12 improvements for a cheap telescope March 6, 2012Posted by aquillam in Astronomy, telescope beginners guide.
Tags: astronomy, observing, telescope, telescope beginners guide
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I seem to be finding a lot of really useful websites lately that I feel the need to share. I hope you’re finding them as useful as I am.
This site offers several tricks to improving a modest telescope, possibly even making something nearly useless into something useful. A wiggly mount could be the end of budding interest in astronomy, and a bad finder can make you want to chuck your ‘scope in the trash. Before you do that, take a look at these suggestions. For a few dollars, you might have something you’ll be happy to take to star parties and show off. There is such a thing as bragging rights for bargain fixes. Ask one of the grey-hairs at your local club about using film canisters for camera adapters sometime!
There is one things you should note though: the only fix for bad optics is to get new optics.
Telescope Dictionary March 18, 2010Posted by aquillam in Astronomy, MichiganAstro, telescope beginners guide.
Tags: astronomy, telescope, telescope beginners guide
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Below is an alphabetical list of terms related to amateur telescopes. References are to other posts in this blog.
I will continue to update this as I think of other terms that I haven’t included. If I missed something, let me know!
- Alt-Az A type of coordinate system or mount. See Alt-Az Mounts.
- Altitude height above the ground. See Alt-Az Mounts.
- Aperture Generally, the width of a telescope. It is usually the width of the objective lens or primary mirror, however some telescopes, especially older ones, may have an aperature stop that permantly limits the telescope aperture.
- Aperture stop A mask that cuts down on the ammount of light the enters a telescope
- Acromatic A lens that corrects for chromatic aberration.See Refracting Telescopes.
- Apocromatic A lens that corrects for chromatic and spherical aberation. See Refracting Telescopes.
- astrophotography Photographing celetial objects. CCDs were invented for professional observers, and are now common in digital cameras.
- Azimuth Direction relative to north. See Alt-Az Mounts.
- Barlow A special lens used in conjunction with an eyepiece to increase magnification. May also be used to mount a camera for photography with more magnification.
- C8 An 8″ schmidt Cassegrain telescope made by Celestron. The “orange tube” was one of the first high quality inexpensive amateur telescopes. (see also, C8 maintance and care)
- Cassegrain See Reflecting Telescopes.
- Catadioptric A system that uses both lenses and mirrors. Usually something like a schmidt cassegrain. See Reflecting Telescopes.
- CCD Charge-coupled Device – the chip that makes up the ehart of a digital camera.
- Clock drive A device that drives an telescope on an equatorial mount to keep it tracking on an object. Technically, a clock drive is mechanical, but some people use the name to refer to any drive.
- Corrector or Corrector plate A sort of lens in a reflecting telescope that corrects problems like spherical aberation. See Reflecting Telescopes.
- Declination or Dec The coordinate that measures the distance of an object from the equator. See Equatorial Mounts
- Diagonal A prism or mirror that redirects the direction of light to make it easy for the observer to look through the telescope.
- Drive A device that keeps a telescope tracking on an object. This is easiest with an equtorial mount, but a computer controlled drive can also drive a telescope on an alt-az mount.
- Dobsonian See Reflecting Telescopes.
- Dobzilla A very large Dobsonian Telescope.
- Equatorial Mount See Equatorial Mounts
- Eyepiece the lens the observer looks though.
- Eye relief The distance back from the eyepiece where the observer can focus and get the full field of view. A long eye relief is especily important if you need to wear eyeglasses.
- Exit pupil The diameter of light coming out of the eyepiece. A wider exit pupil is easier to observe with.
- Field of View The area visible through the telescope. See Basic Telescope Properties.
- Finder ‘scope A small telescope with low magnification and wide field of view attached to a larger telescope to help the observer find objects.
- Focuser The thing that alows the observer to focus the image.
- F-ratio The ratio of the focal length to the aperture. See Basic Telescope Properties.
- Galilean See Refracting Telescopes.
- Light gathering power A measure of the amount of light a telescope can collect. See Basic Telescope Properties.
- Magnification A measure of how much bigger an object appears. See Basic Telescope Properties.
- Meridian a reference line that runs through north and south. Astronomers usually refer to the local meridian, which passes directly overhead. When an object crosses the meridian (transits), it is a its highest altitude. See Alt-Az mounts.
- Newtonian See Reflecting Telescopes.
