Setting Up and Using Your First Telescope
Published on May 12, 2006 at 1:27 pm.
1 Comment.
Filed under amateur astronomy, telescopes.
A while back, while at a public star party, someone came up and asked me a question. Well, OK, so that happens at every star party. Well, this question was from someone who had recently purchases a small telescope. I get these all the time. Many people go to the store, buy a telescope, get it out of the box, and then realize that they don’t know how to use it. I am not sure why this is a surprise. After all, if they bought a piano, do they think that it would come with a ten page instruction booklet explaining how to be a concert pianist? Buying a set of high end pots and pans does not make someone an expert chef. For some reason, though, they think that buying a telescope is different, and that they should be able to set it up, look through it, and see HST quality pictures. Well, folks, it doesn’t work that way! In fact, setting up the telescope often involves adjustments that are not obvious. Very few telescopes can be used effectivly right out of the box. To make matters worse, over and over I see the same thing: the less that someone knows about using a telescope, the more difficult to use telescope that they buy.
So, what makes the telescope easy or difficult to use? Well, the mount is one thing. Telescope mounts come in a couple of different varieties: altitude-azimuth mounts and equatorial mounts. The equatorial mounts are useful because they have one axis parallel to the rotation of the Earth. That means that they only need to turn in one direction, and at a constant rate, to track an object in the sky. Altitude-azimuth mounts need to move on both axies of rotation to track something. Most inexpensive mounts are altitude-azimuth in nature. That means more complicated motion. That is OK, but it means more work for you. Often I tell people that if they want to buy a telescope for astronomical work, an equatorial mount is a worthy investment, even if they are buying a purely manually operated mount. A tracking motor is a very nice accessory. Generally, I tell people that they should never purchase a refractor on anything but an equatorial mount. OK, I have seen some decent refractors on altitude-azimuth mounts, but they are rare. As for reflectors, small ones should also always be on an equatorial mount. The large ones, though, as long as they are used at fairly low power, work well on a Dobsonian mount ( a type of altitude-azimuth mount). Personally, I think that the Dobsonian variant is about the only type of manual altitude-azimuth mount useful for astronomy. There is a caveat here, though. Computer controlled and guided telescopes can operate with pretty much any type of mount. You set them up and then tell them what you want to point at. If you set them up right, then they will automatically point themselves at (or near, more likely) the thing that you wanted to look at. They will then keep the telescope pointed at the object, no matter how complicate the motion needed to do so. Of course, you pay more for that capability, but the prices are dropping very quickly. I imagine that in the not too distant future, the majority of all amateur astronomer telescopes will have this goto capability.
For equatorial mounts, you need to align one axis with the rotational axis of Earth. This is called polar alignment. There are several ways of doing this. I won’t go into them here. If all that you want to do is look at something for a while, then even a rough alignment will do the trick. Altitude-azimuth mounts don’t need polar alignment, so they are easier to set up.
The other important thing is to have a STABLE mount. You also want some way to make fine adjustments. At the last star party, a kid brought a telescope that his parents had bought him for me to show him how to use. Well, it was a cheap refractor, was on an altitude-azimuth mount, and the mount wobbled every time that you touched it. There were no fine adjust controls. So, even if you got the object that you wanted in the field of view, then it would move out of view, and you had no easy way to get it back into the field of view. That meant loosening the screws holding the telescope in position. However, the mount was so loose that any touch to those screws sent the image several fields of view from the eyepiece. There was also a massive amount of backlash in the mount, so you could get it set on something, but tightening the set screws moved the telescope off of the target. Even if you managed to tighten the screws, then when you removed your hand, the weight of the telescope moved it off target … even with the mount locked in place! The kid was really disappointed that I couldn’t show him an easy way to use his telescope. Well, the sad truth is that there WAS no easy way to use that telescope! So, you want a STABLE mount, fine adjust controls, and no slop in the controls. I have seen some where there was so much packing grease in the mount that when you moved the controls, the telescope would keep moving for 5 to 7 seconds after you quit turnig the adjustment knobs! If you have such a problem, then clean the packing grease out of the gears. WD-40 seems to do a good job of washing most of the grease out.
OK, so now you are set, right? Nope. You need to make sure that the finder scope is pointing at the same place as the telescope. Look back at my earlier blog entry on finder scopes to read more about them. This is your sighting aid. You will need it, since the telescope itself sees such a small portion of the sky that you must have it set pretty much on target to start with. That is what the finder is for, but it only works if it is pointed where the telescope is pointed.
