diff --git a/_test/test_experiment/test_experiment_constructor.m b/_test/test_experiment/test_experiment_constructor.m
index 7ebddf0f8c..59778ac14d 100644
--- a/_test/test_experiment/test_experiment_constructor.m
+++ b/_test/test_experiment/test_experiment_constructor.m
@@ -95,7 +95,7 @@ function test_load_save_default_object_creates_default_object(~)
             load(tmpfile, 'expt');
             assertTrue( isempty(expt.instruments));
             assertTrue( isempty(expt.samples));
-            assertEqual(expt.detector_arrays, []);
+            assertEqual(expt.detector_arrays, IX_detector_array.empty);
         end
 
         function test_instruments_setter_updates_value_for_valid_value(~)
diff --git a/_test/test_sqw_class/test_equal_to_tol.m b/_test/test_sqw_class/test_equal_to_tol.m
index 8eac96b27a..d775a2df9d 100644
--- a/_test/test_sqw_class/test_equal_to_tol.m
+++ b/_test/test_sqw_class/test_equal_to_tol.m
@@ -65,7 +65,7 @@ function test_different_sqw_objects_are_not_equal(obj)
                 sqw_copy = obj.sqw_2d;
                 field_name = class_fields{idx};
                 
-                if isstruct(sqw_copy.(field_name))
+                if isstruct(sqw_copy.(field_name)) && ~isempty(sqw_copy.(field_name))
                     sqw_copy.(field_name).test_field = 'test_value';
                 elseif isstring(sqw_copy.(field_name))
                     sqw_copy.(field_name) = 'test_value';
diff --git a/_test/test_sqw_class/test_sqw_constructor.m b/_test/test_sqw_class/test_sqw_constructor.m
index bda7dba358..37091c1599 100644
--- a/_test/test_sqw_class/test_sqw_constructor.m
+++ b/_test/test_sqw_class/test_sqw_constructor.m
@@ -49,7 +49,7 @@ function test_filename_constructor_returns_populated_class(obj)
             assertEqual(numel(sqw_obj.experiment_info.expdata), 85)
             assertEqual(numel(sqw_obj.experiment_info.instruments), 85)
             assertEqual(numel(sqw_obj.experiment_info.samples), 85)
-            assertEqual(numel(sqw_obj.detpar.group), 36864);
+            assertEqual(numel(sqw_obj.experiment_info.detector_arrays.id), 36864);
             assertEqual(numel(sqw_obj.data.pax), 1);
             assertEqual(sqw_obj.data.pix.num_pixels, 100337);
         end
@@ -86,19 +86,32 @@ function test_copy_constructor_returns_distinct_object(obj)
             sqw_obj = sqw(obj.test_sqw_1d_fullpath);
             sqw_copy = sqw(sqw_obj);
             
+            % combine value changes and tests for them
+            % (1) change main_header
             sqw_copy.main_header.title = 'test_copy';
+            assertFalse(equal_to_tol(sqw_copy.main_header, sqw_obj.main_header));
+            
+            % (2) change detpar values now in experiment_info
+            expinf1 = sqw_copy.experiment_info;
+            for i=1:numel(expinf1.detector_arrays), expinf1.detector_arrays(i).azim = 0; end            
+            sqw_copy = sqw_copy.change_header(expinf1);
+            assertFalse(equal_to_tol(sqw_copy.experiment_info, sqw_obj.experiment_info));
+
+            % (3) change entire experiment_info
             sqw_copy = sqw_copy.change_header(Experiment());
-            sqw_copy.detpar.azim(1:10) = 0;
+            assertFalse(equal_to_tol(sqw_copy.experiment_info, sqw_obj.experiment_info));
+
+            % (4) change data arrays
             sqw_copy.data.dax = [2 1];
             sqw_copy.data.pix.signal = 1;
-            
-            % changed data is not mirrored in initial
-            assertFalse(equal_to_tol(sqw_copy.main_header, sqw_obj.main_header));
-            assertFalse(equal_to_tol(sqw_copy.experiment_info, sqw_obj.experiment_info));
-            assertFalse(equal_to_tol(sqw_copy.detpar, sqw_obj.detpar));
             assertFalse(equal_to_tol(sqw_copy.data, sqw_obj.data));
             assertFalse(equal_to_tol(sqw_copy.data.pix, sqw_obj.data.pix));
             
