This photo is of the 7 sisters constellation. Not sure if astrophotography belongs in "nature and wildlife" as that seems to refer to animals and such.

Hope you guys find this interesting. How can I make this image sharper? Or am I stuck with the blur due to the atmospheric pressure/temp changes?

Heres the specs:

Shutter Speed: 4"
Aperture: F/5.6
Focal Length: 400mm(x) 1.6 = 640mm
ISO: 800
Shutter Lockup On
Noise Reduction Off



The tripod was sturdy and held down with a 25lb weight. I left it for 2 minutes after opening the mirror, before I opened the shutter curtain, to reduce even more vibration.

Hopefully i can get some tips on this. Thanks!