The following section will give you tips on saving your image files for display on a website or for emailing them to friends.  There are two problems you would encounter if you were to try emailing an original CCD image to someone:  first, unless the person receiving the file had a CCD image-processing program, they would not likely be able to read the file;  second, with medium or large-format CCD cameras, the file sizes can be quite large (several megabytes).  The solution is to convert the image to a universally-readable format and to compress the image to a reasonable file size (a few hundred KB or less, usually).  The trick is doing this while sacrificing the least amount of image quality as possible.

JPEG Format

Almost every web image (and certainly every astronomical web image) is in JPEG format.  JPEG offers the ability to highly compress a file without sacrificing too much information.  It is also a universal format that can be read by any web browser and by any image viewer program or image processing software.  Photoshop makes saving JPEG files for the internet easy.

Size Considerations

What file size is appropriate for a web image?  Consider your audience:  many people now have high-speed internet connections, but certainly not everyone does.  Most people will be willing to wait a short time to see an impressive astronomical image, but they will not wait for too long!  Given the current state of connection speeds, figure a file larger than 200 KB is likely to take longer to download than many people will be willing to wait.  200 KB might take only 2 seconds on a cable modem, but more often there will be slow servers and heavy web traffic to contend with and a longer wait can be expected even with a high-speed connection.  A dial-up user will have to wait over a minute for a 200 KB image - a long time in a world where fast computer technology has made us so impatient!  You can always have an option for interested viewers to load a higher-resolution image if they so desire.

Consider making thumbnail images of your larger images if you have a website.  A text list will load faster than images, but many people looking at your pictures will not know what in the world IC443 looks like and whether or not they want to wait for a larger picture of it to load.  Thumbnails can very easily be made which show clearly what the larger image will be and still be only about 5-10 KB in size each.  Details for making thumbnails are given below.

As for screen size (or resolution), most new computers have a default resolution of 1024x768 (although 1280x1024 is becoming common).  However, there are still a lot of computers out there which have older operating systems (for example, Windows 98 or earlier) which have a default resolution of 800x600.  Also, many users with less-then-ideal eyesight set their systems to a lower resolution to make the icons and text larger.  It is a cardinal sin in web design to ask your viewers to conform to your site -- rather you should design your site around your expected audience.  If you think most people will be viewing your site at 1024x768 resolution, you can put larger images online, but the files will, of course, be larger.  Most commercial sites (this one included) is set up for 800x600 resolution to accommodate the maximum number of users.  However, most web image galleries for both astronomical and other types of images are set up for the larger standard of 1024x768.

Saving as a JPEG

Take the following example:  say you have an image of the Cone Nebula, taken with your new large-format CCD camera.  The final 16-bit color file is around 60 MB in size, a bit on the large side.  When you save the file in the CCD image processing software it is recommended that you save it as an 8-bit TIFF file to transfer to Photoshop.  (If you are using Photoshop CS, you can keep the file in 16-bit mode, as CS can do most image processing routines on 16-bit files.  Version 6 and 7 can manipulate 16-bit files as well, but will not be able to run as many functions as CS, so for older versions of Photoshop, 8-bit files are recommended.)  Converting to 8-bit will reduce the file to 25 MB, still huge but heading in the right direction.

The first step in Photoshop is to resize the image to a reasonable display size for the web.  Smaller CCD cameras will probably not need to have their images resized, but anything around a megapixel in size or more should be reduced to fit the entire image on screen at once.  The example image of the Cone Nebula above was originally 4000x2700 pixels, about four times the width and height of a typical monitor's resolution.  Resizing to about 750 pixels wide is probably a good target size.  If you think most viewers will have 1024-pixel-wide displays, something closer to 900 pixels is fine.

Select Image > Image Size from the main menu.  In the Image Size window, change the largest dimension (width in the example below) to the chosen final image size.  Make sure Constrain Proportions is checked to keep the image from becoming distorted.  You can pretty much ignore for now the information displayed in the Document Size portion of the window;  this will come in handy when you are ready to print an image.  Bicubic is the best choice for Resample Image as this retains most of the information from the original picture.

In the above example, an image was resized from 4008 pixels wide to only 800.  Notice the file size displayed next to the Pixel Dimensions.  The first number is the resized image file size, while in parentheses is the original larger file size.  By resizing the image to a reasonable display size the file has been reduced more than 10 times from over 30 MB to just 1.2 MB!

Tip:  Most often, large images are reduced to fit onto a typical monitor.  Of course, much of the fine detail visible in the original is lost when the image is shrunk.  In some cases leaving the image slightly oversized is preferable in order to show off all the details, and the user can simply scroll around a bit to see everything.  The newest version of Internet Explorer automatically resizes an image to fit on screen and then allows the user to enlarge the view to full size if desired.  Admittedly, seeing the entire picture at once is probably more aesthetically pleasing, but sometimes more detail is desired.  Often a link is created to a cropped high-resolution section of the picture from the page containing the original full-screen version.

Once the image is resized, it needs to be saved.  Select File > Save As from the main menu or press Shift-Ctrl-S.  In the Save As window, name the file, select JPEG from the Format pull-down menu, and click Save.

Next, the JPEG options window will appear.  This is where you choose the amount of compression you wish to apply to the image.  This is adjusted by moving the slider bar in the Image Options window, by entering a value in the Quality box, or by choosing from the Quality pull-down menu. 

The highest quality is a value of 12, which in this example produces an image size of 320 KB.  By running the slider down to a Quality of 10, the size shrinks to 154 KB.  Keep the preview box checked to see the effect on the image.  By running the slider down to, say, 5, you could produce a 70 KB image, but the quality degrades quickly.  An image quality of 7 often produces a good image, resulting in very little loss of detail while making a reasonably-sized file.  In this example, a quality of 7 shrinks the file size to 80 KB, a very manageable size.

Notice in the Size section of the window that the download time is displayed along with the file size.  You can select from the pull-down menu which modem speed you would like to use as a reference.  An 80 KB image takes about 14 seconds to download over a 56K modem line, well within reason.  On a high-speed modem this image would load in less than 1 second.  Notice we have managed to make the file 750 times smaller than the original 60 MB file!

Thumbnail Images

Creating thumbnail images is a great way for people to get an idea of what your images look like before committing to a relatively long download.  Once your final image is saved, simply resize it to about 150 pixels or so along its longest axis.  This is a good size -- small enough to load rapidly but large enough to show what the general appearance of the image.

A lower JPEG quality may be used for thumbnails as well.  Typically a Quality of 4 or 5 will be sufficient.  A typical thumbnail might be only 5-10 KB or so, meaning a page showing a dozen small images will load in less time than a single large JPEG image.

Tip:  Once you resize the image to make a thumbnail, try using the Unsharp Mask sharpening filter to remove some of the softness that results from shrinking the file.  For a 150-pixel-wide thumbnail, try values of Amount: 100%, Radius: 1.0, and Threshold: 0.

Above:  Unsharp Mask settings for enhancing a thumbnail image.

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