Index was outside the bounds of the array c#, morse converter

Question

I was trying to build a Morse converter, the following is a part of my code of c#, but when I run it it tells me index out of the bounds, could anyone fix it? I'm new for programming:)

private void BTNconvert_Click(object sender, EventArgs e)
{           
    string input = TBinput.Text;
    string[] output=new string[input.Length];
    for (int index=0;index<input.Length;index++)
    {
        index=input.IndexOf('a',index);
        output[index]=".-";                                               
    }

    for (int index = 0; index < input.Length; index++)
    {
        index = input.IndexOf('b', index);
        output[index] = "-...";
    }

    LBcodes.Text = string.Join(" ",output); 

Answer 1

private void BTNconvert_Click(object sender, EventArgs e)
{      
   int index;     
    string input = TBinput.Text;
    string[] output=new string[input.Length];
       index = -1;
        do{
             index=input.IndexOf('a',index+1);
             if(index==-1)break;
             output[index]=".-";     
        }while(true);
        index = -1;
        do{
             index=input.IndexOf('b',index+1);
             if(index==-1)break;
             output[index]="-...";     
        }while(true);
    }

    LBcodes.Text = string.Join(" ",output);

also if you are going to make these loops for all of the characters I would recommand you to do this:

private void BTNconvert_Click(object sender, EventArgs e)
{  

   int index;
   char[] source1 = {'a','b','c',....,'z'} //replace ... with characters
   string[] source2 = {".-","-...",....}   //replace .... with Morse equivalents
    string input = TBinput.Text;
    string[] output=new string[input.Length];
       for(int i=0;i<26;i++){
       index = -1;
        do{
             index=input.IndexOf(source1[i],index+1);
             if(index==-1)break;
             output[index]=source2[i];     
        }while(true);
        }
    }

    LBcodes.Text = string.Join(" ",output);