The white point is usually fixed already by the brightest stars in the image, and if daylight color balance has been used, the white balance of the image will be correct and the stars will have accurate colors. Neutralizing the sky background will take care of any undesirable color cast that the image may have. Setting the black point will maximize the dynamic range of the image.

At this point of adjustments, the image may look fine, and no further adjustments to the brightness or contrast may be necessary.

In some cases the overall image may still be too dark though. In this case we can adjust the brightness of the mid-tones with either a levels or curves adjustment.

Curves Mid-Tone Adjustment

Open the Curves dialog box:
Image > Adjustments > Curves.

Mouse Over

Hold your mouse cursor over the image to see a curves adjustment.

Click on the middle of the straight-line "curve" and drag the point upwards so that the input value of 128 is increased to 157 or so. The middle of the curve represents the midtones of the image. Dragging the curve upwards will lighten these midtones. Dragging it downwards will darken the midtones. Adjust to taste.

Levels Mid-Tone Adjustment

To adjust lighten the mid-tones of the image with a levels adjustment, go to Image > Adjustments > Levels.

Mouse Over

Hold your mouse cursor over the image to see a levels adjustment.

Click on the middle slider and drag the point to the left so that the input value of 1.0 is increased to about 1.15 The middle slider represents the midtones of the image. Dragging the slider to the left will lighten these midtones. Dragging it to the right will darken the midtones. Adjust to taste.

Mouse Over

Hold your mouse cursor over the image to see a comparison of the image before and after a midtones Curves adjustment lightened the midtones.

Curves and levels mid-tone adjustments are the preferred method of adjusting the brightness of an image because it leaves the black and white points unaffected.