The Art of Observing Part 2- Dimmer Objects
Jul 28 '03
The Bottom Line Deep space objects are not what the photos show, but there are good reasons to get a look.
This is the second part of a piece I am writing on what various kinds of objects look like through telescopes. The visual image is extremely different between photographs and real life in astronomy. In this part, I am going to talk about the dimmer objects which really do require a telescope or some form of light amplification to see.
Background
These objects all have the common trait of being difficult to see without special equipment, and are generally difficult to view from the city since light pollution washes them out.
They have a long history of being recognized in the sky dating to ancient times, and will be found under many different kinds of notations. For example, Messier objects, a list of 110 objects from Charles Messier, who listed them as "Things which you may mistake for a comet, but are not." These are, in fact, a mixture of diffuse nebulas, galaxies, planetary nebulas, and star clusters. Messier objects are denotes as "M#" with numbers 1 through 110, M45 for example. All they had in common is they appeared to be fuzzy spots similar to comets in early telescopes.
To the eye, these objects appear to be dim. The human eye has two ways of detecting light- high resolution color sensitive cone cells and light sensitive rod cells. The cone cells are clustered towards the center of the retina and one central spot, the fovea, is 100% cone cells. These occur in three differnent types, each with peak sensitivity to different areas of the spectrum. This is where your most crisp and most color sensitive vision comes from. However, these cells are better for daylight and are limited in their ability to adapt to darkness. The eye overcomes this limitation with a second set of more sensitive rod cells which start mixed in with cone cells in the center of the retina and move out to the edges of the retina which is made up entirely of rod cells. Human night vision, despite what you may have been told, is actually quite good- better than a bear's, for example. However, few modern people ever let their eyes fully dark adapt and thus never get to see what their eyes can do.
When going abruptly from light to dark, the eye will initially be blind. It must do two things to prepare for night vision. First, the iris opens up exposing light to all of the retina. The average diameter in an adult is about 7mm. The eye will also have to adapt the cells to greater light sensitivity than is needed in daylight. For the cone cells, this happens in about five minutes, and this is the point where someone stepping into a darkened theater will initially feel they can find the aisle and walk to a seat. The rod cells take about half an hour to get to peak sensitivity. Since many people resort to flashlights and other sources long before these cells are adapted, they never experience full night vision.
The rod cells have a peak sensitivity in blue green, while daylight vision has peak sensitivity in green. This matches the spectrum of light reaching us in day and night. At night, objects will appear to be a blue-green color, and even an extremely dim blue colored light can illuminate a large area. The eye is less sensitive to red light at night and in the daytime, and this has been commonly been used as a filter for night vision. However,although the rods are less sensitive to red light compared to other colots, the absolute sensitivity between them and cones is almost equal at night. As a result, bright red lights will degrade your night vision. The other effect which causes this is your retina is not just a collection of cells- it is actually part of your brain and is completely integrated. The result is if one type of cell is indicating it can see bright light, it will signal the others to re-adapt to bright light, even though they may not be able to see the color of light used. The military has long since recognized this and instead of red lights, uses blue-green light, which can illuminate areas for night vision with far less light used. This preserves night vision since none of the cells report seeing light bright enough to light adapt, and is also less likely to be seen by an enemy's artificial night vision devices. However, red light has magnetic appeal, and at many astronomy clubs you will find people walking around with 4 D-Cell mag lights with the lens painted clear red with nail polish thinking they are preserving night vision as they blind everyone.
When fully dark adapted, the eye can make out people, objects, and the lay of the ground by starlight with no assistance of any kind. The image will sometimes look a bit grainy as individual cells in the eye are reacting to photons landing on them. You will notice while it is possible to make out the form of a person standing in front of you, it is very difficult to recognize their face. It is relatively easy to find a telescope and where its knobs are, but it isn't so easy to sort out different eyepieces unless they are obvioulsy shaped differently or have obvious high contrast markings.
