This is how Histogram Aid looks like when APT is in DSLR mode (Canon or Nikon). It shows the histograms of R, G and B channels as well as the combined L (luminance) channel. The histogram area is divided in 5 logical sectors. For astro imaging is good to inspect the all the channels and to select exposure / ISO combination that places the peaks in the second or/and third sector. This gives images that after stacking allow easier post-processing. If the peaks are in the left most sector this means that the image is under exposed. If the peaks are in the last two sectors the image is over exposed. Both case will lead to information lost and less options for post-processing. The right exposure / ISO combination depends on various factors like sky glow, object size, object brightens, sensor sensitivity and etc.
APT allows defining separate black/white points for the different R, G and B channels (by highlighting the RGB button) or for the L channel. The RGB mode is very useful when modified DSLR camera is used because gives possibility to make on-screen color balance and to get idea how the post-processed image will look like. It is recommended to use this mode even to un-modified cameras because it takes the most of the signal per every channel.
The "Auto Stretching" is a powerful feature. It makes automatic selection of the optimal black and white points (per L or per RGB) and by this boosting the weak signal. Here is an un-stretched image (black point in very left, white point in very right) taken with modified Canon EOS camera.
This is the same image with "Auto Stretching" feature enabled in RGB mode. The stretching is made per each channel separately in order to get the best from every one. As you can see the object can be seen much better and the image has better color balance.
Channels
Shows the histograms of per R, G and B channels and for the combined L (luminance) channel.
When is active (high-lighted) it allows defining separate black and white points per R, G and B plans. When not is active (high-lighted) the black and white points are only for the L channel.
It also defines if the Auto Stretching works per L or per RGB.
When is active (high-lighted) it makes automatic selection of the black and white points. If the RGB mode is enabled the black/white points are determined per every channel. If RGB mode is disabled black/white points are determined per L channel only.
When is active (high-lighted) the histogram will be calculated for the LiveView stream. Also it applies the Auto Stretching mode or the manually defined black/white points (per L or RGB depending on the RGB mode) on the stream.
Note that it can affect the frames-per-second rate of the LiveView stream when used on slower computer.
Shows where the black point is set. When Auto Stretching is disabled you can set the point manually using the following combinations. Shift+Double Left click set the black point. Shift+Double Right click clears the black point.
Shows where the white point is set. When Auto Stretching is disabled you can set the point manually using the following combinations. Double Left click set the white point. Double Right click clears the white point.
The Histogram is accessible from 
Related videos: 
