How does glass affect taking solar spectra?

How does glass affect taking solar spectra?

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In this case I'm using a CCD camera mounted telescope pointed at the clouds to take solar spectra and was wondering how the telescope being pointed at the clouds through a double gazing (two layer) glass window would affect the spectra, if at all?

you'll need to find the spectral transmissivity of each piece of glass. Most likely they all block UV and large sections of the IR bands. Further, unless you've removed the internal IR filter, your camera won't record any IR. CCDs also cut off in the blue, so no UV will be recorded.

BTW, how are you separating the wavelengths at the camera? Typically one uses a (expensive) grating spectrometer.

Spectrum Scientifics' Store Blog

So we are either a) setting up and cursing our solar projection system, b) cursing at the clouds, c) wondering how we can live to the year 2117, d) wondering why we didn’t make such a fuss for the 2004 Venus transit.

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Projection Solar Astronomy

With the just passed annular eclipse out in the Western part of the USA a couple of weekends ago and the upcoming transition of Venus you might be inclined to buy a fancy solar filter for that telescope.

Well there might be a problem with that. See everyone else had the same idea as you, and that means the manufacturers of such filters are pretty much out of stock! It can even be hard to find just the filter material! What to do?

Well, there is another way of viewing the sun without the use of a filter. It can be tricky and it can be dangerous if proper care is not taken. That method is called projection astronomy. This is where you use the telescope & eyepiece to actually project the image you would normally see with your eye onto a board or other bright surface.

What do you need? You need a telescope with an eyepiece (preferably lower-medium powered), a sunny day, and something to project the image onto.

We came up with this kind of last minute, so we just used a flat box on a clipboard. It had problems with the box seams, but the surface was very bright (brighter than the average piece of printer paper) and so would give a decent image.

Next up, we need a telescope. We used an Orion StarBlast 6 , mostly because that is what we had around the store.

Note that the picture above shows the telescope in action. When setting it up and aiming it you should LEAVE THE DUST COVER ON. This is the best thing for your safety.

Aiming your telescope at the sun is pretty easy, just try to get your tube to make the smallest shadow possible. When you think you are on target, remove the dust cover and see if there is any light coming through the eyepiece. DO NOT look into the eyepiece, ALSO DO NOT LOOK DOWN THE AT THE EYEPIECE. View it from the side. We are not responsible for your losing your eyesight.

Now a further safety warning: Try to avoid getting any body parts in the path of the light coming out of the eyepiece. Hold the board on the edge, work around the telescope, not over it, etc.

So now that you are lined up with the sun, you can see what kind of image you have. Place the projection board about a foot or so away from the eyepiece. Move it closer or further away to try and get it into focus. DO NOT bring it closer than 6 inches from the eyepiece, the light is a little too concentrated there and some types of paper might burn.

Some adjusting will be needed. It is best to move the screen rather than the telescope. If you must adjust the focuser, put the dust cover over the front of the telescope.

So what kind of view so you get? Well, here is a shot of the screen that is a little closer than the last photo:

This didn’t show up too well in the photo, but if you look closely, you can see some sunspots projected on to the board to the right and right/down of the center.

How well will projection astronomy work on the transit of Venus? We don’t know for certain. The sun will be very low in the sky and there may be more distortion or discoloration as a result. But if you have the time and the telescope, this isn’t exactly an expensive experiment! Just remember to be careful! Pointing a telescope at the sun always has risks – just use common sense and keep out of the path of the projected sunlight and you should be fine!

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Next Up in Solar Astronomy – The Transit of Venus June 5th (& 6th)

So, yesterday the Western part of the USA got to enjoy an annular eclipse…NO WE ARE NOT BITTER!

In any case, this is not the only solar event for us this year, because on June 5-6th there is the Transit of Venus!.

A transit is when one of the inner planets (which is pretty much just Mercury and Venus) moves between the Earth and the Sun. A small shadow of that planet can then be seen through a filtered telescope. The last transit of Venus was only just in 2004, but the next will be in 2117! So don’t miss it.

Note: Satellite Photo Image of Venus – your view probably won’t be this crisp!

Remember that looking at the unfiltered sun is very dangerous. Always use a filtered telescope or a projection system to view the sun!

The transit will be visible through the entire United States (and Canada & much of Central America) during the evening/sunset on June 5th. The transit will be in progress when the sun sets. Most of Europe will be able to see the transit on the morning of June 6th. Parts of Asia, Australia and Alaska will be able to see the whole transit.

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The Annular Solar Eclipse – May 20th 2012

Its a bit hard to write about a solar eclipse when the rain is falling, and doubly so when the viewing area is not where you live. But this is a rather important astronomical event and many readers might be in a position to actually see it. We are talking about the Sunday, May 20th annular solar eclipse.

Annular eclipses are when the Moon is positioned in front of the sun, but unlike total eclipses the sun is not completely blocked due to the Moon being further away than in a total eclipse. This means a reduced apparent diameter and results in the “ring of fire’ appearance.

The majority of this eclipse will take place over the Pacific Ocean, and only portions of the Western part of the USA will be able to see it take place close to sundown. Here is a rough map of the areas that can view a portion of this eclipse:

Folks in Alberquerque have all the luck!

Remember that special care should be taken to view an eclipse – view either via a projection method or with a properly made solar filter or eclipse glasses. Do NOT try to view using sunglasses. Even at sundown the sun can be excessively bright and harmful to your eyes.

Enjoy the eclipse if you can!

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Astronomy Hints #15: Filter! Filter!