- Objective lens The large lens at the front of a refracting telescope. The lens that points at the object.
- Observer The person looking through the telescope, a person who regularly looks through a telescope, or an astronomer who observes as opposed to a theorist.
- Pier A permanent mount for a telescope. See Alt-Az Mounts and Equatorial Mounts.
- Primary mirror The mirror in a reflecting telescope responsible for collecting the light.
- Reflector A telescope that uses a mirror to collect light. See Reflecting Telescopes.
- Refractor A telescope that uses a lens to collect light. See Refracting Telescopes.
- Resolution A measure of the detail visible through the telescope.
- Reticle Wires or markings in an eyepiece to help with centering or measurements. Commonly found in the eyepiece of a finder ‘scope.
- Right Ascention or RA The coordinate that corresponds to longitude on the sky, and measures position east or west of the vernal equinox. See Equatorial Mounts
- Schmidt A type of corrector used in reflecting telescopes. See Reflecting Telescopes.
- Secondary The small mirror in a reflector that re-directs the light to a position where it can be observed. See Reflecting Telescopes.
- Shower cap A cover for the end of a telescope similar to a shower cap to protect it from dirt (or sometimes, actually a shower cap placed over the end of a telescope)
- Sidereal Time Time measured by the stars. A sidereal day is 24h, 56 minutes, and the local sidereal time matches the local solar time (the time on your watch) on the autumnal equinox. The RA of the meridian is equal to the sidereal time.
- Star trails The long curved lines that show up in long photographs of the night sky.
- Sucker hole A gap in the clouds big enough to convince you to get your telescope out, even though it will probably be gone by the time you are set up.
- Tripod A three legged mount. See Alt-Az Mounts.
- Wedge A device that attaches to a tripod or pier that tilts the telescope at an angle, usually to make it an equatorial mount.
Telescope – Beginners Guide: Reflector February 12, 2010Posted by aquillam in Astronomy, MichiganAstro.
Tags: astronomy, telescope, telescope beginners guide
Reflectors use a big mirror to collect the light. The biggest reason for using mirrors is because you can support the entire back side of the mirror (note the ring-style support on the Newtonian in the picture at left), which means it is possible to build truly huge mirrors. In fact some of the professional telescopes have flexible mirrors and computer controlled struts that can help correct for atmospheric turbulence.
Some other advantages to this is that no light is absorbed by the mirror (as much as 4% of the light can be reflected at every surface with lenses, and some light is absorbed as it passes through the lens.) Many reflectors are also a “folded” type, which makes them much shorter. Additionally, there are ways to correct the problems with mirrors without using really long focal lengths, which also makes the tubes shorter.
All reflectors must have a primary mirror to collect the light (obviously – that’s why they’re called reflectors!) After that, you can get all sorts of strange configurations. Many of the configurations are designed to solve problems, so lets start with that.
Spherical aberration occurs when spherical mirrors are used. It looks the same as spherical aberration in refractors, and the cause is similar. Light hitting near the edge of the mirror focuses to a slightly different point than the light coming in close to the center.
There are a couple possible solutions to this. One solution is the same as one of the solutions for refractors: use a mirror with a really long focal length. Of course one of the reasons for using mirrors is so you don’t need a really long OTA.
The best solution is to use a hyperbolic mirror. A hyperbolic mirror is a solid of revolution, with the shape of a hyperbola, a conic section that is curved at the center and the edges diverge forever. However, making a hyperbolic mirror that is perfect is hard, which also makes it expensive. The best known hyperbolic mirror is the primary in the Hubble Space telescope. That mirror was made with a small error in the shape, so corrective optics ( frequently refered to as “eyeglasses”) had to be added to all the instruments, many of which had to be replaced. However, even before the instruments were fixed, Hubble was a better telescope than anything on the ground at that time.
Parabolic mirrors also correct spherical aberration, but they introduce coma (see below.)
You can also design a corrector plate that bends the light slightly as it enters the ‘scope so that the light ultimately all comes to the same focus. This of course adds a lens back into the system, so some of the advantages like light loss are added back as well. However, it is a much cheaper alternative to hyperbolic mirrors.
Parabolic mirrors do not suffer from spherical aberration, and are easier/cheaper to make than hyperbolic mirrors, so they are a good alternative to spherical mirrors. If you are viewing something near the center of the field of view, they are great. If you look off-center, the stars may appear elongated, almost comet like. There is a nice image of the Orion nebula with coma at http://portableastronomy.com/i-cubed.htm.
