London — Ancient scientific instruments wouldn't be every woman's idea of a fascinating collection. But for Harriet Wynter, a London antiques dealer, they have provided a rewarding career and made her a name as an expert on such items as 18th-century Persian astrolabes, 19th-century English barometers and Italian telescopes, and 17th-century German sun and moon dials.
Sitting in her booth at the Grosvenor House Antiques Fair here, surrounded by a collection of what she terms ''scientific instruments and curiosities,'' she explains that she sells anything connected with astronomy, navigation, surveying , and optics.
That includes books on the history of science and technology - and such items as sextants, quadrants, drawing instruments, compasses, ship portraits, and sea charts.
Mrs. Wynter claims to be as surprised as anyone at the turn of events that made her a specialist in old scientific instruments. After a divorce, and with two small children to support, she opened a small shop in 1965 to sell decorative things, including pictures and porcelains.
One day, a customer who was a publisher asked her to research and write a book for him on porcelain. She accepted, although she had never written before. The task took eight years and proved her dogged determination to teach herself what she needed to know and to complete a difficult assignment. It also set her own compass on a course of lifetime learning that shows no signs of abating.
''Along the way,'' she explains, ''I got exposed to many other things as well , such as social history. Through social history, I became interested in people and their tools, and from tools it was only a jump to early scientific instruments. Then one day I bought two sextants and knew I had found my business. I promptly bought books on how to teach yourself physics and navigation. I went to navigation school and simply never stopped digging.
''I'm glad, though, that I'm dealing in antiques,'' she confesses. ''I can understand science up to 1800 or so, because it was rather basic. After that, it gets too technical for me. Right now, I am more and more intrigued by 16 th-century things.''
Mrs. Wynter became the first dealer to show only scientific instruments at the Grosvenor House Fair. In 1975 she co-authored an authoritive book on the subject (now out of print) for collectors. In 1980, with her reputation well established, she closed her shop in Chelsea to operate by appointment from her home at 50 Redcliffe Road in London and to sell by mail and through antique shows in England and the United States.
Mrs. Wynter will show at the Theta Charity Show in Houston Sept. 21-25 and will be at the Burlington House Fair in London Oct. 19-29. Half of her business is now done in the US, where she finds people are fascinated with objects having to do with science.
''Scientific instruments,'' she says, ''have a marvelous sculptural quality, a structure and strength that comes from being designed for use.''
Sundials going back to the 16th century are very popular with collectors, she says, because they represent the highest quality of complicated and elaborate workmanship. She contends that some of the most important collections in the world are of sundials. Her stock includes a few early 18th-century sundials priced around (STR)2,000 (about $3,000). Increasingly, she finds, even dealers must pay enormous prices for choice items.
Mrs. Wynter may be an expert, but she keeps her explanations simple. If she really wants to disarm someone, she explains that at heart she is a housewife who loves collecting jelly molds, as well as a few old telescopes and things.