After setting the initial gamma levels by channel, bring up the Levels tool again, and adjust the gamma (middle) and black point (left) sliders for the RGB channel to better visualize the data.

Keep in mind that while adjusting the gamma slider is non-destructive, adjusting the black point slider will irreversibly change the data - well, OK, you can always "undo" adjustments made in Photoshop, but the point is, adjusting the black point clips data, but adjusting the gamma does not clip data.

Adjusting the white point (right) slider also clips the data, and in fact, adjusting the white point on astrophotography images is seldom if ever necessary. The only effect of lowering the white point is to truncate pixel values that were not completely saturated in the original linear image. In just about all cases this is not desirable. Generally, you should avoid making any adjustments at all to the white point slider.

This image is beginning to resemble the Swan Nebula (M17).