One of the more wide-range things to use in astronomy are filters. There are a large number of them and their purpose varies greatly. They can reduce light, help with light pollution, bing out details, or help with astrophotography. Most filters thread on easily to telescope’s eyepieces and can change your viewing experience.

But when choosing filters one must remember this: They are filters, they are designed to remove something, even if it is unwanted. Some folks get the idea, especially with light-pollution filters, that filters make objects being viewed much brighter. But that is not the case. Think of it this way: if you have a kitchen odds are ou might have a water filter in your faucet or some kind of pitcher. When you use this filter you do not make more water by using it, you are merely removing the stuff in the water you do not want. Water filters are actually pretty good because if you put 1 liter of water over a water filter odds are you will end up with very close to 1 liter of clean water. But you won’t end up with 1.1 liters. Sounds obvious, but some folks get the idea that that is what astronomy filters can do. But it is not so. In fact using an astronomy filter on the light from the stars means you are going to lose some of the good light along with the stuff you do not want. If we go back to out water filter you can think of our 1 liter of unfiltered water becoming .9 or even .8 of a liter.

But let us discuss the various types of filters:

Moon Filters

Moon filters are simple neutral density filters (which means they evenly cut down on light across the visible spectrum) that thread onto your eyepiece. They are used because the Moon is actually very, very bright and viewing it in even small telescopes at it can hurt your eyes after a short time (not permanently, mind you). A Moon Filter can make viewing more comfortable. Typically filters allow in 25% of the light, or 12% (for larger telescopes) or in a variable model you adjust yourself .

Solar Filters

Solar filters are the only filters that do not thread onto the eyepiece. The go over front of the telescope. If you find a ‘solar filter’ that is meant to thread onto an eyepiece, destroy it immediately. Those are very dangerous as they can crack letting through sunlight that can damage your eyes. Don’t use them, Don’t keep them – someone else might be tempted. Destroy them.

Most solar filters are simple screens of Mylar that cuts down on 99.999% of the light so that you can safely view the sun. Mostly what you will see is a white disc with some sunspots. It makes for some nice viewing during the high points of the sunspot cycles, during the lows the sun can seem a bit featureless, however.

Another type of Solar Filter is the Hydrogen-Alpha Filter. These allow you to view reddish colored prominences and solar flares. They are very expensive, however (in the thousands of dollars) and they need a certain amount of ‘tweaking’. But they can give very impressive views of solar activity.

Color Filters

Color Filters are used on the planets or the Moon – they cut off too much light to use on deep sky objects. Color filters are used to try and bring out more details on the planets that might get washed out in regular filtering. Details brought out might include the bands on Jupiter, polar caps on Mars, more lunar crater detail, and so on. Color filters can be hit-or-miss among astronomers. Some think they are great, others find them less useful. The field seems rather subjective but if you plan on viewing the Moon & Planets more than anything else you might wish to invest in a set.

Light Pollution Filters

Light Pollution Filters are designed to help astronomers who live in light-polluted suburbs or cities. They are not a substitute for dark skies, but they can certainly help out when options are limited. Light pollution filters help by cutting down on frequencies of light that streetlights, parking lot lights, and other human-made light sources produce, while letting through most of the light that stars, nebulea, and other deep sky objects emit.

This set of quickie photos can give you and idea of the effect of the filters. Here is a shot of a city streetlamp that is on during the daytime:

Street Lamp, through a phone camera, daytime, unfiltered.

And then with an Orion Ultrablock Filer held over the lens:

Same streetlight, with light pollution filter held over the camera lens. Note the difference.

You will notice that the filter helps, but does not completely eliminate the streetlight light, and it does have some effect on the natural background light as well. This is why they are helpful but not a complete solution to dark skies.

Light Pollution filters also are of little use on the Moon (which is bright enough to not be bothered by light pollution) or the major planets (which are similarly unaffected by light pollution). They are of limited effect on the outer planets as those planets emit light over much of the spectrum and get filtered as much as the background light.Some light pollution filters may be referred to as Nebula filters, which are very focused and are even designed to cut down on some of the light from nearby stars.

Astrophotography Filters

There are a huge number of these and their uses could fill a book – a book about astrophotography that is. These filters do things like cut off the IR portion of the spectrum (which messes up CCD chips in digital cameras) or filters out only the all but the Red or Green or Blue part of the spectrum for monochromatic cameras. The number of these filters has expanded vastly in the past few years. Covering these would take a very large entry so we will leave them for now as it is beyond the scope of Astronomy Hints.

Refractor Violet Filters

These filters are for one type of telescope – refractors. The large lenses in these telescope sometimes act as prisms and break up the light into component colors. This is especially noticeable on bright objects like Jupiter or the star Sirius. The effect is that the object being viewed will have a violet colored halo that is affectionately known as ‘purple haze’. Violet-Minus filters cut out this portion of the spectrum without affecting overall viewing much. If you have a larger refractor that sometimes shows the ourple haze you might consider getting one of these filters.

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Paper Vertical Sundials, now where where we….

…oh right! We were telling folks how to get a paper sundial printed out. Well we got ours all printed out and happy. We cut it out and folded it up.

…and realized it would not last through the first drizzle. This would be a problem as summer rains happen almost daily at this point. So what to do?

Well, in our case we printed out another sundial and put it through the lamination machine! Now it was covered in plastic and protect from the elements, at least for a while. If you don’t have a lamination machine (most people don’t) then clear packing tape can help protect your sundial. Its up to you if it is easier to apply the tape before or after you cut it out. We certainly had to put the paper through the laminator before cutting it out (jammed lamination machines tend to burn).

