APOD - Get Astronomy Picture of the Day

• Download Win32 binaries - 333.99 KB
• Download source code - 377.72 KB
• Download utilities (required for source code) - 24.72 KB

Introduction

I am always amazed with space-related images that NASA shares on a daily basis (see http://antwrp.gsfc.nasa.gov/apod/astropix.html). I am a frequent visitor to that site and it was very common (if not always) to download the images to my computer for later offline visualization (we still don't have an always-on world - at least a cost effective one). Moreover, I frequently had several images I wanted to download (all pictures of a month) and it was a tedious clicking process.

Hence, I developed my own automated software for that!

The software and lessons shared here consist of the following:

• Perform HTTP requests to APOD website (for a selected date)
• Retrieve and process HTML code
• Identify and retrieve JPG images contained in HTML (currently only JPG)
• Store JPG image in the same directory as the executable. Image is stored as <year><month><day>.jpg

Moreover, I tried to develop a user friendly window. However, only date selection is allowed at present.

Background

No specific background is required, other than basic knowledge of C# (or any other OO language and principles) and Microsoft Visual Studio.

Note that the code is 'quick' - hence short in documentation and cleanness.

Using the Code

The program is simple. The user is currently allowed to select a specific date. Then three commands are currently allowed:

1. Get image
2. Get all images from selected month (whose date is earlier than selected date)
3. Exit

2 is actually an extension of 1, as I will explain later. 1 does the most complicated stuff.

But before I start, we need to understand how information and pages in APOD are structured - in a very simple and intuitive way: 

APOD base URL is:

• http://antwrp.gsfc.nasa.gov/

The people who run the site are both smart and kind for they provide an extensive archive of pictures (current and past), totally free, which may be accessed as:

• http://antwrp.gsfc.nasa.gov/apod/ap<2-digit year><2-digit month><2 digit day>.html  

For example, image for year 2009, month 09 and day 09 is accessed through the following URL: 

• http://antwrp.gsfc.nasa.gov/apod/ap090916.html  

So, having the rule to fetch pages for any given day, it's a matter of starting using nice C#,.NET and HTTP stuff.  

Let's Start !

Actually action starts when a user presses the 'GET !' button. The click event calls the following code:  

private void buttonGET_Click(object sender, EventArgs e)
{ 
    GetAPODImage(dateTimePicker.Value); 
} 

GetAPODimage method does the main work:

First, we generate the URL for the selected date, following the rule explained above:

string url = GenerateAPOD_URL("http://antwrp.gsfc.nasa.gov/apod/ap",date);

This method builds the URL as follows:

private string GenerateAPOD_URL(string rootURL, System.DateTime date)
{
    return rootURL
            + Get2DigitYear(date)
            + Get2DigitNumberAsString(date.Month)
            + Get2DigitNumberAsString(date.Day)
            + ".html";
}

Note that Get2DigitNumberAsString returns a two digit string. For example, if selected day is 1, it returns ‘01’.

Next, we are going to retrieve the HTML-page:

GetHTTP(url,5000,ref resultHTML); 

GetHTTP method will fetch the HTML page from ‘url’ and store it to ‘resultHTML’. Note that we pass ‘5000’ (i.e., 5 seconds) which indicates the timeout period to wait to fetch the page – this is very important to prevent the possibility to wait a long time for a page. 

GetHTTP code is as follows: 

// create HTTP request
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
req.Timeout = timeout;
req.ReadWriteTimeout = timeout;

// get response
WebResponse resp = req.GetResponse();

// we will read data via the response stream
Stream resStream = resp.GetResponseStream();

int    count      = 0;
byte[] buf = new byte[8192];

resultHTML = "";

do
{
    // fill the buffer with data
    count = resStream.Read(buf, 0, buf.Length);

    // make sure we read some data
    if (count != 0)
    {
        // translate from bytes to ASCII text
        resultHTML += Encoding.ASCII.GetString(buf, 0, count);
    }
}
while (count > 0); // any more data to read
    resp.Close(); 

Straightforward. OK – now, if no exceptions were raised, we must parse ‘resultHTML’ and look for ‘JPGs’ (which are the nice pictures we wish to download): 

GetHTTPFilesByTypeSuffix(resultHTML, ‘.JPG’, ref list); 

I will not go into excessive details in this function, except that work performed is to look for ‘href=’ and fetch the referenced file. It will only consider it valid if it is a ‘.JPG’.
For example: if var ‘resultHTML’ contains “<a href="image/0909/tarantula_gleason.jpg">”, image/0909/tarantula_gleason.jpg will be added to the list (var 'list'). 

Now that we have the image files, we should download and store them on disk (look at the code in bold): 

try 
{
    ....
    //get and save images 
    foreach (string file in list) 
    {
        String source = "http://antwrp.gsfc.nasa.gov/apod/"+file;
        string dest = GetYYYYMMDD(date)+’.JPG’;
        WebClient Client = new WebClient();
        Client.DownloadFile(source, dest); 
        pictureBox.Image = Image.FromFile(dest);
    }

}//end try
catch (Exception ex)
{
    Console.WriteLine("An exception occurred: " + ex.Message);
    pictureBox.Image = Properties.Resources.g_close;
}

Note that:

• After the picture is stored, I show it in pictureBox (in the main window)
• If an error occurs, pictureBox shows an error image

Finally, how do you fetch all images for a selected month (which are earlier than the selected date)? Very simple:

private void buttonGETMONTH_Click(object sender, EventArgs e)
{ 
    DateTime date = dateTimePicker.Value;
    for (int i = 1; date.Month == dateTimePicker.Value.Month; i++ )
    {
        GetAPODImage(date);
        date = dateTimePicker.Value.AddDays(-i);
    }
}

The trick is to initialize temporary variable date to selected date and then decrease n-days until the month is changed. 

That’s it ! I hope you enjoyed the article.

Final Remarks

You may download the win32 binary and run it directly (you need .NET 3.5 Framework). If you open the solution, you'll note a dependency on WeAreUtils (which is also included). This library actually has several of the utilities referred in the article. However, I decided to present a simplified version to make it easier to understand.

If you liked the software, you may find it at SourceForge (http://sourceforge.net/projects/weareapod/)!  Join the project!

History

• 16^th September, 2009: Initial post