diff --git a/inst/cta/src/GCTAAeff2D.cpp b/inst/cta/src/GCTAAeff2D.cpp
index 11544f2..25e9bf9 100644
--- a/inst/cta/src/GCTAAeff2D.cpp
+++ b/inst/cta/src/GCTAAeff2D.cpp
@@ -395,8 +395,9 @@ void GCTAAeff2D::save(const GFilename& filename, const bool& clobber) const
     // Get extension name
     std::string extname = filename.extname("EFFECTIVE AREA");
 
-    // Open or create FITS file (without extension)
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Remove extension if it exists already
     if (fits.contains(extname)) {
diff --git a/inst/cta/src/GCTABackground3D.cpp b/inst/cta/src/GCTABackground3D.cpp
index 0626720..c376003 100644
--- a/inst/cta/src/GCTABackground3D.cpp
+++ b/inst/cta/src/GCTABackground3D.cpp
@@ -448,8 +448,9 @@ void GCTABackground3D::save(const GFilename& filename, const bool& clobber) cons
     // Get extension name
     std::string extname = filename.extname("BACKGROUND");
 
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Remove extension if it exists already
     if (fits.contains(extname)) {
diff --git a/inst/cta/src/GCTAEdisp2D.cpp b/inst/cta/src/GCTAEdisp2D.cpp
index 9c47b5d..7b1eeeb 100644
--- a/inst/cta/src/GCTAEdisp2D.cpp
+++ b/inst/cta/src/GCTAEdisp2D.cpp
@@ -461,8 +461,9 @@ void GCTAEdisp2D::save(const GFilename& filename, const bool& clobber) const
     // Get extension name
     std::string extname = filename.extname("ENERGY DISPERSION");
 
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Remove extension if it exists already
     if (fits.contains(extname)) {
diff --git a/inst/cta/src/GCTAEventList.cpp b/inst/cta/src/GCTAEventList.cpp
index 1a87748..363af53 100644
--- a/inst/cta/src/GCTAEventList.cpp
+++ b/inst/cta/src/GCTAEventList.cpp
@@ -303,8 +303,9 @@ void GCTAEventList::load(const GFilename& filename)
 void GCTAEventList::save(const GFilename& filename,
                          const bool&      clobber) const
 {
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Write event list
     write(fits);
diff --git a/inst/cta/src/GCTAPsf2D.cpp b/inst/cta/src/GCTAPsf2D.cpp
index 0eaba82..6340af0 100644
--- a/inst/cta/src/GCTAPsf2D.cpp
+++ b/inst/cta/src/GCTAPsf2D.cpp
@@ -425,8 +425,9 @@ void GCTAPsf2D::save(const GFilename& filename, const bool& clobber) const
     // Get extension name
     std::string extname = filename.extname("POINT SPREAD FUNCTION");
 
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Remove extension if it exists already
     if (fits.contains(extname)) {
diff --git a/inst/cta/src/GCTAPsfKing.cpp b/inst/cta/src/GCTAPsfKing.cpp
index b32b1ef..9ff5872 100644
--- a/inst/cta/src/GCTAPsfKing.cpp
+++ b/inst/cta/src/GCTAPsfKing.cpp
@@ -423,8 +423,9 @@ void GCTAPsfKing::save(const GFilename& filename, const bool& clobber) const
     // Get extension name
     std::string extname = filename.extname("POINT SPREAD FUNCTION");
 
