ENVI 5: What's the Difference Between "File > Open" and "File > Open As"?

In ENVI 5, there are two options for opening data files; From the File menu, you can choose either Open, or Open As.  What's the difference? 

File > Open is the same as choosing the Open button from the main toolbar, or from inside the Data Manager.  Any of those options can be used to read a limited number of file types that ENVI can automatically recognize.  Those file types are:

• Cartosat
• DPPDB
• DTED
• ENVI
• ERDAS Imagine
• ESRI GRID
• GeoEye-1
• IKONOS
• JPEG
• JPEG2000
• KOMPSAT-2
• Landsat GeoTIFF with metadata
• MrSID
• NITF
• Orbview-3
• QuickBird
• RapidEye
• SPOT DIMAP
• TIFF and GeoTIFF
• Worldview-1 and -2

All of those same file types also have a specific reader than can be found under File > Open As.  In addition, many supported file types are not automatically recognized by File > Open (or the Open buttons).  For those types of files, you need to choose File > Open As to specify the type of file.  ENVI will then use that specific file reader to open the file. 

For example, if you try to open an ASTER dataset, which is distributed in the HDF format, using File > Open, you will get an error message.  But you can read it by choosing File > Open As > EOS > ASTER.