ASTAP ("Astrometric STAcking Program") is a FREE, competent, actively developed stacker, available for all platforms. Stacking and saving your data with these settings is essential to getting good results from StarTools.
Of particular importance is switching off "Auto levels" in the Stack method tab. Any sort of color calibration should be avoided. Once turned off, you will be able to import your ASTAP-exported dataset into StarTools with the second option.
Besides calibration and stacking, do not perform any further operations on the resulting dataset in ASTAP.
If stacking multiple mono datasets for use in a composite, make sure to use one set's finished stack as a reference to stack the others with; StarTools's Compose module requires every dataset to be the same dimensions. Aligning remaining channels against an initial channel during stacking is particularly important to ensure consistency of point spread functions across channels; do not align finished stacks against each other after stacking.
StarTools does more than just help you make "pretty pictures".
StarTools' Detail-aware Wavelet Sharpening allows you to bring out faint structural detail in your images.
As of 1.7, enhanced mouse controls are implemented; Zoom in.
Even the way StarTools displays and scales images, has been created specifically for astrophotography.
Understanding how AutoDev works is key to getting superior results with StarTools.
You can convert everything you see to a format you find convenient. Give it a try!