The night sky quickly opens itself up to the dark adapted eye as the milky way becomes completely obvious and the number of stars appears to expand beyond counting. At this point, dim star clusters, nebulas, and galaxies also become visible as hazy spots in the sky. It isn't easy to determine what is there, but it is obvious something is there. You will notice many things show up off to the side in the visual field and disappear when you look directly at them. This is because the image falls on the sensitive rod cells when looking to one side, and the less sensitive cone cells cannot see it if you look straight at it. This technique is called averted vision, where you look around an object and not directly at it to see it.
Many dim objects are only detectable by telescopes, so finding them is far more difficult than for bright objects, which all can be located by eye, so all you really need to do to find them in a telescope is sight down the barrel. At the same time, their appearance is so different from photographs, it often will take some checking to be sure you are on one. With that said, many amateur astronomers look for these objects bit by bit, and over time, come to see all of the major sights in the night sky for themselves.
Nebulas
Some of the most famous photos of the night sky are taken of Nebulas. These are large clouds of dust and gas and occur in two primary flavors- Planetary Nebulas and Diffuse Nebulas. For a while, even galaxies were called nebulas before it was realized they were something completely unrelated.
-Planetary Nebulas: These are the remnants of dying stars which blow off their outer layers as they run out of nuclear fuel. They are called "Planetary" because they appear to be approximately the same size as planets in the eyepiece. They have nothing to do with planets. Famous examples include M27, the Dumbell Nebula, M57, the Ring Nebula, and M1, the Crab Nebula.
In photographs, these objects appear to be bright rings or splotches in vivid red, green, yellow, and blue. However, their appearance to the eye is totally different. The photographs are taken with long exposures or by stacking up many digital images. The color only becomes visible when a huge amount of light amplification has been done this way.
For the dark adapted eye with a telescope, these will apear to be smoky colored, and while details of their structure may come through, red color doesn't come through as in photographs. These objects are the remains of dying stars which are blowing off their outer layers. Our own sun will become a planetary nebula one day (so we do have a deadline for getting everyone off of this spinning rock). The nebula itself is illuminated by the radiation and light from the star ionizing the gases in the nebula.
At dimensions sometimes a lightyear across, the light hitting this gas is quite dim. So, if you were in a spacecraft right next to a nebula, it would look dim green-gray and blue instead of the brilliant colors depicted in science fiction movies. In some nebulas, such as the Ring Nebula, the dim central star is only visible with a large telescope. Some objects are very obviously blue in color, such as the Eskimo Nebula or the Blue Snowball. Hubble photographs are usually released in a false spectrum completely different from real color, so these are poor guide for what the object looks like (various professionals have pointed to this as possibly "Window dressing."). To give an idea, this is a Hubble photo of the eskimo nebula: http://antwrp.gsfc.nasa.gov/apod/ap020407.html This is an amateur photo of the eskimo nebula in its actual color: http://www.weatherman.com/eskwide2.jpg from Todd Gross' site: www.weatherman.com. When the nebulas are as blue as these, the color is very obvious to the naked eye. Nebulas with red color are mostly a smoky color.
-Diffuse Nebulas
Diffuse nebulas are easily the most extraordinary objects in the sky when seen in dramatic photographs. Like planetary nebulas, Hubble photos are rarely in actual color, but are instead in a false spectrum. A dramatic example is this detail from the Orion Nebula:
Hubble:
http://antwrp.gsfc.nasa.gov/apod/ap990522.html
Amateur actual color from Robert Gendler
http://antwrp.gsfc.nasa.gov/apod/ap020213.html
Spectroscopy and you: The colors of a nebula do tell what it is made from. Red is from ionized hydrogen, while green comes from ionized oxygen. Blue is generally from reflected light, where the light source is between us and the object.
Now, there is a temptation to think because the eye can't show this range of color, it is inferior to film or CCD devices. In actuality, the eye is capable of some things these devices can't do. By the time dimmer details come through on a photograph, brighter items are washed out. So, stars will appear to be grossly oversized blobs when the nebula is visible. The eye, with its incredible response range, can show pinpoint stars and dim details at the same time.