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Remove extension if it exists already
     if (fits.contains(extname)) {
diff --git a/src/obs/GEbounds.cpp b/src/obs/GEbounds.cpp
index 6028a36..c9601b2 100644
--- a/src/obs/GEbounds.cpp
+++ b/src/obs/GEbounds.cpp
@@ -599,31 +599,37 @@ void GEbounds::load(const GFilename& filename)
  * @brief Save energy boundaries into FITS file
  *
  * @param[in] filename FITS file name.
- * @param[in] clobber Overwrite any existing file  (default: false)?
- * @param[in] unit Energy units (default: "keV")
+ * @param[in] clobber Overwrite an existing energy boundaries extension?
+ * @param[in] unit Energy unit
  *
  * Saves energy boundaries into a FITS file. If a file with the given
  * @p filename does not yet exist it will be created, otherwise the method
- * opens the existing file. The method will create (or replace an existing)
- * energy boundaries extension. The extension name can be specified as part
- * of the @p filename, or if no extension name is given, is assumed to be
- * "EBOUNDS".
+ * opens the existing file. Energy boundaries can only be appended to an
+ * existing file if the @p clobber flag is set to `true` (otherwise an
+ * exception is thrown).
  *
- * An existing file will only be modified if the @p clobber flag is set to
- * true.
+ * The method will append a binary FITS table containing the energy
+ * boundaries to the FITS file. The extension name can be specified as part
+ * of the @p filename. For example the @p filename
+ *
+ *      myfile.fits[ENERGY BOUNDARIES]
+ *
+ * will save the energy boundaries in the `ENERGY BOUNDARIES` extension of
+ * the `myfile.fits` file. If the extension exists already in the file it
+ * will be replaced, otherwise a new extension will be created. If no
+ * extension name is provided, the method will use `EBOUNDS` as the default
+ * extension name for energy boundaries.
  ***************************************************************************/
 void GEbounds::save(const GFilename&   filename,
                     const bool&        clobber,
                     const std::string& unit) const
 {
-    // Get extension name
-    std::string extname = filename.extname("EBOUNDS");
-
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Write energy boundaries to FITS file
-    write(fits, extname, unit);
+    write(fits, filename.extname("EBOUNDS"), unit);
 
     // Save to file
     fits.save(clobber);
diff --git a/src/obs/GEnergies.cpp b/src/obs/GEnergies.cpp
index 8d0251d..fe6b7a4 100644
--- a/src/obs/GEnergies.cpp
+++ b/src/obs/GEnergies.cpp
@@ -565,30 +565,36 @@ void GEnergies::load(const GFilename& filename)
 
 
 /***********************************************************************//**
- * @brief Save energies to FITS file
+ * @brief Save energies into FITS file
  *
  * @param[in] filename FITS filename.
- * @param[in] clobber Overwrite any existing energies extension?
+ * @param[in] clobber Overwrite an existing energies extension?
  *
- * Saves energiesinto a FITS file. If a file with the given @p filename does
- * not yet exist it will be created, otherwise the method opens the existing
- * file. The method will create (or replace an existing) energies extension.
- * The extension name can be specified as part of the @p filename, or if no
- * extension name is given, is assumed to be "ENERGIES".
+ * Saves energies into a FITS file. If a file with the given @p filename
+ * does not yet exist it will be created, otherwise the method opens the
+ * existing file. Energies can only be appended to an existing file if the
+ * @p clobber flag is set to `true` (otherwise an exception is thrown).
  *
- * An existing file will only be modified if the @p clobber flag is set to
- * true.
+ * The method will append a binary FITS table containing the energies to the
+ * FITS file. The extension name can be specified as part of the
+ * @p filename. For example the @p filename
+ *
+ *      myfile.fits[ENERGY VALUES]
+ *
+ * will save the energies in the `ENERGY VALUES` extension of the
+ * `myfile.fits` file. If the extension exists already in the file it will be
+ * replaced, otherwise a new extension will be created. If no extension name
+ * is provided, the method will use `ENERGIES` as the default extension name
+ * for energies.
  ***************************************************************************/
 void GEnergies::save(const GFilename& filename, const bool& clobber) const
 {
-    // Get extension name
-    std::string extname = filename.extname("ENERGIES");
-
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Write energies to FITS file
-    write(fits, extname);
+    write(fits, filename.extname("ENERGIES"));
 
     // Save to file
     fits.save(clobber);
diff --git a/src/obs/GGti.cpp b/src/obs/GGti.cpp
index b4ee105..6a31c38 100644
--- a/src/obs/GGti.cpp
+++ b/src/obs/GGti.cpp
@@ -602,31 +602,37 @@ void GGti::load(const GFilename& filename)
 