Once we cut out the laminated sundial and taped it together we put it on the wall. Then we discovered there was a problem. Our store’s outdoor walls don’t have a lot of places where there are not other objects that case shadows on the sundial’s space. Take a look:

OK. so the big shadow you see is from the canopy of the store next to ours. During much of the day it completely covers the sundial’s shadow. But for a few hours the thing does a nice job of giving you the time. Take note that it does show time in a 24 hour clock pattern.

Hopefully you will have better luck than we did and have a non-shadowed space.

A Solar Observing Refresher Course

By: Jeff Medkeff July 17, 2006 0

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Solar filters are typically made with a specially coated Mylar or glass substrate mounted in a cell that fits snugly over the front of the telescope. Such filters offer safe white-light views of the Sun, revealing sunspots, faculae, limb darkening, and a host of lesser features.

Sky & Telescope photo by Craig Michael Utter.

"Solar Filter Safety.") Viewing the Sun also demands extra vigilance when it comes to equipment. Never leave a telescope or binoculars unattended, especially when children are about. It takes only a moment of inattentiveness to create a dangerous situation.

Welder's glass of shades 12 through 14 are popular and safe solar filters, easily obtained at welding-supply outlets. Most observers prefer shades 13 or 14 the solar image through a number-12 filter is uncomfortably bright.

Sky & Telescope photo by Chuck Baker.

The solar surface, seen in white light, includes sunspots, faculae, prominences, and other features associated with the magnetic fields that result from the motion of ionized matter in the Sun's interior.

Closeup of a sunspot. This image, taken by the Swedish 1-meter Solar Telescope on La Palma in the Canary Islands, resolves the filaments making up the spot's penumbra with a resolution of 0.1 arcsecond, courtesy of an advanced adaptive-optics system. The black-and-white images have been colorized here to emphasize contrasts. The smallest resolved structures are about 90 kilometers across.

Courtesy Royal Swedish Academy of Sciences.

Various types of solar filters are helping increase the popularity of solar observing. Here, Sky & Telescope staff members demonstrate several different ways of safely studying the Sun.

Sky & Telescope: Craig Michael Utter.

Small telescopes are especially suited to the solar-projection method. Seen here is a simple projection system made from a cardboard box with a piece of white paper as a projection surface. A surprising amount of detail can be seen with this setup.

Sky & Telescope photo by Craig Michael Utter.

"Observing The Sun By Projection.") An eyepiece is placed in the telescope's focuser and used to project an image of the Sun onto a convenient flat surface. Telescopes with folded light paths, such as Newtonians or Schmidt-Cassegrains, are not recommended since the converging beam of light can produce enough heat to damage internal components.

When it comes to eyepieces for projecting the Sun's image, the much-maligned Huygenian design is a good choice because it does not contain cemented elements that can be damaged by the Sun's intense heat. Most solar projection is done onto white paper or card stock. But no matter how white the screen, it must be adequately shaded from direct sunlight and other extraneous light in order for the viewer to see the finest details in the solar image. This powerful technique enables a 4-inch telescope to produce a usable image of the Sun 30 inches across. The size and brightness of the Sun's image depend mainly on the distance between the eyepiece and the viewing surface — the farther away it is, the larger and dimmer the image.

Sunspots are cooler regions of the solar surface caused by intense localized magnetic fields that bring the upward convection of internal material to a virtual standstill. Although they appear almost black, this is merely a contrast effect. If it were possible to place a modest-size sunspot into the night sky, it would shine 10 times brighter than the full Moon!

Even the casual observer will soon learn that sunspots come in a wide variety of shapes and sizes. While the simplest sunspots are isolated dark areas, larger spots are quite dramatic. Complex spots feature a dark central region called the umbra surrounded by a gray penumbra. The penumbra normally appears as a smooth fringe, but under steady seeing conditions it may exhibit radial patterns or knots of light and dark. During those fleeting moments of good seeing you may also see tiny circular sunspots 2 arcseconds in diameter or smaller. These are called pores. Sometimes they erupt into full-fledged spots but usually they simply disappear — sometimes after a lifetime of only a few minutes.

The Wilson effect, which is greatly exaggerated in this diagram, shows how the umbra and penumbra of a typical sunspot become more and more symmetric as they near the center of the Sun's disk.

Sky & Telescope illustration.

This sketch by Jeffery Sandel shows a complex sunspot group. Unlike the traditional method wherein the sunspots' positions are plotted and their outlines traced directly on the projected solar image, he draws them freehand on a blank 8-inch-diameter circle. This results in exaggerated sunspot sizes.

Solar activity varies with an 11-year cycle. As the cycle progresses, activity rises and falls, and with it the amount of detail visible on the Sun. At solar minimum, the Sun often appears nearly featureless, completely free of sunspots. At maximum, however, there can be hundreds of sunspots arranged in a half dozen or more groups and plenty of faculae. Obviously, the most exciting time to observe the Sun is in the years surrounding solar maximum. The last solar maximum was in 2000, and NOAA's Space Weather Prediction Center forecasts the next maximum for May 2013. So there's no better time than now to become a daylight astronomer!

ED glass in binoculars

ED means that the glass has Extra-low Dispersion. That by itself has no meaning as far as color correction and no bearing on the final color correction of a doublet, triplet or multi-element lens. Dispersion does not govern the color correction of a lens. The term "ED" does not imply any particular level of color correction. There are extra-low dispersion glasses that produce bad color correction, and there are high dispersion glasses that produce excellent color corrections.

