The Hansen Planetarium is offering two-for-the-price-of-one admission to the Saturday morning (11 a.m.) performance of "Dr. Zap Explains it All." Both Dr. Zap and the Zap `90 show end at the end of October, explains spokesperson Doug Lowe. Since the Dr. Zap show is written for children, planetarium officials decided to offer the special rates to encourage families to see a Zap show while there is still time.