 
 /***********************************************************************//**
- * @brief Save Good Time Intervals intervals to FITS file
+ * @brief Save Good Time Intervals into FITS file
  *
  * @param[in] filename FITS filename.
- * @param[in] clobber Modify any existing FITS file? (default: false)
+ * @param[in] clobber Overwrite an existing Good Time Interval extension?
  *
  * Saves Good Time Intervals into a FITS file. If a file with the given
  * @p filename does not yet exist it will be created, otherwise the method
- * opens the existing file. The method will create (or replace an existing)
- * Good Time Interval extension. The extension name can be specified as part
- * of the @p filename, or if no extension name is given, is assumed to be
- * "GTI".
+ * opens the existing file. Good Time Intervals can only be appended to an
+ * existing file if the @p clobber flag is set to `true` (otherwise an
+ * exception is thrown).
  *
- * An existing file will only be modified if the @p clobber flag is set to
- * true.
+ * The method will append a binary FITS table containing the Good Time
+ * Intervals to the FITS file. The extension name can be specified as part
+ * of the @p filename. For example the @p filename
+ *
+ *      myfile.fits[GOOD TIME INTERVALS]
+ *
+ * will save the Good Time Intervals in the `GOOD TIME INTERVALS` extension
+ * of the `myfile.fits` file. If the extension exists already in the file it
+ * will be replaced, otherwise a new extension will be created. If no
+ * extension name is provided, the method will use `GTI` as the default
+ * extension name for Good Time Intervals.
  ***************************************************************************/
 void GGti::save(const GFilename& filename, const bool& clobber) const
 {
-    // Get extension name
-    std::string extname = filename.extname("GTI");
-
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
     // Write GTI to FITS object
-    write(fits, extname);
+    write(fits, filename.extname("GTI"));
 
     // Save to file
     fits.save(clobber);
diff --git a/src/support/GNodeArray.cpp b/src/support/GNodeArray.cpp
index 0905ad0..8f9641a 100644
--- a/src/support/GNodeArray.cpp
+++ b/src/support/GNodeArray.cpp
@@ -721,26 +721,35 @@ void GNodeArray::load(const GFilename& filename)
 
 
 /***********************************************************************//**
- * @brief Save nodes to FITS file
+ * @brief Save node array into FITS file
  *
  * @param[in] filename FITS filename.
- * @param[in] clobber Overwrite any existing nodes extension? (default: false)
+ * @param[in] clobber Overwrite an existing node array extension?
  *
  * Saves node array into a FITS file. If a file with the given @p filename
  * does not yet exist it will be created, otherwise the method opens the
- * existing file. The method will create (or replace an existing) node array
- * extension. The extension name can be specified as part of the @p filename,
- * or if no extension name is given, is assumed to be "NODES".
+ * existing file. Node arrays can only be appended to an existing file if the
+ * @p clobber flag is set to `true` (otherwise an exception is thrown).
  *
- * An existing file will only be modified if the @p clobber flag is set to
- * true.
+ * The method will append a binary FITS table containing the node array to
+ * the FITS file. The extension name can be specified as part of the
+ * @p filename. For example the @p filename
+ *
+ *      myfile.fits[NODE ARRAY]
+ *
+ * will save the node array in the `NODE ARRAY` extension of the
+ * `myfile.fits` file. If the extension exists already in the file it will be
+ * replaced, otherwise a new extension will be created. If no extension name
+ * is provided, the method will use `NODES` as the default extension name
+ * for the node array.
  ***************************************************************************/
 void GNodeArray::save(const GFilename& filename, const bool& clobber) const
 {
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Open or create FITS file (without extension name since the requested
+    // extension may not yet exist in the file)
+    GFits fits(filename.url(), true);
 
-    // Write GTI to FITS object
+    // Write node array to FITS object
     write(fits, filename.extname("NODES"));
 
     // Save to file
diff --git a/src/xspec/GArf.cpp b/src/xspec/GArf.cpp
index d63f52e..3ba40e2 100644
--- a/src/xspec/GArf.cpp
+++ b/src/xspec/GArf.cpp
@@ -252,11 +252,18 @@ const double& GArf::at(const int& index) const
  * @brief Load Auxiliary Response File
  *
  * @param[in] filename File name.
+ *
+ * Loads the Auxiliary Response File from the `SPECRESP` extension of the
+ * FITS file.
  ***************************************************************************/
 void GArf::load(const GFilename& filename)
 {
-    // Open FITS file
-    GFits fits(filename);
+    // Clear response
+    clear();
+
+    // Open FITS file (without extension name as the user is not allowed
+    // to modify the extension names)
+    GFits fits(filename.url());
 
