ED glass blanks

Does anyone know if there is a diameter or thickness limit to pouring ED glass? I've heard the blanks can't be larger than 8 inches. Thoughts? =Doug Bullis, Santa Fe

>>Does anyone know if there is a diameter or thickness limit to pouring ED glass>>

ED glass is not really "poured" in the old fashioned sense. Ohara is the principal source of high quality ED glass that can be purchased at this time for amateur telescopes. All the other sources are really only set up for high volume production for the camera industry and will not respond to low quantity purchase requests.

The Ohara ED material is made in a continuous furnace. The glass is heated inside the furnace using gas and electric power. There is a spigot at the bottom from which flows a continuous stream of glass onto a slow moving conveyor, a couple hundred ft long. The conveyor has sides of about 180mm width, which constricts the molten glass to that size as it slowly moves along. The glass thickness is regulated by the conveyor speed and spigot flow rate.

The conveyor carries the long continuous strip through a coarse annealing oven which brings the temperature down from 2000 degrees to room temperature. At the end of the process, a worker marks and cuts the strips into rectangular pieces, approximately 180mm wide and 250 - 300mm long. The strips are on the order of 40mm thick. From this the customer can order round plano-plano blanks made, tested for high homogeneity and guaranteed to a certain quality level. The strips will cost between $2000 to $3000 each depending on weight and will yield just one blank for a 175mm ED lens, or a few blanks suitable for 130-140mm diameter ED lenses. The blanks must still be fine annealed in order to work for a real astronomical lens, and that costs further dough.

Alternatively, if one wishes to purchase untested blanks at rock bottom prices, one can cut the strip up into odd size equal weight pieces and re-melt them into roughly shaped pressings for 100mm thru 180mm diameter lens shaped near-sized blanks suitable for grinding and polishing. Since these pressings are not plano-plano, but already have curves on them, and since they are re-pressed, Ohara cannot test them for homogeneity and cannot guarantee the high quality levels needed for astronomical objectives. The customer then takes the chance that these pressings will be high enough quality to allow diffraction limited performance. The price for a 130mm untested pressing will be a couple hundred, versus well over one thousand for the tested pieces.

It is possible to buy pressings and use them in the coarse annealed state, and perhaps get suitable results, but more often then not they will have too high internal stresses. Normally one would order them fine or precision annealed and at least have very low internal stress. That adds to the cost of course, but annealing does not change the internal homogeneity, and that will always be an unknown. How bad can it be? It can be so bad that the entire batch of 50, 100, 250 or 500 blanks that were delivered show 2 to 5 waves of astigmatism and must be thrown into the landfill. On another day they may all be good enough or almost perfect. The chance is one the lens maker always has to take and live with the consequences. The glass manufacturer never takes the glass back, nor will they ever give any refunds.

Have fun,

Rolando