Basic LRGB workflow part three
Before stretching the Luminance window there is an optional step of deconvolution. Whether you decide to carry out deconvolution will depend on your data and the target. Please see the separate tutorial here.
Stretching the Luminance image
The stretching of the Luminance is done with the Histogram Transformation tool. There other other stretching options - Masked Stretch and Exponential Transformation. However I prefer the bulk of the basic stretching to be done with an iterative approach using the Ht tool.
I do use Exponential Transformation at a later stage if needed when carrying out enhancements to the image in its final stages.
First, deactivate the STF stretch on the menu bar. Click the tick on the HT tool to track progress then the preview window. Enlargen this window by dragging the bottom corner.
Next, drag the righhand slider across left towards the righhand edge of the histogram. Click the square apply button, this applies the histogram stretch to the image window.
Then reset the HT tool by clicking the diagonal cross icon on the bottom right of the HT tool window.
This step repeats the process above except do not move the slider as far to the left: you want to avoid moving it into or underneath the histogram at the moment.
Do not forget to rest the HHT tool after you have applied the adjustment.
As a result of this iterative adjustment, the lefthand edge of the histogram is beginning to move rightwards.
We will want to ensure that we maintain some distance from this lefthand edge but to not let it expand excessively.
You will see that the righthand slider is now beginning to move under the outer edge of the histogram as we succesively reveal more detail.
If necessary move the lefthand slider rightwards until the Shadows reads a few pixels or back off left until the reading is back to zero.
After the fifth stretch had been applied I assessed that the Luminance was sufficiently stretched and I closed the preview window and minimised the HT tool.
The temptation when you are processing is to try and eek out every last faint detail of the target. With time and practise you will learn at what stage the image has achieved its optimum balance of stretch - I use web references to help along with a mental reminder that not every astronomical feature across the frame has the same brightness. A range of contrasts and brightnesses is natural.