+            % detpar is now empty struct array, should be unchanged in the
+            % copy
+            assertTrue(equal_to_tol(sqw_copy.detpar, sqw_obj.detpar));
+            
+            % (5) check entire sqw
             assertFalse(equal_to_tol(sqw_copy, sqw_obj));
         end
         
diff --git a/_test/test_sqw_class/test_sqw_copy.m b/_test/test_sqw_class/test_sqw_copy.m
index 5d59cca6c2..c200fd1067 100644
--- a/_test/test_sqw_class/test_sqw_copy.m
+++ b/_test/test_sqw_class/test_sqw_copy.m
@@ -28,19 +28,36 @@ function test_copy_returns_distinct_object(obj)
             sqw_copy = copy(sqw_obj);
             
             sqw_copy.main_header.title = 'test_copy';
-            sqw_copy = sqw_copy.change_header(struct([]));
+            assertFalse(equal_to_tol(sqw_copy.main_header, sqw_obj.main_header));
+            
+            % want to test expinfo/instruments and expinfo/detpar
+            % separately, so make 2 copies of expinfo
+            expinf1 = sqw_copy.experiment_info;
+            expinf2 = sqw_copy.experiment_info;
+
+            % all instruments changed with new name to make expinf different
+            for i=1:numel(expinf1.instruments), expinf1.instruments{i}.name = 'copy'; end
+            sqw_copy = sqw_copy.change_header(expinf1);
+            assertFalse(equal_to_tol(sqw_copy.experiment_info, sqw_obj.experiment_info));
+
+            % all detpar detectors changed with new azim value to make expinf different
+            for i=1:numel(expinf2.detector_arrays), expinf2.detector_arrays(i).azim = 0; end
+            sqw_copy = sqw_copy.change_header(expinf2);
+            assertFalse(equal_to_tol(sqw_copy.my_detpar(), sqw_obj.my_detpar()));
+            %{
+            % old version of detpar change with separate detpar left for
+            % reference to see what was intended
             dtp = sqw_copy.my_detpar();
             dtp.azim(1:10) = 0;
             sqw_copy = sqw_copy.change_detpar(dtp);
-            sqw_copy.data.dax = [2, 1];
-            sqw_copy.data.pix.signal = 1;
+            %}
             
             % changed data is not mirrored in initial
-            assertFalse(equal_to_tol(sqw_copy.main_header, sqw_obj.main_header));
-            assertFalse(equal_to_tol(sqw_copy.experiment_info, sqw_obj.experiment_info));
-            assertFalse(equal_to_tol(sqw_copy.my_detpar(), sqw_obj.my_detpar()));
+            sqw_copy.data.dax = [2, 1];
+            sqw_copy.data.pix.signal = 1;
             assertFalse(equal_to_tol(sqw_copy.data, sqw_obj.data));
             assertFalse(equal_to_tol(sqw_copy.data.pix, sqw_obj.data.pix));
+            
         end
         
         function test_copy_excluding_pix_returns_empty_pix_data(obj)
diff --git a/horace_core/algorithms/head_horace.m b/horace_core/algorithms/head_horace.m
index 1940852814..e6277c4117 100644
--- a/horace_core/algorithms/head_horace.m
+++ b/horace_core/algorithms/head_horace.m
@@ -108,6 +108,7 @@
                 [data.main_header,exper_block,data.detpar,data.data] = ...
                     loaders{i}.get_sqw('-legacy','-nopix','-verbatim');
                 data.header = exper_block.header;
+                data.detpar = exper_block.detpar;
             else
                 data         = loaders{i}.get_data('-verbatim','-nopix');
                 if isa(data,'data_sqw_dnd')
diff --git a/horace_core/sqw/@Experiment/Experiment.m b/horace_core/sqw/@Experiment/Experiment.m
index d083b6dcb1..4598e7d09a 100644
--- a/horace_core/sqw/@Experiment/Experiment.m
+++ b/horace_core/sqw/@Experiment/Experiment.m
@@ -3,7 +3,7 @@
 
