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Images - Size, Resolution, and Focus

"An image is an optical representation of an object produced by light rays from the object being refracted or reflected by a lens or mirror." - author George Abell in Exploration of the Universe.

[An image is] "An optically formed duplicate, counterpart, or other representative reproduction of an object, especially an optical reproduction of an object formed by a lens or mirror." - American Heritage Dictionary.

Oppy, the Dalmatian
This is an image of a Dalmatian.

A galaxy
This is an image of a galaxy.

Both of these imageshave one thing in common: They are optical light images. This is what the objects look like with our eyes (or, in the case of the galaxy, with our telescope-aided eyes). But an image can be made out of any kind of electromagnetic radiation. You just have to have the right kind of detector to 'see' the kind of radiation you want to study. For example, if I want to make an X-ray image of my hand, I can put a piece of X-ray sensitive film under my hand and shoot X-rays into the top of my hand. What I get is an X-ray image of my hand that my eyes can interpret. What I see is not what my hand looks like to my eyes, but what my hand would look like if, instead of optical light, my eyes could see X-rays! We do the same thing with stars and galaxies and such. We use a detector that can see the X-rays they emit and then, in a very clever way, interpret what we have measured so that we can produce an image of what the objects would look like if our eyes could see X-rays.

An image is just a way for scientists to plot things or draw things so that they can look at them. Most images show the brightness of an object in the spatial domain, i.e., how many photons are coming from a specific location in space. The ability of a detector to tell one location from a nearby location is called spatial resolution. Three properties of an image -- size, resolution, and brightness -- are the most important properties of an image to a scientist. From size, we learn about astronomical scales, like how big the Moon is. From brightness, we learn the amount of energy that an object is producing, and then we may be able to figure out HOW it is producing that energy. Finally, higher resolution lets us know things like whether or not a planet has rings or if there are two stars close to each other, versus one star by itself.

Looking at images of the same object made with different parts of the electromagnetic spectrum is a very important tool for scientists. They can learn something unique about the object at every different wavelength, and then they can figure out "the big picture" of what is really going on with that object by putting what they learn from each wavelength together into a model that can explain everything they see! For example, in the figure below, we see four images of the Crab Nebula. The radio image tells us about the magnetic fields and free electrons in the Nebula. The optical image tells us about the hydrogen in the Nebula and more about the free electrons moving in the magnetic field of the pulsar. The UV image tells us about the cooler electrons, while the X-ray image tells us about the very hot electrons, which are clearly coming from the collapsed central object in the Nebula.

The Crab Nebula at multiwavelengths

    * Use Hera to analyze images.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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