     // Get ARF table
     const GFitsTable& table = *fits.table("SPECRESP");
@@ -268,7 +275,7 @@ void GArf::load(const GFilename& filename)
     fits.close();
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -279,21 +286,30 @@ void GArf::load(const GFilename& filename)
  * @brief Save Auxiliary Response File
  *
  * @param[in] filename File name.
- * @param[in] clobber Overwrite existing file? (defaults: false)
+ * @param[in] clobber Overwrite existing file?
+ *
+ * Saves the Auxiliary Response File into a FITS file. If a file with the
+ * given @p filename does not yet exist it will be created. If the file
+ * exists it can be overwritten if the @p clobber flag is set to `true`.
+ * Otherwise an exception is thrown.
+ *
+ * The method will save the `SPECRESP` binary FITS table into the FITS file
+ * that contains the values of the Auxiliary Response File.
  ***************************************************************************/
 void GArf::save(const GFilename& filename, const bool& clobber) const
 {
-    // Open FITS file
+    // Create FITS file
     GFits fits;
 
     // Write ARF into file
     write(fits);
 
-    // Close FITS file
-    fits.saveto(filename, clobber);
+    // Save to file (without extension name since the requested extension
+    // may not yet exist in the file)
+    fits.saveto(filename.url(), clobber);
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -304,6 +320,19 @@ void GArf::save(const GFilename& filename, const bool& clobber) const
  * @brief Read Auxiliary Response File
  *
  * @param[in] table ARF FITS table.
+ *
+ * Reads the Auxiliary Response File from a FITS table. The true energy
+ * boundaries are expected in the `ENERG_LO` and `ENERG_HI` columns, the
+ * response information is expected in the `SPECRESP` column.
+ *
+ * The method will analyze the unit of the `SPECRESP` column, and if either
+ * `m^2` or `m2` are encountered, multiply the values of the column by 
+ * \f$10^4\f$ to convert the response into units of \f$cm^2\f$. Units of the
+ * `ENERG_LO` and `ENERG_HI` columns are also interpreted for conversion.
+ *
+ * See
+ * http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html#tth_sEc4
+ * for details about the Auxiliary Response File format.
  ***************************************************************************/
 void GArf::read(const GFitsTable& table)
 {
@@ -349,6 +378,18 @@ void GArf::read(const GFitsTable& table)
  * @brief Write Auxiliary Response File
  *
  * @param[in] fits FITS file.
+ *
+ * Writes the Auxiliary Response File into the `SPECRESP` extension of the
+ * FITS file. An existing extension with the same name will be removed from
+ * the FITS file before writing.
+ *
+ * The method writes the boundaries of the true energy bins into the
+ * `ENERG_LO` and `ENERG_HI` columns, response information will be written
+ * into the `SPECRESP` column.
+ *
+ * See
+ * http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html#tth_sEc4
+ * for details about the Auxiliary Response File format.
  ***************************************************************************/
 void GArf::write(GFits& fits) const
 {
@@ -405,7 +446,7 @@ void GArf::write(GFits& fits) const
 /***********************************************************************//**
  * @brief Print Auxiliary Response File
  *
- * @param[in] chatter Chattiness (defaults to NORMAL).
+ * @param[in] chatter Chattiness.
  * @return String containing Auxiliary Response File information.
  ***************************************************************************/
 std::string GArf::print(const GChatter& chatter) const
diff --git a/src/xspec/GPha.cpp b/src/xspec/GPha.cpp
index 93d7ff1..abcf660 100644
--- a/src/xspec/GPha.cpp
+++ b/src/xspec/GPha.cpp
@@ -330,16 +330,18 @@ void GPha::fill(const GEnergy& energy, const double& value)
  *
  * @param[in] filename File name.
  *
- * Loads the Pulse Height Analyzer spectrum from a FITS file. If an EBOUNDS
- * extension is given, the energy boundaries information is also loaded
+ * Loads the Pulse Height Analyzer spectrum from the `SPECTRUM` extension
+ * of the FITS file. If the file contains also an `EBOUNDS` extension the
+ * energy boundaries of all Pulse Height Analyzer channels are also loaded.
  ***************************************************************************/
 void GPha::load(const GFilename& filename)
 {
-    // Clear any existing models
+    // Clear spectrum
     clear();
 
-    // Open FITS file
-    GFits fits(filename);
+    // Open FITS file (without extension name as the user is not allowed
+    // to modify the extension names)
+    GFits fits(filename.url());
 