This effect is very obvious on the Orion nebula, where the central portion is entirely lost in the photograph to wash-out. The nebula looks very different to the dark adapted eye. The wing-like shape of the edges is very objous, as is the texture of the cloud. The central stars, called the trapezium, are easy to resolve as points, while a camera simply cannot do this trick. As a result, the visual image has depth and texture missing from photographs, though the color is in shades of green-gray to blue.
Some nebulas are enormous objects in the sky several times the size of the full moon. Examples of these include the veil nebula and Bernard's loop- an enormous arc of nebula expanding by the Orion Nebula. These features are indeed enormous, and many are major star forming regions. The Great Orion Nebula is only 1200 light years away and is the trapezium at its center is the first visible group of stars from a large cluster of brand new stars which will become visible over the next few thousand years as their newly ignited nuclear fires push the nebula away from them.
Orion is the brightest diffuse nebula in the sky, but is a rich training ground for finding these objects. Other famous nebulas, such as the Horsehead at the end of the three stars in Orion's belt, are nearby. The Horsehead is actually a shadow from a dust cloud in front of a glowing nebula. While this is a popular photographic subject, the only feature I have been able to see on this object by eye has been a bit of a notch where the horsehead is, but no actual detail of the "Head" shape.
Nebulas become all but impossible to find in light polluted areas. Many of them are simply lost in the sky glow caused by artificial lights wasting energy by shining up in the sky. Strangely enough, some nebulas are still visible if the image is dimmed. An easy way to do this is by boosting magnification. In a 90mm scope sited in the countryside away from city lights, the Orion nebula is an incredible sight at low magnification. In the city, the nebula will be a slightly brighter fuzzy spot in the sky when viewed at the exact same magnification.
Going to higher magnification will cut down the field of view and also dim the image. Doubling the magnification cuts the image brightness in half. Unlike a camera, the human eye is not a linear transducer- in other words, the eye is looking for differences between objects and will magnify this difference. In this case, by dimming the image, the relative brightness of the nebula becomes more apparent and its structures will become obvious when the object was only barely detectable with a brighter image. This general observation is the root of this popular observing addage: Large scopes are best in the countryside, small scopes are best in the city. Scopes up to around 5" in diameter are favored in cities since they have less susceptibility to light pollution because they amplify light less. Scopes around 8" in diameter are in a sort of transition between small and large aperatures. They can be used in the city with some technique. My personal favorite is still a 5" scope since it does well no matter where it is, and I can pick it up and move it in one piece with the mount.
A second approach is to use light pollution filters, which are designed to cut some colors of light and pass the colors in astronomical objects. To be frank, I haven't seen much difference between these in practice and magnifying the image since everything appears darker with the filter. These will let large scopes- 10" or 250mm diameter and larger function a bit better in the city, but in my experience, they really aren't doing much better than a small scope at this point.
Star Clusters
These objects are some of the less well understoon but beautiful objects in the sky. In a telescope, they can appear as anything between glowing fuzzballs to sugar spilled in a heap to a field of brilliant diamonds on velvet. They range in size from a small spot to several times the width of the moon.
Open Clusters
These are groups of stars close to each other, but not in an obvious formation. In the winter sky, the Hyades and Plieades are the most prominent, with the Pieades easily taking the title as the most famous star cluster. These are generally groups of stars numbering up to in the hundreds. They occur in two general ways. First, there are groups of stars which have just formed and are blowing the nebula they formed out of away, or have completely swept it away. The Pieades still has whisps of clouds left and these brand new stars burn with intense blue-white fire. If I were to name one cluster as the most beautiful, it would be this one. The stars will fit into the field of view of a telescope with a focal length up to 1250mm and appear to be diamonds on velvet in a dark sky. The Hyades are a second, here, with the stars already starting to spread out. Six of them are in a roughly triangular pattern with two at each corner, and this is an interesting view.
