even if you can afford a large instrument you might not want to haul it around for club star parties and such. Too often buyers get “aperture-fever” and buy big without thinking about the “Where” question of where are you going to be observing from. DRO Scopes of All Sizes and Shape With all the advertisements and hype in the astronomical world it isn’t hard to be confused by the myriad of choices out there. Given the fundamental knowledge that we have just discussed you should know that there are really only three basic types of telescopes to choose from; Refractor Reflector Catadioptric DRO Principals of Three Telescope Designs: Refractors The refractor was the first type of instrument to be turned towards the heavens nearly 400 years ago, then called a “spyglass”. These are the stereotypical instruments that one thinks of when the word telescope is first heard. These telescopes are made with the primary or objective lens mounted at one end of a long tube structure and an eyepiece mounted in the opposite end. Light travels from the distant object through the objective lens, where it is refracted to a focus at the opposite end of the tube where the eyepiece magnifies the image. It is common to use a 90 degree mirror in the last part of the light path to project the light to a more convenient position where the eyepiece is inserted for viewing. This is called a diagonal mirror or diagonal for short. SkyWatch / Gregg Dinderman DRO Refractor Cont. In their current day implementations these instruments are often sought after by those wishing to observe the Lunar surface or planetary details. These instruments can offer crisp, high-contrast views that can take high magnifications. In fact when well made, a refractor can offer the finest images obtainable by any other telescope type for the same aperture. Another advantage to a refractor is that their construction is such that they are more optically rugged and require little if any optical alignment adjustments and are a good choice for “pick up and go” instruments for the field. This convenience comes at a price, however, as these are far more expensive to produce and buy than the other types of telescopes available. Additionally, when the aperture becomes large the instrument becomes quite long and unwieldy (typically 10 to 15 times the length of the aperture diameter) requiring very large mounts to support and position the instrument. With such large mounts; the use of high magnifications can be quite tricky as even small vibrations get transmitted and seemingly amplified through the mount. DRO Principals of Three Telescope Designs: Reflector The second type of telescope is the reflector. A reflector uses mirrors to direct the incoming light to a focal point where an eyepiece is used to magnify the image. Light enters the open end of the telescope tube and is reflected by a curved mirror (dished out) at the back of the tube to a small flat mirror mounted near the tube opening and then towards the side of the tube where the eyepiece is conveniently placed. Classical Newtonian reflecting telescope. SkyWatch / Gregg Dinderman DRO Reflector Cont. If you want the largest aperture for your money then the reflector is the telescope for you. When well made and maintained a reflector can provide sharp, clear images of all manner of celestial objects at a fraction of the cost of an equal-aperture refractor. The tube of a Newtonian reflector is considerably more manageable too; its length is seldom more than eight times the diameter of the primary mirror. So an 8” reflector tube is about 4 feet long and easily fits in the back of a small car. The reflector’s low center of gravity when placed on a stable mount will position the eyepiece at a very convenient height for just about any sky orientation. DRO Reflector Cont. For the best value of all, one should strongly consider a particular type of reflector called a “Dobsonian”. These reflectors have their optical tubes placed on a simple, sturdy, low-profile alt-az mounting and are very easy to transport to the field. There are Dobsonians in the ranges of 4 to 30 inches in aperture and are really the ultimate instrument for the casual observer. Periodic cleaning and infrequent realignment of the optical components of a reflector type telescope often lessen the appeal for a lesser mechanically inclined observer. Having an open tube construction; dust and dirt will accumulate on the optics and infrequent cleaning will be required. The aluminized surface of the primary mirror usually needs recoated at about a ten year interval but this changes based upon the conditions in which the instrument is used. DRO Catadioptric This is the “best of both worlds” and the third type of telescope to consider. This type of telescope came about because of the desire to capture the best features of the refractor and the reflector in a combined design. As such these telescopes use both lens and mirrors to accomplish their task. SkyWatch / Gregg Dinderman DRO Catadioptric Cont. The greatest appeal of these instruments is that in their commonly encountered form (Schmidt-Cassegrain and Maksutov-Cassigrain) they are very compact. Their tube length is normally only two to three times their aperture due to a folding of the light path through the telescope. The smaller tubes can use smaller and consequently more manageable mounts and tripods. With the compound construction of these instruments there comes the occasional optical alignment maintenance associated with the mirrors. The field of view of these telescopes is often very small and the secondary mirror that sits in the light path slightly degrades the views of planets and the Moon. Even with these minor drawbacks a well made catadioptric telescope will deliver very fine images of a large variety of astronomical objects. One other advantage of the catadioptric is that it is a sealed tube instrument and resists dust and dirt deposition on the primary mirror. DRO Catadioptric Cont. Many people seeking a highly versatile and portable (for the aperture) telescope that can be used for all sky subjects and astrophotography will tend to use some sort of compound instrument. In short they are excellent general-purpose telescopes that can be used with a wide variety of accessories. On a cost scale, the catadioptric is positioned between the reflector and the refractor. DRO Telescope Mounts The best telescopes in the world are practically useless unless they are attached to a stable mount that allows it to be directed in any desirable part of the sky and has the ability to follow a celestial object smoothly and precisely. A stable mount is one that when lightly tapped will have its image steady in under a second at moderate to high magnification. DRO Telescope Mounts Cont. There are two basic types of mounts encountered; pan & tilt (alt-az) and equatorial. Alt-az mounts are similar to a photographic tripod’s pan & tilt head in that movement is up and down (altitude) and left an right (azimuth) Equatorial mounts have two axis as well but one of them is aligned with the rotational axis of the Earth. DRO Telescope Mounts Cont. Alt-Az: The previously mentioned Dobsonian telescope mount is of this type. It is constructed of simple materials such as