$$\def\|{&}\DeclareMathOperator{\D}{\bigtriangleup\!} \DeclareMathOperator{\d}{\text{d}\!}$$

[529]

## 1. Artificial Satellites

An artificial satellite is a built thing that orbits around a celestial object. Most artificial satellites are very expensive, so they must be useful, or else nobody would want to spend the money to build them. Here are a few things that satellites can do that we cannot do as well from the ground:

1. See a large part of the Earth at the same time (up to half of the Earth at a given time). That can be handy to take measurements from a large part of the Earth at the same time (e.g., weather satellites), or to broadcast to or receive from a large part of the Earth (e.g., navigation satellites and communication satellites).
2. See parts of the Earth of which it is dangerous or difficult or to you not allowed to travel there across the ground (e.g., military satellites).
3. Look at the stars the whole time, instead of only when it is night and only when the weather is good.
4. Look at the Sun the whole time, instead of only during about half of the time, if the weather is good.
5. Take much sharper pictures of the starry sky because you're not looking through the turbulent and shimmering atmosphere.
6. See kinds of radiation from the Universe that do not pass (well) through the atmosphere of the Earth (e.g., infrared radiation and X-rays).
7. Travel to other moons or planets and take close-up pictures and measurements.

[394]

## 2. Satellites See Colors

The filters that satellites use to take pictures in particular colors are selected based on what the images are meant for (and on how expensive the filters are). A weather satellite uses filters that make clouds or water vapor or other weather features stand out well, so that you can recognize them easily. A military satellite may use filters that allow you to see things at night, or that make buildings or other man-made things stand out well. An agricultural satellite uses filters that make different kinds of crops and vegetation stand out well in different ways, so that you can tell which crop or type of vegetation it is. Astronomical satellites may use filters that allow you to see particular kinds of atoms or gases of a particular range of temperatures.

The filters don't all need to be for visible light, either. There are satellites that can see infrared radiation or ultraviolet radiation or X-rays. Each different satellite usually has a different set of filters on board, and they do not all need to be used to take the same pictures.

[188]

## 3. How Many Man-Made Satellites Are There In Space?

It is estimated that about 8000 satellites have been launched into space so far. An organization in the U.S.A. that monitors satellites reports 5643 satellites launched until and including November, 2003, but that does not include secret satellites and also does not include satellites that are too small or too far away. Of those satellites, 2974 were still in orbit at the end of November 2003, and 2669 had fallen back into the atmosphere of the Earth where almost all of them burned up.

When a satellite is launched, then sometimes parts of the rocket that brought the satellite into space also keeps floating through space. If an astronaut loses a screwdriver during a space walk, then that screwdriver orbits the Earth for a while, too, and so does garbage from the International Space Station. So, besides the satellites there are also many pieces of space junk that orbit around the Earth. At the end of November 2003, 9275 pieces of space junk floating around the Earth were large enough to be detected from Earth.

[168]

## 4. Weather Satellite Pictures

How you should interpret a weather satellite picture depends very much on what kind of light or other radiation is recorded in the picture. Some pictures are based on ordinary light (of all colors or just specific colors or spectral lines), and other pictures are based on various kinds of infrared or ultraviolet light. Each different color is differently sensitive to the circumstances in the atmosphere, such as the temperature at a certain height, or the presence of water vapor, or the amount of sunlight that is reflected. Weather satellite designers on purpose select different colors that are differently sensitive to the circumstances, so that they get as many independent measurements as possible to use to predict the weather. In general, you can figure out for weather pictures available on the internet by which instrument they were recorded and what kinds of radiation they record.

If they are visible-light picutres, then white spots indicate where much sunlight was reflected. That might be because there are clouds there, or snow, or ice, or (if it is a black-and-white picture) perhaps because there is a desert there.

If it is an infrared picture, then then often tell where the water vapor is at great altitudes, and where the cold fronts and warm fronts (or perhaps both) are.

Making accurate weather predictions is not easy, even if you have satellite pictures and know what kind of radiation they recorded. That's why different weather predictors don't always agree about what the weather will be like, even if they have big computers and many weather satellite pictures available.

You should go to a meteorologist or a weather bureau for specific advice about predicting the weather from satellite pictures.

You can usually find books about the weather in bookstores and libraries, and they often contain weather satellite pictures with explanations.

[133]

## 5. Interferometry

Interferometry is a technique for measuring very small differences in distances that waves travel. Those could be sound waves, or surface waves (like waves on water), or electromagnetic waves (like light or radar waves), or other wave phenomena. Using interferometry, you can combine measurements from different telescopes such that it appears as if those measurements came from one giant telescope that was so big that it reached all the way from the location of the first telescope to the location of the second telescope. For example, if you combine measurements from two telescopes that are 2 kilometers apart, then you can see details in the results just as if you had been using a single telescope that is 2 kilometers in size.