     // Get PHA table
     const GFitsTable& pha = *fits.table("SPECTRUM");
@@ -362,7 +364,7 @@ void GPha::load(const GFilename& filename)
     fits.close();
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -373,30 +375,32 @@ void GPha::load(const GFilename& filename)
  * @brief Save Pulse Height Analyzer spectrum
  *
  * @param[in] filename File name.
- * @param[in] clobber Overwrite existing file? (default: false)
+ * @param[in] clobber Overwrite existing file?
  *
- * Saves Pulse Height Analyzer spectrum and energy boundaries into a FITS
- * file. If a file with the given @p filename does not yet exist it will be
- * created, otherwise the method opens the existing file. The method will
- * create (or replace an existing) Pulse Height Analyzer spectrum and energy
- * boundary extensions.
+ * Saves the Pulse Height Analyzer spectrum and energy boundaries into a
+ * FITS file. If a file with the given @p filename does not yet exist it
+ * will be created. If the file exists it can be overwritten if the
+ * @p clobber flag is set to `true`. Otherwise an exception is thrown.
  *
- * An existing file will only be modified if the @p clobber flag is set to
- * true.
+ * The method will save two binary FITS tables into the FITS file: a
+ * `SPECTRUM` extension that contains the channel values of the Pulse
+ * Height Analyzer spectrum and an `EBOUNDS` extension that contains the
+ * energy boundaries for all channels.
  ***************************************************************************/
 void GPha::save(const GFilename& filename, const bool& clobber) const
 {
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Create FITS file
+    GFits fits;
 
     // Write PHA into file
     write(fits);
 
-    // Close FITS file
-    fits.save(clobber);
+    // Save to file (without extension name since the requested extension
+    // may not yet exist in the file)
+    fits.saveto(filename.url(), clobber);
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -406,10 +410,18 @@ void GPha::save(const GFilename& filename, const bool& clobber) const
 /***********************************************************************//**
  * @brief Read Pulse Height Analyzer spectrum
  *
- * @param[in] table PHA FITS table.
+ * @param[in] table FITS table.
  *
  * @exception GException::invalid_value
  *            Mismatch between PHA file and energy boundaries.
+ *
+ * Reads the Pulse Height Analyzer spectrum from a FITS table. The channel
+ * values are expected in the `COUNTS` column of the table. All other
+ * columns are ignored.
+ *
+ * See
+ * https://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/spectra/ogip_92_007/node5.html
+ * for details about the Pulse Height Analyzer spectrum format.
  ***************************************************************************/
 void GPha::read(const GFitsTable& table)
 {
@@ -417,17 +429,16 @@ void GPha::read(const GFitsTable& table)
     const GFitsTableCol* col_data = table["COUNTS"];
 
     // Extract number of channels in FITS file
-    int length = table.integer("NAXIS2");
+    int length = col_data->length();
 