Other open clusters are actually groups of stars in orbit around each other which have been together in orbit in the galaxy for billions of years. One of the best examples of these is double cluster between Cassiopia and Perseus. These appear to be a fuzzy spot in the sky. It is obvious enough to the naked eye to find them. They will show up together as a dramatic pair of clouds of stars in a telescope with a 1000mm focal length at low power. With larger aperatures, more stars are visible. Unlike a cluster of new stars, the ancient stars in an orbiiting star cluster are the long-term survivors of stars in a sweet spot of yellow-white color.
There are actually quite a few open clusters in the sky from many different catalogs. When I go stargazing with my wife, she will set up the small 90mm scope and go hunting for things, and will typically find a half dozen fairly nice looking clusters in an evening. If you go looking along the milky way, you will find many of these.
Globular Clusters
While it would be logical to think of the opposite of an open cluster being a closed cluster, these are actually a bit different entirely. The globular clusters are large numbers of stars (thousands) in orbit as a ball of stars in space. These are so ancient that they have pushed back the age of the universe to accomodate them. These are all round in shape, and take on different appearances depending on the diameter of the telescope, the distance to the object, and their actual size.
In small 80mm telescopes, most globular clusters look like a dim fuzzy ball. Some near ones, such as the Beehive, are so close that they look more like an open cluster though the circular shape is very obvious in some telescope finders, such as a 9X50mm finder.
For distant globular clusters, they will have different appearances depending on what is looking at them. A mid sized telescope around 8" in diameter will often show what is called a "Sugar appearance" on famous objects like M13, the Hercules cluster. This is a gray fuzzy ball in scopes around 90mm with a few stars able to be resolved. In an 5" to 8" scope, this takes on a sort of grainy appearance where it looks a little like a small pile of sugar. In a really large scope, on the order of 24", this object becomes a brilliant ball of stars with each one appearing to be a point of light. Large scopes, if placed out in the countryside are real winners for visual observations on dim objects.
Galaxies
These are extremely photogenic objects, but are generally very difficult to observe by eye. The brightest galactic objects are the Megellanic clouds, two small galxy pieces in orbit around the Milky Way. Unfortunately for people in the northern hemisphere, these are only visible in the Southern Hemisphere. One of these days I hope to get a trip to Australia where I can get out into the desert and get a good look at these.
Nearby galaxies such as M31, the Andromeda Galaxy, are visible to the naked eye in dark skies. They will appear as strangely indistinct fuzzy patches in the sky. The Andromeda galaxy is quite large in the sky, but it is 2 million lightyears away. It is far, far more distant than any nebulas or star clusters visible in the Milky Way. In photographs, Andromeda is an extraordinary sight with its millions of stars appearing as a glowing band with a few brighter ones standing out in this outstanding photo from Robert Gendler:
http://antwrp.gsfc.nasa.gov/apod/ap021021.html
Through a telescope, this object will, at first, simply look like a smudge. Some details such as the darker dust bands obscuring part of the core will come through, but the object's outer dim bluer areas are much more difficuult to see. To give some idea how much the visual part of the image is clustered to the core area, the small orbiting sub-galaxies are actually as listed as separate Messier objects.
Other instances have very different appearances. M51, near the Big Dipper's handle, is just barely visible by averted vision in a completely dark location. In photographs, two galaxies can be seen in the midst of collision. In a photograph, it is easy to see what they are:
http://antwrp.gsfc.nasa.gov/apod/ap020710.html
By eye, this formation looks completely different. The overall impression is more of a stack of three gray clumps near each other- the cores of the two galaxies and the knot formed between them. The rest of the structure is more hinted at than anything. The phrase "Hinted at" generally means if you stay on the object and continue moving your eye around the field, you will begin to get the impression something is there, but what it is and how it is shaped never really pops out. Galaxies really need large scopes to bring out any recognizable detail.