This combining of the observations of different telescopes is not easy, so not everybody does it. You need very accurate and therefore expensive equipment, and it takes a whole lot of calculations to properly combine interferometric measurements. So far, in astronomy, this technique has been used mostly for radio waves, for example by the Westerbork Synthesis Radio Telescope in the Netherlands, by the Very Large Array in New Mexico, and by the VLBI-network that interferometrically links telescopes in different countries, but people are starting to use it on light waves as well now, for example by the Keck and Gemini telescopes.

The receiving surface area of a few small telescopes at 2 kilometers from each other is of course very much smaller than the receiving surface area of a telescope that is itself 2 kilometers in size, so you may be able to use small telescopes and interferometry to see things that are as small but not things that are as dim as those that a single telescope of 2 kilometers in size can see.

[112]

## 6. Large Telescopes

You can find a list of the largest optical telescopes at //www.seds.org/billa/bigeyes.html. The Keck I telescope, which is currently the largest together with Keck II, is operational since 1993. Keck II followed in 1996.

[68]

## 7. Telescope Orientation

To be able to see a certain star or planet or the Moon through a telescope, all you need to do is to point the telescope at that star or planet or the Moon, just like for a pair of binoculars. To practice, try it on something relatively big and bright, like the Moon. If you have a telescope with an equatorial mount, then the telescope tube is attached to a think slanted axle, and then it is best if that axle points exactly at the celestial pole (which is close to the Pole Star, if you're in the north). The manual of the telescope explains how this works. If you don't have a (more expensive) equatorial mount but rather a (less expensive) alt-azimuth mount, then select the orientation that allows the most comfortable viewing (while still showing the star that you're interested in).

[69]

## 8. Stance when Watching the Stars

Stances that can be recommended for watching through a telescope are those that you can keep comfortably for a long time. Which ones those are depends on your telescope and on where in the sky the star is that you want to watch. Because the starry sky slowly rotates, you cannot watch through the telescope while seated, at least not if you want to follow your subject along the sky with the telescope, so usually you'll watch through the telescope while standing or kneeling.

[64]

## 9. Resolution

The resolution of some optical instrument such as a telescope or a pair of binoculars or an eye indicates the size of the smallest things you can still recognize if you use that instrument. It is more difficult to see smaller things, so people say that the resolution is better if you can see smaller things, and worse if you can see only larger things.

Resolution is often measured in arcseconds. An arcsecond is a measure of angle. You can express any angle in arcseconds, though 1 arcsecond is a very small angle and therefore only commonly used for things that appear very small in the sky. For example, the diameter of the Moon in the sky is about 1800 arcseconds, which is the same as half a degree.

The question is: what does it mean when someone says that their telescope has "bigger resolution"? Does that mean that it resolves more, i.e., can see smaller details, or that the size of the things it can resolve is greater, i.e., it can see only larger details? This can get confusing. It is best to talk of "better" or "worse" resolution, or perhaps of "greater" or "less" resolution, but not of "bigger" or "smaller" resolution.

[113]

In general, the resolution of a telescope is proportional to the wavelength of the radiation that is being recorded. So, the Hubble Space Telescope, like other telescopes, can get the best resolution at the smallest wavelengths. However, great resolution is useless if you can't actually see or record that kind of radiation, so you have to take into account the range of wavelengths that are visible to the instrument. Most optical telescopes can see visible light and perhaps a little bit beyond the visible colors into the ultraviolet or infrared. They get the best resolution at the blue/ultraviolet end of that range, where the wavelengths are smaller than at the red/infrared end. It can make a difference of about a factor of two in resolution.

[181]

Suppose that you can just distinguish two things that are a distance $$b$$ apart (from the center of the one to the center of the other, perpendicular to the direction to the telescope), when they are at distance $$d$$ from the telescope. Then you can calculate the resolution of the telescope using the following formula (which assumes that $$d$$ is much greater than $$b$$):

$$r = 57.3° \frac{b}{d} = 206265″ \frac{b}{d}$$

The resolution $$r$$ is then the smallest angle that you can just barely resolve with that telescope. For example, if you can resolve two things at 0.03 m from each other when they are 115 m from the telescope, then the telescope has a resolution of 57.3° × 0.03 / 115 = 0.015° or 54″ (arc seconds).

The resolution of a good telescope depends on the diameter of the opening or lens where the light enters the telescope. If that diameter is $$s$$ centimeters, then the best resolution that is theoretically possible is equal to $$0.0036/s$$ degrees, but most telescopes don't even get close to that limit.

languages: [en] [nl]

//aa.quae.nl/en/antwoorden/instrumenten.html;
Last updated: 2020-07-18