     properties(Access=private)
         instruments_ = {}; %IX_inst.empty;
-        detector_arrays_ = []
+        detector_arrays_ = IX_detector_array.empty;
         samples_ = {}; % IX_samp.empty;
         expdata_ = IX_experiment();
     end
@@ -19,6 +19,7 @@
     properties(Dependent,Hidden)
         % property providing compatibility with old header interface
         header
+        detpar
     end
     properties(Constant,Access=private)
         fields_to_save_ = {'instruments','detector_arrays','samples','expdata'};
@@ -90,6 +91,10 @@
                     'Must give all of detector_array, instrument and sample or the structure representing them')
             end
         end
+        
+        function add_detector_array(obj,detarr)
+            obj.detector_arrays_(end+1) = detarr;
+        end
         %
         function oldhdrs = convert_to_old_headers(obj,header_num)
             % convert Experiment into the structure suitable to be
@@ -295,6 +300,15 @@
             head = [head{:}];
             head = rmfield(head,{'instrument','sample'});
         end
+        
+        function dp = get.detpar(obj)
+            if numel(obj.detector_arrays)>0
+                dp = obj.detector_arrays(1);
+                dp = dp.convert_to_old_detpar();
+            else
+                dp = struct();
+            end
+        end 
     end
     methods(Access=protected)
         %------------------------------------------------------------------
diff --git a/horace_core/sqw/@Experiment/private/build_from_old_headers_.m b/horace_core/sqw/@Experiment/private/build_from_old_headers_.m
index 75c1fae9c1..e8395919e5 100644
--- a/horace_core/sqw/@Experiment/private/build_from_old_headers_.m
+++ b/horace_core/sqw/@Experiment/private/build_from_old_headers_.m
@@ -106,7 +106,7 @@
 
 % Construct the new header Experiment object
 % update with expdata, which maybe should go in the Experiment constructor
-obj = Experiment([], instruments, samples);
+obj = Experiment(IX_detector_array.empty, instruments, samples);
 obj.expdata = expdata;
 
 end % function build_from_old_headers
diff --git a/horace_core/sqw/@rundatah/private/calc_sqw_.m b/horace_core/sqw/@rundatah/private/calc_sqw_.m
index a4491b19ab..b63612b7ea 100644
--- a/horace_core/sqw/@rundatah/private/calc_sqw_.m
+++ b/horace_core/sqw/@rundatah/private/calc_sqw_.m
@@ -138,7 +138,8 @@
 % ----------------------------------------------------------------------
 d.main_header=main_header;
 d.experiment_info=header;
-d.detpar=det0;
+d.experiment_info.detector_arrays(end+1) = IX_detector_array(det0);
+d.detpar=struct([]);
 d.data=data_sqw_dnd(sqw_datstr);
 d.runid_map = containers.Map(id,1);
 
@@ -193,7 +194,7 @@
 else
     sample = obj.sample;
 end
-header = Experiment([],obj.instrument,sample);
+header = Experiment(IX_detector_array.empty,instrument,sample);
 
 
 uoffset = [0;0;0;0];
diff --git a/horace_core/sqw/@sqw/spe.m b/horace_core/sqw/@sqw/spe.m
index ed582b1c4b..748788e608 100644
--- a/horace_core/sqw/@sqw/spe.m
+++ b/horace_core/sqw/@sqw/spe.m
@@ -51,7 +51,7 @@
 else
     ne=numel(w.experiment_info.en)-1;    % number of energy bins
 end
-ndet0=numel(w.detpar.group);% number of detectors
+ndet0=numel(w.experiment_info.detector_arrays(1).id);% number of detectors
 
 tmp=w.data.pix.get_data({'detector_idx', 'energy_idx', 'signal', 'variance'})';
 tmp=sortrows(tmp,[1,2]);    % order by detector group number, then energy
@@ -63,7 +63,7 @@
 end
 
