This letter was asking question on using telescope mirrors made of cast aluminium or other light metal, which could be cheap and easy to make and also what kind and size of grating, its makers and price to building a solar observatory.

The letter is from C. V. Ramam, Raman Research Institute to M. K. Vainu Bappu, Nizamiah Observatory asking about using telescope mirrors made of cast aluminium and about diffraction grating as a preliminary to building solar observatory.