Susan:
I don't have a lot of experience with this unit but I do have tons of experience using Nuarks fliptops and contact units. That little light probe in the back should have a phillips head screw in the middle. If you loosen it up ever so slightly you can turn it so the aperature changes. If the time is too slow open it, and vice versa. There is no harm in doing this as that is it's function. Nuark equipment is functional and sometimes clumsy. They also have a long history of changing suppliers for things like daughter boards and not telling everybody in their employ. You should be able to do what your looking to with it though.