The term "reduced secondary spectrum" is used to describe a lens that has better than achromat correction, but it will still have residual color. If a lens color error is not specified as apochromatic, you can pretty much bet on the lens still being a simple achromat. In many cases the term ED is nothing more than a marketing ploy.

From the notes of a world renowned lens designer
“The term ED means Extra-low Dispersion. A lot of glass has extra-low dispersion (generally any glass having a Vd value > 70). Not all glasses having this property will produce measurably better color correction than a standard crown-flint combination achromat. Measurably better would be better than 1 part in 2000 color error over the C-F spectrum (red to blue-green). Therefore, you can easily claim to have a (Semi-ED) lens if you use the common design FK5 crown and SF1 flint, yet have exactly the same color error as any normal achromat.

It is not ED that produces better color correction. It is the fact that some glasses, notably the ones using fluorite elements in their construction, have a property known as abnormal dispersion. This property allows two dissimilar materials (such as crown and flint) to have opposite and equal color errors which cancel when they are combined. Most (not all) of these glasses with abnormal dispersion also are Extra-low Dispersion glasses, but the two properties should not be confused”.
Roland Christen

Color correction of two different achromat scopes can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.

Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens.

From this, you can see how easy it may be to market a small, not so fast, ED scope to the viewing public and have it declared at least by potentially half the viewers that is is nearly color free, and yet it is still an achromat.

ED lenses can be doublets or triplets. Even in a well made ED doublet with well-matched ED glass, a longer F ratio will show better color correction. For instance an ED doublet 80mm f/7 will have less color than an ED 80mm f/5. Generally, no ED doublet with lower Abbe# ED glass will match the performance of higher Abbe# glass.

OK, so how does this relate to ED glass in Binoculars?

Setting aside what ED really means to the end user, take a moment to think about where ED glass is employed in the design and what may be the overall result. In an optical system, color error is far more dependant on the correction of the objective lens than the eyepiece. The ratio of contribution to (longitudinal) color error can be found by comparing the focal length of the objective versus the focal length of the eyepiece. Some may recognize this as magnification. So, at 10x the objective contributes 10x the color error as the eyepiece. There are many eyepieces labeled ED, however, their total contribution to color correction is quite small. In a 20x binocular ED color correction contribution from an eyepiece is limited to correcting 5% of the longitudinal CA error.

So, having two binoculars of apparently the same quality, why is it one binocular can look so much better than another? Refer to the portion above about bias. Color correction of two different achromat scopes (two different binoculars) can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.

Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens. Relate that to a 50mm or 40mm binocular and the color error is already very small due to aperture.

Furthermore, it is not uncommon at all for a binocular to have internal vignette that reduces aperture to something even smaller than the nominal stated based on objective size. Well, at the same time aperture is reduced (benefical towards reducing false color), this increases the focal ratio (also benefical towards reducing false color), reducing some aberrations, including color error. For two equal sized (nominal) binoculars, but one having greater vignette than the other, a portion of the false color is slightly suppressed by the greater internal vignette.

Many designs especially binocular designs, are faster than desirable for optimum ED color correction and their color performance suffers as a result. In the almost uncountable scope discussions that we have, it has been said data consistently shows better designs incorporate higher Abbe# ED glass in an objective of focal ratio significantly more than twice the diameter in inches. For instance, as relates to a binocular, the WO22x70 Apo (apprx f/5.8 to f/6) is specified as APO and as using FPL-51 ED glass (a lower Abbe# ED glass) in a doublet configuration. This binocular still shows some false color. It remains questionable that it produces what would be considered an apochromatic image.

There are many testimonies about scopes using FPL-51 ED glass that still show moderate to significant false color. Triplets using ED glass of various types have consistently proven to have great visual color correction. The Tak Astronomer 22x60 4 element Flourite objective shows no noticable visual color error.

So here we have not only many reasons why it may be quite easy to market ED in a scope and show good results, even though the results might not be entirely a result of a better combination of ED glass, but also we have a number of issues that are specifically binocular related that would tend to lessen even moreso the degree of performance improvement resulting from ED use in a binocular.

Don’t be so overwhelmed by what you read on some websites about the outstanding performance of ED binoculars. There are just as many, if not more, fine non-ED binoculars. How often have you seen reviews of ED binoculars in which the author isolated various tests and reported contribution to overall performance from such aspects such as focal ratio, aperture size, vignette, etc. Could it be that the ED marketed models performance is not entirely due to the inclusion of some ED in the design? I leave that to you to decide.

5.6 The Doppler Effect

The last two sections introduced you to many new concepts, and we hope that through those, you have seen one major idea emerge. Astronomers can learn about the elements in stars and galaxies by decoding the information in their spectral lines. There is a complicating factor in learning how to decode the message of starlight, however. If a star is moving toward or away from us, its lines will be in a slightly different place in the spectrum from where they would be in a star at rest. And most objects in the universe do have some motion relative to the Sun.

Motion Affects Waves

In 1842, Christian Doppler first measured the effect of motion on waves by hiring a group of musicians to play on an open railroad car as it was moving along the track. He then applied what he learned to all waves, including light, and pointed out that if a light source is approaching or receding from the observer, the light waves will be, respectively, crowded more closely together or spread out. The general principle, now known as the Doppler effect, is illustrated in Figure 1 .

Doppler Effect.

