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Following on from Steffen, stopping down a terrestrial lens will increase your depth of field making objects nearer and further from the actual focus point also appear to be in-focus. For astro work we are always focussed at infinity. No need for and increase in depth of field.
For astro work we want the widest aperture available, as it allows us to gather more light in a shorter period of time. This minimises issues with tracking/guiding due to shorter exposure times.
Additionally it allows us to better capture deep sky targets such as nebulae without over-exposing the neighbouring stars.
Can be a difficult concept to grasp, but extended objects such as nebulae follow the standard photographic laws of capture, i.e. longer exposure increases the brightness of the image captured.
Stars are different. They are point sources of extremely bright light and will quickly saturate your detector even with only very short exposures. Longer exposures (necessary to capture extended objects) cause the pin-point stars to bloat and bleed and will reveal the imperfections in your optical train very quickly.
At another level, the F number of your astro lens is an indication of the angle of view that can be achieved through it. For two say 8 inch diameter scopes, and F4 will allow a much wider field of view than an F10. If you are looking to image bright objects such as planets ( which are also small) then an F10 would be more appropriate than an F4 lens, but for capturing the nebulosity of say the Rosette nebula, then an F4 would be better.
Hope This Helps
Brian
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