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Nikon NEF RAW data minimum value clamp

When I use DCRAW to extract the RAW pixel values from a NEF file made by my D5300 I notice that there seems to be a minimum value clamp = 588 applied to the pixel values. I am an astrophotographer so generating master BIAS and DARK frames is part of my image calibration method. Both BIAS and DARK frames are images with the lens cap in place so that no light enters the camera. I believe that Nikon uses a Darkness value of 600 in their NEF files. The 588 minimum value clamp causes a huge number of pixel values in my individual DARK frames to be clamped at 588. I believe this limits my ability to generate useful master DARKs which are median or averaged versions of 10 or 20 DARK frames.

Is the 588 minimum value clamp imposed on the pixel values when the camera writes the NEF file or is the 588 clamp imposed by DCRAW due to NEF file standards? I would like to be able to remove this minimum value clamp or, at least, lower it to 0. Can you enlighten me?

Peter W