The best f-ratio for astrophotography depends on your preferences and the characteristics of the telescope and camera. A lower f-ratio, or faster f-number, is better for astrophotography because it results in a wider field of view and a brighter image. This can be especially useful for capturing large objects such as nebulae and galaxies, which may require longer exposure times to capture sufficient light. Generally, a focal ratio of around f/5 to f/8 is a good choice for most types of astrophotography, as it provides a wide field of view and a relatively bright image while still allowing for some detail and resolution.