particle board and Teflon friction pads resulting in a light-weight, low center of gravity mount, that glides smoothly in both axis with fingertip control. A Newtonian reflector mounted in this way is extremely easy to setup and operate, easily transported and a great value as well. SkyWatch / Gregg Dinderman DRO Telescope Mounts Cont. Equatorial: For telescopes that are to be dedicated to astronomical use and where astrophotography is in the future, serious consideration must be given to using an equatorial mount to counteract the effect of the Earth’s rotation. When properly setup the observer need only turn one axis to keep a celestial object centered in the eyepiece. Many mounts come with a motor to take care of this automatically. SkyWatch / Gregg Dinderman DRO Telescope Mounts Cont. Is one mount better than another? Not really as each has its own strengths and drawbacks. For the casual observer an alt-az mount and particularly a Dobsonian is the best choice as it is highly portable and quickly set up for viewing. If amateur astrophotography of celestial objects is the intended use then an equatorial mount in one form or another is really the only practical solution. Polar alignment, although not a difficult procedure, does take some time before observing can begin but becomes much easier with practice. For this reason, serious astrophotography is done with an equatorially mounted telescope on a permanent pier-type structure that has been previously and rigorously polar aligned. DRO Telescope Positioning Manual, Driven or Go-To positioning: Manual; as the name implies, the telescope is manually slewed or positioned to point to the object and has to be manually repositioned, usually only as the object moves near the edge of the field of view to compensate for the Earth’s rotation. Driven; normally manually positioned and then a motor drive takes over to compensate for the Earth’s rotation. This requires a polar type mount to “track” the object. Go-To; all the rage now and becoming much improved as technology advances. The latest offerings have global positioning systems built into them so that they know the current location, elevation, date and time; that the control software requires to perform it’s mathematical calculations needed for Go-To functionality. Earlier models require manual input of the aforementioned parameters. DRO The Finder When using medium or high powers a telescope will show you a very small window on the sky. This can make finding faint objects a frustrating process without the aid of a finder. As the name suggests, these are observing aids that assist you in locating celestial objects, and all scopes, irrespective of type, should be equipped with one. DRO The Finder Cont. In their most common occurrence they look like a miniature telescope. They are normally mounted near the main eyepiece and have a cross-hair (reticule) on which to place the desired object. The front aperture should be at least 25mm (1”) and if possible, larger as the larger aperture will allow finding fainter objects less difficult. There are designs that consist of a laser or LED circular projection on a non-magnified sky background that many find convenient and natural but these can’t be used except to find objects that are visible to the naked eye. That said they can easily be used to “star hop” to the deep sky object instead. DRO The Finder Cont. Some individuals prefer to have their finder fitted with a right angle prism or mirror so that they can see in the finder and without much head repositioning, see in the main eyepiece. Some prefer to use the finder straight-through and sight “generally” with the eye that is not looking through the finder to assist in finding those fainter objects. The choice is of course the individuals and it is advised that the new telescope purchaser take advantage of a local club’s star party to determine which is best suited to them. DRO The Local Astronomy Club I am sure that it is no surprise to anyone that the local astronomy club can offer a significant amount of help in finding the right telescope and accessories that fit your individual needs and desires. Don’t ever be afraid to ask for advice and take advantage of the “star parties”, that clubs routinely hold, in helping you make the right choice for you. There are more often then not a wide variety of telescope types available for you to observe through and talk to the owners to get ideas about how each instrument operates. DRO Everything Has a Price While it may be tempting financially, resist the temptation to buy the cheapest telescope available. Many of these instruments are of poor quality either optically or mechanically, and frequently both, and will inevitably lead to disappointment and frustration. If you have a budget of just $200.00 then you should really consider a good pair of binoculars instead. That said, there are a great number of quality instruments that can be obtained secondhand and an experienced member of the club may be able to help “weed” through the offerings and assist you in a sound purchase. Are you reasonably good with your hands? Then why not make your own instrument from purchased optics and local hardware. A Dobsonian can be made in a short time and offer excellent views as well as the pride associated with self-construction. DRO … Price Cont. Even if you are a beginner fortunate enough to have a sizable disposable income, do not buy the largest, most expensive telescope you can find. If you’re just learning to identify the constellations, then many of the advanced features that such an instrument possesses will not likely be of any use to you. Remember that there is more to a telescope than a tube, mounting and steady tripod. Be sure to save some for those accessories that make the instrument all that it can be; Eyepieces to give a larger range of magnification. Filters to aid in light pollution and to bring out those fine details in the planets. And what of astrophotography? That is an entire topic in itself that I leave for future discussions in this series. DRO Concluding Thoughts So is there a perfect telescope out there just waiting for you? Actually, there is; it’s the one that you’ll use most often! An optically perfect massive refractor is of no use if you can’t carry it outside for use, and the largest Dobsonian will not show you the faintest galaxies if the only place you can use it is in a light-polluted parking lot near the center of town. Consider carefully what you consider to be your primary observing interest, where you are likely to observe, and what you consider “portable”. Remember that each telescope has it’s own strengths and weaknesses; an ideal instrument for detecting fine detail on the planets may not be the best at catching faint views of a distant galaxy. DRO Final Thoughts Get with members of the local club and attend the star parties, this is really the best way to decide what is right for you. A telescope is a big investment to most people, and the universe is not going away soon, so take your time over the purchase. When you buy an instrument that is right for you, you’ll possess the key to unlock a universe of wonders. It’s a clear night, so what are you waiting for? DRO Questions, Comments? Got any?