Old search index available while indexing?

Is it possible to let the old search index be available while lucene reindexes.
My collection is quite large and it takes about 11 hours to reindex every time. I update the search index only once per week (at night) but I wonder if it wouldn't be possible to still use the old index during this process?
My collection is quite large and it takes about 11 hours to reindex every time. I update the search index only once per week (at night) but I wonder if it wouldn't be possible to still use the old index during this process?