Figure 1. (a) A source, S, makes waves whose numbered crests (1, 2, 3, and 4) wash over a stationary observer. (b) The source S now moves toward observer A and away from observer C. Wave crest 1 was emitted when the source was at position S1, crest 2 at position S2, and so forth. Observer A sees waves compressed by this motion and sees a blueshift (if the waves are light). Observer C sees the waves stretched out by the motion and sees a redshift. Observer B, whose line of sight is perpendicular to the source’s motion, sees no change in the waves (and feels left out).

In part (a) of the figure, the light source (S) is at rest with respect to the observer. The source gives off a series of waves, whose crests we have labeled 1, 2, 3, and 4. The light waves spread out evenly in all directions, like the ripples from a splash in a pond. The crests are separated by a distance, λ, where λ is the wavelength. The observer, who happens to be located in the direction of the bottom of the image, sees the light waves coming nice and evenly, one wavelength apart. Observers located anywhere else would see the same thing.

On the other hand, if the source of light is moving with respect to the observer, as seen in part (b), the situation is more complicated. Between the time one crest is emitted and the next one is ready to come out, the source has moved a bit, toward the bottom of the page. From the point of view of observer A, this motion of the source has decreased the distance between crests—it’s squeezing the crests together, this observer might say.

In part (b), we show the situation from the perspective of three observers. The source is seen in four positions, S1, S2, S3, and S4, each corresponding to the emission of one wave crest. To observer A, the waves seem to follow one another more closely, at a decreased wavelength and thus increased frequency. (Remember, all light waves travel at the speed of light through empty space, no matter what. This means that motion cannot affect the speed, but only the wavelength and the frequency. As the wavelength decreases, the frequency must increase. If the waves are shorter, more will be able to move by during each second.)

The situation is not the same for other observers. Let’s look at the situation from the point of view of observer C, located opposite observer A in the figure. For her, the source is moving away from her location. As a result, the waves are not squeezed together but instead are spread out by the motion of the source. The crests arrive with an increased wavelength and decreased frequency. To observer B, in a direction at right angles to the motion of the source, no effect is observed. The wavelength and frequency remain the same as they were in part (a) of the figure.

We can see from this illustration that the Doppler effect is produced only by a motion toward or away from the observer, a motion called radial velocity. Sideways motion does not produce such an effect. Observers between A and B would observe some shortening of the light waves for that part of the motion of the source that is along their line of sight. Observers between B and C would observe lengthening of the light waves that are along their line of sight.

You may have heard the Doppler effect with sound waves. When a train whistle or police siren approaches you and then moves away, you will notice a decrease in the pitch (which is how human senses interpret sound wave frequency) of the sound waves. Compared to the waves at rest, they have changed from slightly more frequent when coming toward you, to slightly less frequent when moving away from you.

Color Shifts

When the source of waves moves toward you, the wavelength decreases a bit. If the waves involved are visible light, then the colors of the light change slightly. As wavelength decreases, they shift toward the blue end of the spectrum: astronomers call this a blueshift (since the end of the spectrum is really violet, the term should probably be violetshift, but blue is a more common color). When the source moves away from you and the wavelength gets longer, we call the change in colors a redshift. Because the Doppler effect was first used with visible light in astronomy, the terms “ blueshift ” and “ redshift ” became well established. Today, astronomers use these words to describe changes in the wavelengths of radio waves or X-rays as comfortably as they use them to describe changes in visible light.

The greater the motion toward or away from us, the greater the Doppler shift. If the relative motion is entirely along the line of sight, the formula for the Doppler shift of light is

where λ is the wavelength emitted by the source, Δλ is the difference between λ and the wavelength measured by the observer, c is the speed of light, and v is the relative speed of the observer and the source in the line of sight. The variable v is counted as positive if the velocity is one of recession, and negative if it is one of approach. Solving this equation for the velocity, we find v = c × Δλ/λ.

If a star approaches or recedes from us, the wavelengths of light in its continuous spectrum appear shortened or lengthened, respectively, as do those of the dark lines. However, unless its speed is tens of thousands of kilometers per second, the star does not appear noticeably bluer or redder than normal. The Doppler shift is thus not easily detected in a continuous spectrum and cannot be measured accurately in such a spectrum. The wavelengths of the absorption lines can be measured accurately, however, and their Doppler shift is relatively simple to detect.

The Doppler Effect

We can use the Doppler effect equation to calculate the radial velocity of an object if we know three things: the speed of light, the original (unshifted) wavelength of the light emitted, and the difference between the wavelength of the emitted light and the wavelength we observe. For particular absorption or emission lines, we usually know exactly what wavelength the line has in our laboratories on Earth, where the source of light is not moving. We can measure the new wavelength with our instruments at the telescope, and so we know the difference in wavelength due to Doppler shifting. Since the speed of light is a universal constant, we can then calculate the radial velocity of the star.A particular emission line of hydrogen is originally emitted with a wavelength of 656.3 nm from a gas cloud. At our telescope, we observe the wavelength of the emission line to be 656.6 nm. How fast is this gas cloud moving toward or away from Earth?


Because the light is shifted to a longer wavelength (redshifted), we know this gas cloud is moving away from us. The speed can be calculated using the Doppler shift formula:

Check Your Learning

Suppose a spectral line of hydrogen, normally at 500 nm, is observed in the spectrum of a star to be at 500.1 nm. How fast is the star moving toward or away from Earth?