Coma occurs because light coming in to a parabolic mirror from a little off-center doesn’t come to a nice focus. Instead it focuses in more of a line, with a sharp edge close to the center.
Types of Reflectors
The simplest possible reflector, the prime focus has no secondary mirror. It is just a large primary and with an equipment package at the focal point. The best prime focus telescopes use hyperbolic mirrors. Most professional telescopes are this type (see for example, the popular picture of Edwin Hubble in the observer’s cage at Palomar.) However, they are a really impractical design for amateurs. Unless you happen to be a really really wealthy amateur.
The first reflecting telescopes were designed by Issac Newton, around the same time he was experimenting with poking himself in the eye (section 58 – not for the faint of heart!) Newton realized he could use a large mirror instead of a lens to collect the light, then place a plane mirror just before the focal point to direct the light off to the side where he could observe it.
Newton’s first telescope used a spherical mirror, but he changed his design after Christiaan Huygens suggested that he use a parabolic mirror to avoid spherical aberration, so maybe it would be better to call it a Huygens-Newtonian telescope. The standard Newtonian telescope is a parabollic primary mirror with a plain secondary mirror angled at 45º so the light exits out the side of the telescope. The secondary is placed just short of the focal length of the primary, so the length of the OTA is roughly the focal length of the primary.
Since coma gets worse the farther off-center the object is, most Newtonians are relatively narrow. Also, since the OTA has to be roughly the focal length of the primary, they tend to be shorter focal length mirrors, which also helps reduce coma.
The eyepiece of a Newtonian is at the top of the tube, which makes them ideal for alt-az and particularly Dobsonian style mounts. Some of them (frequently reffered to as “Dobzillas”) are big enough to need ladders. See for example http://samples.briskbuild.biz/deepskyranch/dobzie/dobzie.html, http://www.flickr.com/photos/25411368@N07/3302802968/, and http://www.cruxis.com/scope/scope1070.htm.
At about the same time Newton was designing his telescope, a Frenchman named Cassegrain was designing a different reflecting telescope. His design used a concave primary and a convex secondary to send the light back out a hole in the primary. This folded up design made for a short tube, and the eyepiece out the back would have made the telescope easier to point. However it took some time before the design caught on, probably because Newton and Huygens absolutely trashed the design.
The major problem of the Cassegrain telescope is that it relies on two spherical mirrors. To really have a good Cassegrain telescope, you need some sort of corrective optics. With modern materials however, a corrector plate is relatively easy to make, and Schmidt-Cassegrain and Maksutov-Cassegrain are now very common amateur ‘scopes.
Modern Cassegrains have many advantages. They are compact, with OTAs much shorter than the focal length, which makes them highly portable. Since the eyepiece is out the back, it some people find it easier to point the telescope, and the telescopes can be mounted on equatorial mounts without worrying about where the eyepiece might end up (except if you want to look toward the north celestial pole.) They are also easy to balance if you want to add heavy equipment like a camera.
The Newtonian and Cassegrain telescopes (and Galilean and Keplerian refractors) were invented in the 17th century. Astronomers had to wait until the 20th century for the next real leap in telescope technology, when optician Bernhard Schmidt invented a corrector plate that could correct spherical aberration.
The simplest Schmidt telescope is the Schmidt camera, which is basically a prime focus telescope with a spherical mirror and corrector plate. In the mid-20th century many professional observatories built a Schmidt camera for large photographic surveys.
In the 1960s Tom Johnson developed a method of mass-manufacturing a good quality Schmidt-Cassegrain telescope. It became the Celestron orange-tube (which I am very familiar with.) Since then, the Schmidt-Cassegrain has become a very popular design for amateur telescopes.
Just a few years after Schmidt, a Russian named Maksutov designed a Cassegrain style telescope with a simpler corrector plate. The Maksutov corrector plate is a simple meniscus, with an aluminized spot on the inside to act as the secondary. This eliminates any problems of having to mount the secondary mirror. However, the corrector plate is limited in how much correction it can do, so the focal length of a Maksutov-Cassegrain is generally fairly long. Also, the field of view tends to be smaller than an equivalent Schmidt-Cassegrain.