I have always found galaxies to be difficult objects. And, given they are the most distant objects known, this is reasonable. While some star clusters and nebulas can overcome the glare of light pollution, precious little is left to see on galaxies. These objects are the most sensitive to light pollution and benefit from large aperature scopes the most, so they are in a way the first casualties of gas station lights.
Comets
These objects are a mixture of rock, dust, water ice, and perhaps ices of various frozen gasses or hydrocarbons. Unlike the movies, they aren't on fire, covered with spire, or somehow all out to crash into the earth's cities. They fall into two major groups- periodic comets, which are in an orbit close enough for us to see them on a regular basis, and first time comets which zip by from the distant reaches of the sun's grasp.
All comets are all descended from objects which formed in the early days of the solar system, but were too far out to get incorporated into a larger planet. At the same time, the extreme cold let them keep large amounts of water, methane, and other compounds which ordinarily would boil off into the vacuum of space. At some point, a nudge from the gravity of another object cause them to lose the slow orbital motion which kept them out from the Sun and sent them falling into the inner solar system. This fall may have taken thousands of years, but once started, the encounter with our neighborhood was inevitable.
Comets are distinguished from other solar system objects by their wild orbits which take them close to the Sun, where they heat up and start to boil off their ices and blow dust off. The result is they grow a tail pointed away from the sun thousands of miles long as they close in. While the appearance for comets varies, they are some of the most exciting dim objects in the sky because of how unpredictable they are.
-Periodic Comets
A periodic comet would have come close to another object as it neared the Sun on its first perhaps over many passes, such as Jupiter, which bent its orbit from one which may take tens of thousands or even millions of years to return into a regular orbit. Examples of regular comets include the first recognized regular comet- Halley's Comet. While Halley's has an orbit taking 76 years, several comets are in orbits where they are never completely out of view.
In general, periodic comets are more important for the historical importance of recognizing the reliability of their visits. Thus far in my lifetime, all have managed to be fairly dim smudges in the sky, with the coma around the head of the comet a bit brighter than the tail. With some good fortune, I may still be in observing shape in 2061 when Halley's comes back. I hear that performance should make up for the last time this comet came by.
A periodic comet is requently a dim object in a telescope or binoculars, and may in fact be totally invisible to the unaided eye. The actual parent body is very small, so a large telescope will be needed to see it. However, even modest 5" scopes can show these objects, and are instruments used by successful amateur comet hunters. When periodic comets move away from the sun, the gray smudge of their tails fades and they become points. When they move closer to the sun, the tail will reappear. In a long exposure photograph, the tail of a comet will become brilliant and bright. Even the brightest comets, which are visible with no aid in the city, are much dimmer than the photographic image.
New Comets
Comets which haven't been circling the sun at close range over and over have the potential to become extremely bright. The reason for this is they haven't had the majority of their water and other material boiled off and blown away on previous passes by the Sun. As a result, these objects have two very clear defining characteristics:
(1) You never know when one is going to show up.
(2) You don't know what you're going to get when one does come by.
Lots of new comets come streaking into the solar system. A really bright one like Hale Bopp only pops in every decade or so, but ones bright enough to find with binoculars come through fairly often.
Because these are not periodic objects, people don't know they are coming until someone sights it. At this point, various observations will be put together to try to predict the orbit. You can find data on incoming comets at www.heavens-above.com. This site can actually serve as one stop shopping for a lot of observing data, from predicting when the space station will fly over your area to where the planets are at the moment.
Bright comets are obvious even in city skies, and nothing is better for looking at them than some good 7X50mm binoculars. They can be so large in the sky that telescopes will only show a small segment of them. The best advice I can give is when one is around, make sure you get a look.
 |
|
|
|
|
|
|
Epinions.com ID:
Pirich
|
 in Electronics |
 - Top 500 |
|
Member: Rich W.
Location: Tucson, AZ
Reviews written: 137
Trusted by: 40 members
About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
|
|
|
© 1999-2009 Shopping.com, Inc.