-    // Check whether column length is okay
+    // Check whether column length is consistent with energy boundaries
     if (m_ebounds.size() > 0) {
         if (m_ebounds.size() != length) {
-            std::string msg = "Number of channels in PHA file ("
-                              ""+gammalib::str(length)+") mismatches the"
-                              " number of energy boundaries ("
-                              ""+gammalib::str(m_ebounds.size())+") that"
-                              " are defined in the GPha instance.\n"
-                              "Please define the correct energy boundaries.";
+            std::string msg = "Mismatch between the "+gammalib::str(length)+
+                              " channels in the PHA file and the "+
+                              gammalib::str(m_ebounds.size())+" energy "
+                              "boundaries. Please correct either the energy "
+                              "boundaris or the PHA file.";
             throw GException::invalid_value(G_READ, msg);
         }
     }
@@ -449,6 +460,20 @@ void GPha::read(const GFitsTable& table)
  * @brief Write Pulse Height Analyzer spectrum
  *
  * @param[in] fits FITS file.
+ *
+ * Writes the Pulse Height Analyzer spectrum into `SPECTRUM` and `EBOUNDS`
+ * extensions of the FITS file. Extensions with these names will be removed
+ * from the FITS file before writing.
+ *
+ * The columns `CHANNEL`, `COUNTS`, `STAT_ERR`, `SYS_ERR`, `QUALITY`,
+ * `GROUPING`, `AREASCAL`, and `BACKSCAL` will be written into the `SPECTRUM`
+ * extension, but only the `CHANNEL` and `COUNTS` columns will be filled with
+ * values. Note that the channels start from 1 in the Pulse Height Analyzer
+ * spectrum.
+ *
+ * See
+ * https://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/spectra/ogip_92_007/node5.html
+ * for details about the PHA file format.
  ***************************************************************************/
 void GPha::write(GFits& fits) const
 {
@@ -516,7 +541,7 @@ void GPha::write(GFits& fits) const
 /***********************************************************************//**
  * @brief Print Pulse Height Analyzer spectrum
  *
- * @param[in] chatter Chattiness (defaults to NORMAL).
+ * @param[in] chatter Chattiness.
  * @return String containing Pulse Height Analyzer spectrum information.
  ***************************************************************************/
 std::string GPha::print(const GChatter& chatter) const
diff --git a/src/xspec/GRmf.cpp b/src/xspec/GRmf.cpp
index 0bcf91d..3efa0c3 100644
--- a/src/xspec/GRmf.cpp
+++ b/src/xspec/GRmf.cpp
@@ -369,20 +369,25 @@ GEbounds GRmf::emeasured(const GEnergy& etrue) const
  * @brief Load Redistribution Matrix File
  *
  * @param[in] filename File name.
+ *
+ * Loads the Redistribution Matrix File from the `MATRIX` extension of the
+ * FITS file. If the file contains also an `EBOUNDS` extension the energy
+ * boundaries of all measured energies are also loaded.
  ***************************************************************************/
 void GRmf::load(const GFilename& filename)
 {
-    // Clear any existing models
+    // Clear matrix
     clear();
 
-    // Open FITS file
-    GFits fits(filename);
+    // Open FITS file (without extension name as the user is not allowed
+    // to modify the extension names)
+    GFits fits(filename.url());
 
     // Read measured energy boundaries
-    m_ebds_measured.load(filename);
+    m_ebds_measured.load(filename.url());
 
     // Get RMF table
-    const GFitsTable& table = *fits.table(filename.extname("MATRIX"));
+    const GFitsTable& table = *fits.table("MATRIX");
 
     // Read RMF data
     read(table);
@@ -391,7 +396,7 @@ void GRmf::load(const GFilename& filename)
     fits.close();
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -402,24 +407,36 @@ void GRmf::load(const GFilename& filename)
  * @brief Save Redistribution Matrix File
  *
  * @param[in] filename File name.
- * @param[in] clobber Overwrite existing file? (default: false)
- * @param[in] unit Energy unit (default: "keV")
+ * @param[in] clobber Overwrite existing file?
+ * @param[in] unit Energy unit.
+ *
+ * Saves the Redistribution Matrix File into a FITS file. If a file with the
+ * given @p filename does not yet exist it will be created. If the file
+ * exists it can be overwritten if the @p clobber flag is set to `true`.
+ * Otherwise an exception is thrown.
+ *
+ * The method will save two binary FITS tables into the FITS file: a
+ * `MATRIX` extension that contains the Redistribution Matrix File elements,
+ * and an `EBOUNDS` extension that contains the measured energy boundaries
+ * for all matrix. The @p unit argument specifies the unit of the true
+ * energies that will be saved in the `MATRIX` extension.
  ***************************************************************************/
 void GRmf::save(const GFilename&   filename,
                 const bool&        clobber,
                 const std::string& unit) const
 {
-    // Open or create FITS file
-    GFits fits(filename, true);
+    // Create FITS file
+    GFits fits;
 
     // Write RMF into file
     write(fits, unit);
 
-    // Save to file
-    fits.save(clobber);
+    // Save to file (without extension name since the requested extension
+    // may not yet exist in the file)
+    fits.saveto(filename.url(), clobber);
 
     // Store filename
-    m_filename = filename;
+    m_filename = filename.url();
 