Because the light is shifted to a longer wavelength, the star is moving away from us:

You may now be asking: if all the stars are moving and motion changes the wavelength of each spectral line, won’t this be a disaster for astronomers trying to figure out what elements are present in the stars? After all, it is the precise wavelength (or color) that tells astronomers which lines belong to which element. And we first measure these wavelengths in containers of gas in our laboratories, which are not moving. If every line in a star’s spectrum is now shifted by its motion to a different wavelength (color), how can we be sure which lines and which elements we are looking at in a star whose speed we do not know?

Take heart. This situation sounds worse than it really is. Astronomers rarely judge the presence of an element in an astronomical object by a single line. It is the pattern of lines unique to hydrogen or calcium that enables us to determine that those elements are part of the star or galaxy we are observing. The Doppler effect does not change the pattern of lines from a given element—it only shifts the whole pattern slightly toward redder or bluer wavelengths. The shifted pattern is still quite easy to recognize. Best of all, when we do recognize a familiar element’s pattern, we get a bonus: the amount the pattern is shifted can enable us to determine the speed of the objects in our line of sight.

The training of astronomers includes much work on learning to decode light (and other electromagnetic radiation). A skillful “decoder” can learn the temperature of a star, what elements are in it, and even its speed in a direction toward us or away from us. That’s really an impressive amount of information for stars that are light-years away.

Key Concepts and Summary

If an atom is moving toward us when an electron changes orbits and produces a spectral line, we see that line shifted slightly toward the blue of its normal wavelength in a spectrum. If the atom is moving away, we see the line shifted toward the red. This shift is known as the Doppler effect and can be used to measure the radial velocities of distant objects.

For Further Exploration


Augensen, H. & Woodbury, J. “The Electromagnetic Spectrum.” Astronomy (June 1982): 6.Darling, D. “Spectral Visions: The Long Wavelengths.” Astronomy (August 1984): 16 “The Short Wavelengths.” Astronomy (September 1984): 14.Gingerich, O. “Unlocking the Chemical Secrets of the Cosmos.” Sky & Telescope (July 1981): 13.Stencil, R. et al. “Astronomical Spectroscopy.” Astronomy (June 1978): 6.


Doppler Effect: A shaking bug and the Doppler Effect explained.Electromagnetic Spectrum: An introduction to the electromagnetic spectrum from NASA’s Imagine the Universe note that you can click the “Advanced” button near the top and get a more detailed discussion.Rainbows: How They Form and How to See Them: By meteorologist and amateur astronomer Joe Rao.


Doppler Effect: ESA video with Doppler ball demonstration and Doppler effect and satellites (4:48).How a Prism Works to Make Rainbow Colors: Short video on how a prism bends light to make a rainbow of colors (2:44).Tour of the Electromagnetic Spectrum: NASA Mission Science video tour of the bands of the electromagnetic spectrum (eight short videos).

Introductions to Quantum Mechanics

Ford, Kenneth. The Quantum World. 2004. A well-written recent introduction by a physicist/educator.Gribbin, John. In Search of Schroedinger’s Cat. 1984. Clear, very basic introduction to the fundamental ideas of quantum mechanics, by a British physicist and science writer.Rae, Alastair. Quantum Physics: A Beginner’s Guide. 2005. Widely praised introduction by a British physicist.

Collaborative Group Activities

  1. Have your group make a list of all the electromagnetic wave technology you use during a typical day.
  2. How many applications of the Doppler effect can your group think of in everyday life? For example, why would the highway patrol find it useful?
  3. Have members of your group go home and “read” the face of your radio set and then compare notes. If you do not have a radio, research “broadcast radio frequencies” to find answers to the following questions. What do all the words and symbols mean? What frequencies can your radio tune to? What is the frequency of your favorite radio station? What is its wavelength?
  4. If your instructor were to give you a spectrometer, what kind of spectra does your group think you would see from each of the following: (1) a household lightbulb, (2) the Sun, (3) the “neon lights of Broadway,” (4) an ordinary household flashlight, and (5) a streetlight on a busy shopping street?
  5. Suppose astronomers want to send a message to an alien civilization that is living on a planet with an atmosphere very similar to that of Earth’s. This message must travel through space, make it through the other planet’s atmosphere, and be noticeable to the residents of that planet. Have your group discuss what band of the electromagnetic spectrum might be best for this message and why. (Some people, including noted physicist Stephen Hawking, have warned scientists not to send such messages and reveal the presence of our civilization to a possible hostile cosmos. Do you agree with this concern?)

Review Questions

Thought Questions

With what type of electromagnetic radiation would you observe:

  1. A star with a temperature of 5800 K?
  2. A gas heated to a temperature of one million K?
  3. A person on a dark night?

Figuring for Yourself


These Astronomical Glass Plates Made History

O n a clear Christmas morning atop Mount Wilson, before the first tentacles of dawn struck the Los Angeles sprawl 5,700 feet below, George Willis Ritchey was capturing the most spectacular view of the “Great Nebula of Orion” anyone had ever seen. For close to four hours, he had been standing at the base of an enormous, steel-framed telescope, making minute adjustments as the machine tracked the nebula across the night sky.

The year was 1908, and the 60-inch reflector, which Ritchey had engineered and newly built, was the largest and most powerful in the world. As its huge curved mirror collected the nebular light, the incoming photons slowly exposed the emulsion on a photographic glass plate roughly the size of an iPad. Later, an assistant would develop the negative and label it “Ri-0”—the inaugural scientific image from Ritchey’s state-of-the-art scope.

The Great Nebula of Orion: This is a digital print of a photographic plate from the Ritchey 60-inch telescope at Mount Wilson Observatory, made in 1908.

