Sort the indices to get consecutive columns adjacent to each other:
idx = sort([86:90:1800,87:90:1800,88:90:1800,89:90:1800,90:90:1800])
eta(:,idx)
Or without having to write that long list:
>> vec = 86:90;
>> C = arrayfun(@(b)b:90:1800, vec, 'Uni',0);
>> idx = sort([C{:}])
or generate them in the correct order in the first place:
>> vec = 86:90;
>> idx = bsxfun(@plus,vec(:),0:90:1800-90);
>> idx = idx(:).'