 % Get the indexing of detector group in the detector information
-[lia,ind]=ismember(group,w.detpar.group);
+[lia,ind]=ismember(group,w.experiment_info.detector_arrays(1).id);
 
 signal=NaN(ne,ndet0);
 err=zeros(ne,ndet0);
diff --git a/horace_core/sqw/@sqw/sqw.m b/horace_core/sqw/@sqw/sqw.m
index 24d0460f86..3f8526e263 100644
--- a/horace_core/sqw/@sqw/sqw.m
+++ b/horace_core/sqw/@sqw/sqw.m
@@ -97,7 +97,11 @@
         wout = noisify(w,varargin);
 
         function dtp = my_detpar(obj)
-            dtp = obj.detpar_x;
+            if ~isempty(obj.detpar_x)
+                dtp = obj.detpar_x;
+            elseif ~isempty(obj.experiment_info.detector_arrays)
+                dtp = obj.experiment_info.detector_arrays(1).convert_to_old_detpar();
+            end
         end
 
         function obj = change_detpar(obj,dtp)
@@ -258,8 +262,19 @@
 
     methods(Static)
         function obj = loadobj(S)
-            % boilerplate loadobj method, calling generic method of
+            % modified boilerplate loadobj method, calling generic method of
             % saveable class
+            % the generic code is preceded by the conversion of old-style
+            % detpar info into the new detector arrays. This allows loading
+            % of .mat files without converting them on disk..
+            if isfield(S,'experiment_info') && isfield(S,'detpar')
+                if ~isempty(S.detpar) && isempty(S.experiment_info.detector_arrays)
+                    dd = IX_detector_array(S.detpar);
+                    S.experiment_info.detector_arrays = IX_detector_array.empty;
+                    S.experiment_info.detector_arrays(end+1) = dd;
+                    S.detpar = struct([]);
+                end
+            end
             obj = sqw();
             obj = loadobj@serializable(S,obj);
         end
@@ -275,6 +290,8 @@
 
         % take the inputs for a cut and return them in a standard form
         [proj, pbin, opt,args] = process_and_validate_cut_inputs(obj, ndims_source, return_cut, varargin);
+        
+        
         function obj = from_old_struct(obj,S)
             % restore object from the old structure, which describes the
             % previous version of the object.
@@ -307,6 +324,18 @@
                         end
                         ss = rmfield(ss,'header');
                     end
+                    
+                    % assuming that detpar exists and is not empty, convert
+                    % its contents into a detector array. There are test
+                    % datasets with empty detpar and those should be left
+                    % unconverted, they seem to be testing other items in
+                    % the sqw and ignoring the detpar.
+                    if isfield(ss,'experiment_info') && isfield(ss,'detpar') && ~isempty(ss.detpar)
+                        dd = IX_detector_array(ss.detpar);
+                        ss.experiment_info.detector_arrays(end+1) = dd;
+                        ss = rmfield(ss,'detpar');
+                    end
+                    
                     if isfield(ss,'data_')
                         ss.data = ss.data_;
                         ss = rmfield(ss,'data_');
diff --git a/horace_core/sqw/file_io/@dnd_binfile_common/dnd_binfile_common.m b/horace_core/sqw/file_io/@dnd_binfile_common/dnd_binfile_common.m
index cdf89daab1..21e39d184b 100644
--- a/horace_core/sqw/file_io/@dnd_binfile_common/dnd_binfile_common.m
+++ b/horace_core/sqw/file_io/@dnd_binfile_common/dnd_binfile_common.m
@@ -256,7 +256,8 @@ function check_obj_initated_properly(obj)
         function check_error_report_fail_(obj,pos_mess)
             % check if error occured during io operation and throw if it does happened
             [mess,res] = ferror(obj.file_id_);
-            if res ~= 0; error('SQW_FILE_IO:io_error',...
+            if res ~= 0
+                error('Horace:SQW_FILE_IO:io_error',...
                     '%s -- Reason: %s',pos_mess,mess);
             end
         end
diff --git a/horace_core/sqw/file_io/@sqw_binfile_common/get_sqw.m b/horace_core/sqw/file_io/@sqw_binfile_common/get_sqw.m
index 63a7e48724..459c3f9b1f 100644
--- a/horace_core/sqw/file_io/@sqw_binfile_common/get_sqw.m
+++ b/horace_core/sqw/file_io/@sqw_binfile_common/get_sqw.m
@@ -75,7 +75,14 @@
 % Get detector parameters
 % -----------------------
 if ~(opts.head||opts.his)
-    sqw_struc.detpar = obj.get_detpar();
+    dp = obj.get_detpar();
+    if isa(headers, 'Experiment')
+        dp = obj.get_detpar();
+        headers.detector_arrays(end+1) = IX_detector_array(dp);
+        sqw_struc.detpar = struct([]);
+    else
+        sqw_struc.detpar = obj.get_detpar();
+    end
 end
 
 % Get data