You will need a FITS viewer for astronomical images.  The images will have extensions such as .fts or .fits or .fit .  Sometimes, however, images will be compressed and therefore will need to to be unzipped before use.

There are a large number of these viewers available; they range in price from free to several hundreds of dollars.  A; fairly complete list is here.
Two particularly nice free ones are iris and DS9

You will also need a reasonably fast internet connection as FITS images can be as large as 1 MB in size.

Astronomical images are decentralized in location.  They generally reside at the web site of the organization or satellite which produced the image, although there are a few central repositories.  Two such repositories are

Skyview    provides access to 30 sky surveys
                 The front page of Skyview gives quick access to 5 surveys
                  (the optical DSS, the x-ray ROSAT and HEAO, the infrared IRAS, and the radio 408 MHz).
                 Use the advanced interface page to access other surveys.
                 Click for information (wavelength band, resolution, etc.) about individual surveys.

Aladin       entirely Java based; can run as an applet over the web or as a stand-alone on your hard drive

Both Skyview and Aladin provide reasonably friendly interfaces.  Both allow you to view a FITS image interactively with a web browser, without downloading the image to a local drive.  Skyview allows you to download and save FITS images to your local drive.  Aladin only provides that option if you are running the stand-alone Java option.  In general images provided by different satellites or observatories will not have the same image size, so the user should pay close attention to the angular size of the image, which is generally provided in the associated information.

However, there are many more sites with publicly accessible astronomical data that have recently come on-line that are not part of either of the above 2 atlases.  Examples of these are Chandra X-ray Observatory and the Sloan Digital Sky Survey (SDSS).  Instructions for accessing images for these sites are provided below.

Many atlases will retrieve images if given the common name of an object (if it is relatively well known, for example Cas A or the Crab Nebula), but all atlases will retrieve images based on the object's coordinates (right ascension and declination).

Here is an example of how to find images of the same object at multiple wavelengths using the supernova remnant Cas A:

1) first obtain some images from the front page of Skyview:

a) select the RASS3 (RoSat) option (this was a low-resolution x-ray imaging satellite) and then download the FITS image to a local drive
    note that this image is centered on 23 23 25.40, + 58 48 37.96 (2000) and is 3.75 degrees square

b) select the IRAS (InfraRed All Sky survey) and download the FITS image;
    note that the image is centered on 23 23 25.40, + 58 48 37.96 (2000) and is 7.5 degrees square

    IRAS has observations at 12, 25, 60, and 100 microns (m) and has a resolution of 1.5 arcminutes

c) select the DSS
    this image is also centered on 23 23 25.40, + 58 48 37.96  and is 0.15 degrees square

    DSS has observations at      and has a resolution of 1.7 arcseconds

2) Now use the advanced interface of Skyview to obtain a VLA (radio) image of Cas A:

a) type in Cas A (or its coordinates) in the "Coordinates or Souce" box
b) click on VLA NVSS (1.4 GHz)   [this is NRAO Very large array Sky Survey at a wavelength of 21 cm
c) click on the Submit Request Now bar
d) after the image pops up in another window you can download the FITS image by clicking on the FITS bar

the resolution of the NVSS is 45 arcseconds

3) next, let's load a Chandra x-ray image; this procedure will be more complicated

a) first you'll need to retrieve an ObsId (Observation Identification) number for your object; again, we will use Cas A as an example:
to do this, go to the Chandra Data Archive Retrieval Interface  and then select Sort by RA & Dec
(this may take awhile)

b) then locate an ObsId that matches the RA and Dec of your object; in this case, many ObsId numbers fall close to the RA and Dec given above (23 23 25.40, + 58 48 37.96) ; note that they also happen to be labeled Cas A

c) click on the ObsId link (in the left column) and new screen comes up with the ObsId centered in Select ObsId box;  under Select Data Product Category, check the first box, "fully processed science products (level=2) "  and then click the "Browse Archive and Retrieve Data Products"

d) another screen opens which lists the relevant files and allows you to select the files you want retrieved; the file you want will end with evt2.fits; highlight the name of the file you want in the white box and then click on "RETRIEVE from Archive"

e) a new screen opens, which gives you two options (ftp and a more direct "click to download") to obtain the zipped FITS file (generally a .tar file)

f) after the .tar file is downloaded, it must be unzipped (with WINZIP, for example) before being readable by a FITS viewer

g) what's the scale of this image?  in general, the Chandra ACIS instrument produces images that have a scale of 0.5" (arcseconds)/pixel
(you can verify this by scanning across the image and noticing both the change in pixels and the change in declination);

4) Now let's look for Sloan Digital Sky Survey images (in the visible part of the spectrum):

a) access the SDSS server
b) click on Search under Skyserver Tools in the left box
c) click on Radial under Search in the list in the left-hand column
d) insert the coordinates in the ra and dec boxes; note the required format below the Submit button
e) click on the Submit button

For the Cas A coordiantes given above (ra = 350.86, dec = 58.81, both in degrees) no images will be found.  SDSS has yet to release any images for this region of the sky.

5) Solar images (along with velocity- and magneto-grams) are available here.     You may find it helpful to read the Readme file first.

Questions?  Suggestions?  email me