You can use various noise reduction programs after the fact, but they will probably not do as good a job subtracting the specific noise signature of your camera as the built in correction. Here is an idea which might save you some time over the long run:
1) Turn off the noise subtraction feature in the camera
2) Take a 45 minute dark exposure (lens cap on) in the field, after the camera has reached temperature equilibrium.
3) Take a 45 minute real image as usual.
4) Subtract the dark image from the real image later in Photoshop.
5) Compare to automatic camera method, to see if noise reduction is just as good.
So far, you have gained nothing, except an understanding of what the dark image noise looks like and how the correction works. However, if you take more than one 45 minute exposure per night, you will save time, since you can use one dark image to correct all real images. You might even be able to use the same dark image days or weeks later, at least if temperature is similar.
Also, this technique might be useful for cameras which do not have the automatic subtraction feature.