Don't discount the mail order stores. I've used Eagle Optics, their salespeople have great expertise (they often own the equipment) and they also have a useful booklet which they will send free of charge. Eagle Optics 2120 West Greenview Dr. Middleton, Wi 53562-2547 tele: 800-289-1132 technical assistance: 608-836-6568 Good Luck, Gail Frantz MD