Today, Ri-0 is one of more than 200,000 astronomical plates archived at the main offices of the Carnegie Observatories, in Pasadena, California. Made between 1892 and the early 1990s using telescopes at Mount Wilson, Palomar (near San Diego), Las Campanas (in Chile), and Kenwood (in Chicago) observatories, the plates range in size from centimeter-square slivers to pieces as large as a desktop computer screen. 1

This collection, the second largest in the United States, includes some of the most important observations in astronomy in the last century. It’s these images, for instance, that sparked Edwin Hubble’s realization of the expanding universe, that led George Ellery Hale to discover the sun’s magnetic field, and that provided the empirical basis for theories of how stars and galaxies form.

Here is a sampling of the most famous, and most striking, shots.

Schrödinger’s Cat When Nobody Is Looking

Some of the most perplexing topics in physics revolve around quantum theory. The quandary is seen most famously in the Schrödinger’s cat question and the issue of information loss in black hole evaporation. Richard Feynman said, “I think that I can safely. READ MORE

The sun’s magnetic field

Solar Magnetism: George Ellery Hale made this photographic plate using the Snow solar telescope at Mount Wilson Observatory, in 1908.

In early 1908, the solar astronomer and telescope engineer George Ellery Hale began tinkering with specialized photographic plates that were sensitive to red (long) wavelengths of light. He was particularly interested in observing the sun in the red wavelength known as H-alpha, an important signature of a star’s atmosphere. It took him a month to perfect the technique. The plate above was his first clear image, which revealed strange swirls surrounding sunspots, which Hale called flocculi. Although he (wrongly) hypothesized that the flocculi were gas tornadoes full of whirling electrons, the discovery led him to (rightly) conclude that the sun generated a magnetic field.

Sun spectrum: A photographic plate from the 60-foot solar tower at Mount Wilson Observatory, made in 1917.

Hale went hunting for direct evidence of the sun’s magnetic field using a spectrograph, which separates light into a frequency spectrum, as represented by a series of vertical lines. To spread out these lines so that he could see them in detail, Hale placed an enormous, 30-foot spectrograph in a concrete well beneath the brand new 60-foot solar telescope at Mount Wilson. He captured the projected spectra on a 17-inch-long glass plate, like the one depicted above by an unknown photographer, possibly Hale. When he compared the spectral lines from the surface of the sun with lines from its sunspots, he saw that the sunspots split some of the lines into multiples while also polarizing the light. (In the above plate, the split spectral lines are labeled K and H.) This splitting, known as the Zeeman effect, provided the first confirmation of a magnetic field beyond Earth.

Hubble’s famous “VAR!” revelation

It’s a galaxy!: Edwin Hubble made this photographic plate of the Andromeda “nebula” using the 100-inch Hooker telescope at Mount Wilson Observatory, in 1923.

One night in the fall of 1923, Edwin Hubble took a 45-minute exposure of what was then called the Andromeda nebula. At the time, astronomers were debating whether the spiral smudges, or “nebula,” they were seeing in their telescopes were small star clusters within our own galaxy, the Milky Way, or much larger, distant “island universes.” Hubble hoped to settle the debate once and for all.

When he developed the plate, he thought he saw a “nova,” or stellar explosion, on the outskirts of one of Andromeda’s spiral arms. He labeled the tiny black dot “N.” But when he compared the plate with other photographs taken on different dates, he realized that the star was actually a Cepheid variable, a kind of star that brightens and dims on a regular schedule. By measuring its period and luminosity relative to other known variables, Hubble could then calculate its distance, thus revealing that Andromeda was a huge stellar system far outside the Milky Way. In his excitement, Hubble crossed out the “N” and wrote “VAR!” in its place.

More Cepheid variables: This is a digital print of another of Hubble’s photographic plates of Andromeda. He made it using the 100-inch Hooker telescope at Mount Wilson Observatory, in 1924.

Hubble devoted dozens of plates to observing Andromeda in search of more Cepheid variables that would confirm his original discovery. Like many astronomers of his time, he adorned these plates with colorful notations—circles and arrows that identify candidate Cepheid variables, reference stars, and other notable objects. He numbered each confirmed Cepheid variable in order, often followed by exclamation points, as if he couldn’t contain his excitement. In this digital print of a plate made in early 1924, you can make out the notation “V4. ” in the lower left corner.

“My god, it’s full of stars!”

Stellar sensation: Walter Baade made this photographic plate of Andromeda using the 100-inch Hooker telescope at Mount Wilson Observatory, in 1943.

Although Hubble had proven that Andromeda was a massive galaxy, likely full of hundreds of billions of stars, it took two decades for astronomers to finally resolve the stars in its dense, central region. The first image [above], taken on a 5-by-7 inch plate, came about under unusual circumstances.

In 1943, at the height of World War II, the astronomer Walter Baade was at work on Mount Wilson. Being a German national, Baade was barred from war duties, and so spent his nights peering at the sky above Los Angeles, which was delightfully dark due to wartime brownouts. One night, he aimed the observatory’s 100-inch telescope at Andromeda, capturing for the first time individual stars in its nucleus. This shot laid the groundwork for Baade’s classification of stars into two types: young, hot stars that occupy a galaxy’s spiral arms, and their older, cooler relatives in the galaxy’s heart.

Nearly a century later, the image still astounds. When NASA astronomer Jane Rigby visited the Carnegie archive last year, she examined the plate under a loupe. “My god, it’s full of stars!” she exclaimed.