     // Return
     return;
@@ -430,6 +447,18 @@ void GRmf::save(const GFilename&   filename,
  * @brief Read Redistribution Matrix File
  *
  * @param[in] table RMF FITS table.
+ *
+ * Reads the Redistribution Matrix File from a FITS table. The true energy
+ * bins are expected in the `ENERG_LO` and `ENERG_HI` columns. Information
+ * for matrix compression are stored in the `N_GRP`, `F_CHAN` and `N_CHAN`
+ * columns, and the matrix elements are stored in the `MATRIX` column.
+ *
+ * The method automatically decompresses the Redistribution Matrix File and
+ * stores the matrix elements in a spare matrix.
+ *
+ * See
+ * http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html#tth_sEc3
+ * for details about the Redistribution Matrix File file format.
  ***************************************************************************/
 void GRmf::read(const GFitsTable& table)
 {
@@ -497,7 +526,23 @@ void GRmf::read(const GFitsTable& table)
  * @brief Write Redistribution Matrix File
  *
  * @param[in] fits FITS file.
- * @param[in] unit Energy unit (defaults to "keV")
+ * @param[in] unit Energy unit.
+ *
+ * Writes the Redistribution Matrix File into `MATRIX` and `EBOUNDS`
+ * extensions of the FITS file. Extensions with these names will be removed
+ * from the FITS file before writing.
+ *
+ * The true energy bins are written into the `ENERG_LO` and `ENERG_HI`
+ * columns. The units of the energies in these columns is specified by the
+ * @p unit argument. Information for matrix compression are stored in the
+ * `N_GRP`, `F_CHAN` and `N_CHAN` columns, and the matrix elements are
+ * written into the `MATRIX` column.
+ *
+ * The measured energy bins are written into the `EBOUNDS` extension.
+ *
+ * See
+ * http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html#tth_sEc3
+ * for details about the Redistribution Matrix File file format.
  ***************************************************************************/
 void GRmf::write(GFits& fits, const std::string& unit) const
 {
@@ -629,7 +674,7 @@ void GRmf::write(GFits& fits, const std::string& unit) const
 /***********************************************************************//**
  * @brief Print Redistribution Matrix File
  *
- * @param[in] chatter Chattiness (defaults to NORMAL).
+ * @param[in] chatter Chattiness.
  * @return String containing Redistribution Matrix File information.
  ***************************************************************************/
 std::string GRmf::print(const GChatter& chatter) const
diff --git a/test/test_GObservation.cpp b/test/test_GObservation.cpp
index 930064c..fee1db3 100644
--- a/test/test_GObservation.cpp
+++ b/test/test_GObservation.cpp
@@ -252,6 +252,24 @@ void TestGObservation::test_ebounds(void)
     test_assert(!containment.contains(GEnergy(200.0, "MeV"), GEnergy(2000.0, "MeV")), "Energy bin [200,2000] MeV should not be contained.");
     test_assert(!containment.contains(GEnergy(80.0, "MeV"), GEnergy(2000.0, "MeV")), "Energy bin [80,2000] MeV should not be contained.");
 
+    // Remove test file
+    GFilename filename("test_ebounds.fits");
+    filename.remove();
+
+    // Check saving
+    containment.save("test_ebounds.fits");
+    GEbounds load1("test_ebounds.fits");
+    test_value(load1.size(), 1, "GEbounds should have 1 element.");
+    test_value(load1.emin().MeV(), 100.0, 1.0e-10, "Minimum energy should be 100.");
+    test_value(load1.emax().MeV(), 1000.0, 1.0e-10, "Maximum energy should be 1000.");
+
+    // Check saving in a different extnsion
+    ebds.save("test_ebounds.fits[ENERGY BOUNDARIES 2]", true);
+    GEbounds load2("test_ebounds.fits[ENERGY BOUNDARIES 2]");
+    test_value(load2.size(), 1, "GEbounds should have 1 element.");
+    test_value(load2.emin().MeV(), 1.0, 1.0e-10, "Minimum energy should be 1.");
+    test_value(load2.emax().MeV(), 10.0, 1.0e-10, "Maximum energy should be 10.");
+
     // Return
     return;
 }
@@ -379,6 +397,12 @@ void TestGObservation::test_gti(void)
     test_value(gti.tstart().secs(), 1.0, 1.0e-7, "Start time should be 1.");
     test_value(gti.tstop().secs(), 1000.0, 1.0e-7, "Stop time should be 1000.");
 
+    // Remove test file
+    GFilename file1("test_gti1.fits");
+    GFilename file2("test_gti2.fits");
+    file1.remove();
+    file2.remove();
+
     // Check saving in and loading from FITS file
     gti.save("test_gti1.fits", true);
     GGti test1("test_gti1.fits");
@@ -480,6 +504,24 @@ void TestGObservation::test_gti(void)
     test_value(test5.tstart().secs(), 1.0, 1.0e-7, "Start time should be 1.");
     test_value(test5.tstop().secs(), 10000.0, 1.0e-7, "Stop time should be 10000.");
 