Now, you are set to use the telescope, right? Well, sort of. Next you want to select the right eyepiece. The magnification of the telescope is the ratio of the telescope’s focal lenght to that of the eyepiece. The longer focal length eyepiece, the lower the magnification. Normally, that is what you are after: low magnification to start with. It is tough enough finding something, so low power lets you see the biggest portion of the sky, and that makes it easiest to find something. Then, you can go to higher power if you want. Often, you find the clearest views at low power, however. That is the other problem that most beginners make. They try to use the highest power available. How do you know what the focal length of the eyepiece is? It should be written on it somewhere. You can’t go by the physical size of the eyepiece. That is often related to the focal length, but it is not always true that the largest eyepiece has the longest focal length. It depends somewhat on the composition of the eyepiece. There are different types, most using varying number of lenses in them. This can make for all the eyepieces being the same length, the longest focal length being the longest eyepiece, or even the shortest focal length being the longest eyepiece. So, you need to read the focal length from the eyepiece itself. Also, the higher the magnification, the dimmer an object normally will look. So keep that in mind, too.
OK, you’ve got low power, a stable mount, aligned finder, …, you’re ready to go, right? Yep. Not the fun starts. It takes practice and skill to find something. It is MUCH tougher than it looks. I’d recommend something easy for a first attempt — like the Moon. It is amazing how long it takes someone first using a telescope to find the Moon. Then, you can go for something bright, like Jupiter. Take your time. Get used to finding bright things first, then dim things. Eventually, you can go for things too dim to see. Huh. How can you find something too dim to see? Simple. You can look on a star chart. You might find that the object of interest M-X for example, is located just north of a little triangle of stars that is east of a fairly bright star that you can see with the naked eye. So, you point the telescope towards the bright star. Get it centered in both the telescope and the finder scope. Then, looking through the finder, you slide over to the little triangle of stars. Then, you move to where the cross hairs of the finder point right where the star chart says that the object is located. Now, if everything worked right, when you look in the telescope, you can see the object. Normally, you just missed it, but by using the fine adjust knobs to move the telescope a couple of fields of view in every direction, you’ll find it. This procedure is called “star hopping.”
Now, if you have one of those cool computer controlled telescopes, you just input M-X and it will point towards it all by itself. Well, that is in theory. There are a few telescopes that have GPS systems so that you turn them on, and they know how they are oriented and can align themselves. Most, however, you have to do the work. This means pointing the telescope at two or three known stars. The telescope then knows how it is oriented and can then point to something else. But to point at those stars, you need to know which stars are which and how to find them. That means doing all of the steps in the paragraph above. So, you still need to learn to star hop.
Finally, an new amateur astronomer came to me at a recent star party and asked about the little number scales along the axies of the telescope. This person had already learned to align the finder and to star hop. But, she wanted to know about what we call “setting circles.” For a polar aligned equatorial mount, the axies of rotation of the telescope also correspond to the motion east and west in the sky, and north and south in the sky. These are along the grid lines of the celestial coordinate system. So, you can use this alignment to more easily find something. The celestial equivalent of latitude is called declination. The celestial equivalent of longitude is called right ascension. So, you would just have to point the telescope at a known star. You then can adjust the movable circles on the telescope to read the correct right ascension and declination for the star. Then, you just adjust the telescope until the right ascension and declination are reading the same as object M-X that you wanted to look at. If everything went well, then when you look in the telescope, you will see M-X. Again, how well this works depends on how well polar aligned the telescope is in the first place and how well you set the circles. Any errors in either will make for errors in the result. Often it is good to set circles from a star fairly close to the object of interest. Also, the longer the time since when you set the circles to the corret right ascension and declination of the reference star, then the more time that error can creap into the system. Also, on many older systems, the setting circles don’t track, so they need to be set immediately before you use them. So, do this procedure quickly. The longer the time delay, the less likely that you’ll point the telescope at the right spot in the sky. Really, setting circles is what the computers are doing in the computer controlled telescopes.
So, that is how you set up and use a telescope.
-Astroprof
Astroprof’s Page » Your first telescope — Part One on December 27, 2006 at 8:21 pm: 1
[…] Every year, after Christmas or after a birthday, people ask me for help with their new telescope. It is wonderful and exciting to get a telescope as a gift, but if it’s your first telescope, then you often don’t have any idea how to use it. Any instruction manual that comes with it either is incomplete, not telling you enough, or assumes some prior knowledge. Now, for me, that is fine. But for anyone new to astronomy, it is a bit daunting. So, I thought that I’d post a series on how to use your new telescope. I have posted a previous entry on setting up your first telescope, but it may be useful to go over some of this again. First of all, let’s get the terminology down. A telescope that focuses light using only lenses is a refractor. A telescope that focuses light primarily with a mirror is a reflector. A telescope that focuses light using a mirror and a lens is called a catadioptric telescope. The distance at which the light is focuses is called the objective focal length. The objective element is the part of the telescope focusing the light, and it is either a mirror or a lens. The objective aperture is the diameter of the objective element. Telescopes are often described by the ratio of the objective focal length to the objective aperture, and we call this the f-ratio. The lower the f-ratio, typically the larger the field of view, and the higher the f-ratio, the more magnification a given eyepiece provides. For most beginners, the f-ratio isn’t all that important. For astrophotographers, though, it matters. The lower the f-ratio, the shorter exposure will be needed. For visual observations, a low f-ratio will make something look brighter and smaller with a particular eyepiece, but this isn’t really a major issue, since you can just change eyepieces. […]