A supernova in a strange galaxy

Now you see it …: Halton “Chip” Arp made this pair of photographic plates showing the sudden appearance of a supernova using the 200-inch Hale telescope at Palomar Observatory, in 1964 [left] and 1971 [right].

The late astronomer Halton “Chip” Arp is best known for his 1966 Atlas of Peculiar Galaxies, for which he photographed hundreds of galaxies with strange shapes and behaviors. Among these was Stephan’s Quintet, five closely interacting galaxies undergoing violent collisions as far away as 300 million light-years. In one remarkable shot, taken five years after the Atlas’s publication at Palomar Observatory, in San Diego, he identified a supernova [labeled “SN” in the right image], which had exploded a month before. (The other arrows on this plate point to reference stars, which Arp used to calculate the supernova’s coordinates in space.) In an earlier shot [left], taken in 1964, this brilliant blast is noticeably absent.

A sweeping galactic survey

Too many to count: This image shows just a small section of a large photographic plate depicting hundreds of galaxies in or near the Virgo Cluster. It was made using the du Pont telescope at Las Campanas Observatory, in Chile, in 1980.

In the late 1970s and early 1980s, Allan Sandage, a onetime assistant to Hubble, and his collaborators conducted the first exhaustive survey of the Virgo Cluster, a bundle of galaxies that comprise the heart of the supercluster containing our own Milky Way. To do this, the astronomers made 67 enormous, 20-inch-square photographic plates, which together produced a catalogue of 2,096 galaxies. The image above shows just a small fraction of one plate, which Sandage made at Las Campanas Observatory, in Chile, in 1980. He painstakingly located and measured each galaxy one-by-one, noting their catalog number and magnitude directly onto the plate in red and green ink.

Notes for Astronomers

While it is possible to project an image of the sun through telescope optics onto a paper, it can damage your instrument. The sunlight can heat up optics in just a few minutes, damaging eyepiece coatings and even melting the cement that holds eyepiece optics together.

Also avoid so-called solar eyepieces that may come with less expensive telescopes. They are highly dangerous, as intense heat from incoming unfiltered sunlight can hit the eyepiece and cause the lens to crack, allowing the magnified sunlight to hit your eye.

Follow Andrew Fazekas, the Night Sky Guy, on Twitter and Facebook.


P.A. acknowledges STFC support from grant numbers ST/R004285/2 and ST/T000384/1 and support from the International Space Science Institute, Bern, Switzerland to the International Teams on ‘Implications for coronal heating and magnetic fields from coronal rain observations and modeling’ and ‘Observed Multi-Scale Variability of Coronal Loops as a Probe of Coronal Heating’. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 647214). P.T. was also supported by contracts 8100002705 and SP02H1701R from Lockheed-Martin to the Smithsonian Astrophysical Observatory (SAO), and NASA contract NNM07AB07C to the SAO. P.A. thanks I. De Moortel, R. Rutten and B. De Pontieu for valuable discussion and J. A. McLaughlin for the suggested name of the nanojet. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway). IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research Center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre. SDO is part of NASA’s Living With a Star Program. All data used in this work are publicly available through the websites of the respective solar missions. This work used the [email protected] facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility ( The [email protected] equipment was funded by BEIS capital funding via STFC capital grants ST/P002293/1 and ST/R002371/1, Durham University and STFC operations grant ST/R000832/1. The DiRAC component of CSD3 was funded by BEIS capital funding via STFC capital grants ST/P002307/1 and ST/R002452/1 and STFC operations grant ST/R00689X/1. DiRAC is part of the National e- Infrastructure.

Atmospheric Evolution

  • Condensation of H2O into the oceans.
  • Locking up of CO2 into carbonaceous rocks
  • Formation of O2 by photosynthesis in plants & algae
  • CO2 content of the atmosphere is regulated by a complex balance cycle.
  • Increases in O2 and methane (CH4) from "biomass" (plants and animals)
  • Human activity (fuel burning & agriculture)

HYDROLOGY | Ground and Surface Water

Hydrological Cycle

Powered by solar energy , the hydrological cycle is the endless movement of water from one reservoir to another in the Earth system ( Figure 4 ). Water evaporates into the atmosphere from open waters such as oceans and lakes, from soil moisture in the unsaturated zone, and from the water table. Plants lose water to the atmosphere through the process of transpiration. These two processes, evaporation and transpiration, are collectively known as evapotranspiration. Water falls back to the Earth's surface as precipitation in the form of snow or rain. Upon reaching the surface, water flows overland as runoff to streams or infiltrates to the subsurface to become ground water. In the subsurface, water infiltrates through soils, recharges the ground water table and joins the ground water flow system. Ground water takes its course through geological basins of various scales, and some eventually makes its way to the oceans while some accumulates in inland aquifers. The rates of water flow between reservoirs within the hydrological cycle vary spatially and temporally in the Earth's system. As a result, the residence time of water – the time water remains in a reservoir since recharge – in different reservoirs varies from hours in the near-surface soil to tens of thousands of years in rocks several kilometers deep in the crust.

Figure 4 . The hydrologic cycle. The water table is the boundary between the unsaturated zone above and the saturated zone below. Upon reaching the land surface, precipitation either infiltrates soil to replenish ground water or flows overland as runoff to open water bodies. Water evaporates from open water bodies at the Earth's surface, soil moistures in the unsaturated zone, and the water table. Transpiration occurs over vegetated lands. Ground water flows through the vast domain of the subsurface and returns to the oceans.


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