+    // Remove test file
+    GFilename filename("test_gti.fits");
+    filename.remove();
+
+    // Check saving
+    test4.save("test_gti.fits");
+    GGti load1("test_gti.fits");
+    test_value(load1.size(), 4, "GGti should have 4 intervals.");
+    test_value(load1.tstart().secs(), 1.0, 1.0e-7, "Start time should be 1.");
+    test_value(load1.tstop().secs(), 10000.0, 1.0e-7, "Stop time should be 10000.");
+
+    // Check saving in a different extnsion
+    test3.save("test_gti.fits[GOOD TIME INTERVALS]", true);
+    GGti load2("test_gti.fits[GOOD TIME INTERVALS]");
+    test_value(load2.size(), 1, "GGti should have 1 interval.");
+    test_value(load2.tstart().convert(GTimeReference(51544.5, "s")), 0.0, 1.0e-7, "Start time should be 0.");
+    test_value(load2.tstop().convert(GTimeReference(51544.5, "s")), 100.0, 1.0e-7, "Stop time should be 100.");
+
     // Return
     return;
 }
@@ -984,6 +1026,10 @@ void TestGObservation::test_energies(void)
         test_value(energies[i].MeV(), double(i));
     }
 
+    // Remove test file
+    GFilename filename("test_energies.fits");
+    filename.remove();
+
     // Save and reload energies
     test_try("Saving and loading");
     try {
@@ -1012,6 +1058,21 @@ void TestGObservation::test_energies(void)
         test_try_failure(e);
     }
 
+    // Save and reload energies in another extension
+    test_try("Saving and loading in another extension");
+    try {
+        energies.save("test_energies.fits[NEW ENERGIES]", true);
+        energies.clear();
+        energies.load("test_energies.fits[NEW ENERGIES]");
+        test_try_success();
+    }
+    catch (std::exception &e) {
+        test_try_failure(e);
+    }
+    for (int i = 0; i < 4; ++i) {
+        test_value(energies[i].MeV(), double(i));
+    }
+
     // Check linear energies
     energies.clear();
     energies.set_lin(3, GEnergy(1.0, "MeV"), GEnergy(3.0, "MeV"));
diff --git a/test/test_GSupport.cpp b/test/test_GSupport.cpp
index 334e032..38f11a4 100644
--- a/test/test_GSupport.cpp
+++ b/test/test_GSupport.cpp
@@ -381,6 +381,30 @@ void TestGSupport::test_node_array(void)
     test_value(nodes.wgt_left(), 0.25, 1.0e-6, "Expected weight 0.25");
     test_value(nodes.wgt_right(), 0.75, 1.0e-6, "Expected weight 0.75");
 
+    // Remove test file
+    GFilename filename("test_nodes.fits");
+    filename.remove();
+
+    // Check saving
+    nodes.save("test_nodes.fits");
+    GNodeArray load1("test_nodes.fits");
+    test_value(load1.size(), 3, "GNodeArray should have 3 nodes.");
+    test_value(load1[0], 1.0, 1.0e-7, "First node should be 1.");
+    test_value(load1[1], 3.0, 1.0e-7, "First node should be 3.");
+    test_value(load1[2], 4.0, 1.0e-7, "First node should be 4.");
+
+    // Check saving in a different extnsion
+    nodes.append(10.0);
+    nodes.append(11.0);
+    nodes.save("test_nodes.fits[NODE ARRAY]", true);
+    GNodeArray load2("test_nodes.fits[NODE ARRAY]");
+    test_value(load2.size(), 5, "GNodeArray should have 5 nodes.");
+    test_value(load2[0], 1.0, 1.0e-7, "First node should be 1.");
+    test_value(load2[1], 3.0, 1.0e-7, "First node should be 3.");
+    test_value(load2[2], 4.0, 1.0e-7, "First node should be 4.");
+    test_value(load2[3], 10.0, 1.0e-7, "First node should be 10.");
+    test_value(load2[4], 11.0, 1.0e-7, "First node should be 11.");
+
     // Return
     return;
 }