Rough grinding was carried on today, as per schedule. Our mirror grinding room is a bit small and so it creates a little problem while we were 「walking around the table」. Anyway we managed to finish the 5 hour of rough grinding, 1 hour running ahead of the original schedule.
Important data arrived – after totally 6 hours of rough grinding we had generated a f/11 curve on the 24 inch mirror which means that 30.8 cubic inch of glass had been removed away. Simple calculation shows that the glass removal rate is about 5 cubic. inch per hour. To obtain a f/3.5 curve 97 cubic inch of glass has to be removed, meaning that we require totally about 20 hours of rough grinding time. It sound quite logic and fits my intuition.
5 lbs of #40 carbo was consumed in 4 hours. The carbo consuming rate is about 1.3 lb per hour. Hence to produce the f/3.5 curve at least 26 lbs #40 carbo is needed, a cost of HK$ 520, 12% the total construction cost of the whole telescope. It also sounds right.
We have to make such calculation to monitor how much we are going spent and to allocate the right time for the student』s shift.
Another 4 hours of roughing is scheduled tomorrow and hopefully a f/7 curve will be arrived at after the roughing, as per calculation.
Below is the formula used to calculate the volume of glass has to be removed for a given f/number of any mirror:-
Volume of glass to be removed = (3.142)(r)^3/(16 x f/no.)
Chan Yuk Lun