In the 1960s, Questar made a beautiful (but expensive) Mak-Cass telescope, which is now a coveted collector’s item. Recently, some very compact, lightweight, and much less expensive mak-cass telescopes have made this a popular design again.
Telescope Beginners Guide: Alt-Az mounts January 26, 2010Posted by aquillam in Astronomy, MichiganAstro, telescope beginners guide.
Tags: astronomy, telescope, telescope beginners guide
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The altitude-azimuth system
Alt-Az mounts work in the altitude-azimuth coordinate system. Although the words sound big, this is the one you usually use when you’re just talking to somebody. For example,a friend says “I saw this really bright light just above the horizon in the east this morning. Do you know what it was?” You reply “Of course! Venus is very bright and low in the east in the morning right now.” In this case, “low” is the altitude, and “in the east” is the azimuth.
Altitude is in degrees above the horizon. It is generally assumed that the zenith is at 90º, and the horizon at 0º. Of course what you can actually see can be affected by your location: a lot of trees may limit you to within a few degrees of the zenith. Colloquially, we may also use “high” or “low” for the altitude, but since different people have different ideas about where high stops and low begins, that’s not as helpful as actual degrees.
Azimuth is the direction along the horizon, such as east, or southwest. For more precise mounts, it is measured in degrees away from due north going clockwise. That means due east is actually 90º, and an azimuth of 200º would be SSE.
Examples of mounts
This is one of the easiest mounts to make. The most common design is a turntable with an axis for altitude adjustment. Since azimuth is along the horizon, it is important that the mount be horizontal, or level. The simplest version, or at least the cheapest, is probably cardboard box mount. You turn the box to rotate in azimuth, and rotate the telescope to rotate in altitude.
The most common place you are likely to encounter an alt-az mount is a tripod for cameras or small ‘scope. a standard tripod has a mount that rotates in azimuth and has a handle for making quick adjustments in altitude.
A very popular mount, especially for do-it-yourselfers is the Dobsonian. A quick search for “home built dobsonian” turns up dozens of examples. The body of a Dobsonian mount is actually just a box on a turntable. Usually, a pivot attaches to the OTA and rests in a cradle in the box. A carefully made Dobsonian may be accurate enough for setting circles and even a drive mechanism, which helps mitigate some of the disadvantages of alt-az mounts. One thing to note though is that these mounts really only work with Newtonian style telescope, where the eyepiece comes out the side of the tube near the top. If it’s a very big telescope, that may mean you’ll need a ladder to observe.
Most “tabletop” scopes are alt-az mounts. Dobsonians are common because the turntable can sit on the table without adding much of any height to the telescope. The OTAs are usually light enough that only one support arm is needed.
Advantages and Disadvantages
- Easy set-up. Alt-az mounts are the easiest mount to set up. Dobsonian types you just plop down on a level surface, pop in an eyepiece, and you’re observing. If it’s a tripod, you’ll have to extend the legs, but you can make adjustments if the ground isn’t level.
- Easy to operate. They work the same way you usually think about looking for things: up and over, so beginners usually find alt-az mounts much easier to aim.
- Cost. As with anything, you can get some very fancy, expensive alt-az mounts. But you can’t beat a cardboard box for price.
- Easy to make. If you like building things yourself, a Dobsonian is the way to go. You just have to make sure everything is square and you’ll have a working mount.
Tracking is hard. Unless you are observing the pole, any target you set the ‘scope on will move. Unless you are observing at the equator, the target will move at an angle, so both the altitude and azimuth will change. Even harder, the rate they change at depends on where the target is in the sky. A star close to the pole may only change 5 degrees in altitude over the course of the night, but a star that rises in the northeast may cover almost 90º over the course of the night. Images of star trails illustrate this. Near the pole, stars don’t move much over the time it takes to capture the image, so the trails are short, but farther from the pole the stars move more, and the trials are longer. Modern telescope mounts overcome this problem by having a computer control the tracking, but that means if you want an alt-az mount that will track, you’ll have to spend the money on a computerized mount.
- Coordinates are location dependant. The north celestial pole is at about 45º altitude, 0º azimuth for people along most of the US Canadian border, but only about 30º altitude for people on the US Mexico border. And of course, most objects change in azimuth as the night goes on. This means if you want to get the coordinates of an object like a comet, you’ll have to know your latitude and what time you’ll be looking for it. Again, the computer controlled mounts can fix some of that. The computer can calculate where it should point as long as it knows where you are and what time it is.