StarTools

Here are some notes relating to using this module. It is not the only way to use the module - please experiment and provide feedback.
Please let me know if anyone sees any errors or has any additional advice they think helpful so this post can be improved.
I will update this post as needed.

Lens module
Purpose:

• To correct common lens distortions such as chromatic aberration and coma.

Description:
For a general overview see Lens: Distortion Correction and Field Flattening
The Lens module in StarTools corrects for "simple" radial distortion, which can alleviate some of the worst effects of coma.
It also allows for re-alignment of the Red, Green and Blue colour channels to remove some of the worst effects of transverse (lateral) chromatic aberration. Axial (longitudinal) chromatic aberration can be controlled using the Filter or Decon modules.
Useful Sources
The Unofficial guide is also a good source of help. It relates to version 1.3.5 so there may have been some changes. The notes below relate to StarTools version 1.4.

When to use:

• After the Bin module (if used).
• Before the Crop module - the Lens module requires the centre of the image to be the same as when the image was taken.
• Stacking artefacts can be asymmetrical and require some re-centering of the image. This is easier to do in the Lens module than by using the Crop module.

Example Workflow (v1.5):
AutoDev-{Band/Lens}-Bin-Crop-Wipe-AutoDev(or Develop)-{As needed: Decon/Sharp/Contrast/HDR/Flux/Life}-Color-{Filter}-Denoise-{If needed: Layer/Magic/Heal/Repair/Synth}
Key: {...} optional modules

Example Workflow (v1.6):
AutoDev-{Band/Lens}-Bin-Crop-Wipe-AutoDev (or Develop)-{Contrast/HDR/Sharp/Decon/Flux/Life}-Color-{Entropy/Filter}-Denoise (or Denoise 2)-{Layer/Shrink/Heal/Repair/Synth/Stereo 3D}
Key: {...} optional modules

Example Workflow (v1.7):
{Compose}-AutoDev-{Lens}-Bin-Crop-Wipe-AutoDev (or FilmDev)-{Contrast/HDR/Sharp/Decon/Flux}-Color-{Shrink/Filter/Entropy/SuperStr}-Track/NR-{Layer/Heal/Repair/Synth/Stereo 3D}
Key: {...} optional modules

Example Workflow (v1.8):
{Compose}-AutoDev-{Lens}-Bin-Crop-Wipe-AutoDev (or FilmDev)-{Contrast/HDR/Sharp/SVDecon}-Color-{Shrink/Filter/Entropy/SuperStr/NBAccent/}-Track/NR(Unified-Denoise)-{Flux/Repair/Heal/Layer/Synth/Stereo 3D}
Key: {...} optional modules

Method:
This is a way of using the module which should give good results in most cases:
To correct coma:

1. Increase the Curvature Linked setting until the stars in the corners are circular.
2. Adjust Center X and Center Y to correct if the centre of the lens is slightly off-centre.

To correct chromatic aberrations:

1. Adjust the Red Shift X and Red Shift Y until the red fringe of the star has moved into the stars centre.
2. Adjust the Blue Shift X and Blue Shift Y until the blue fringe of the star has moved into the stars centre.

What result to look for:

When correcting coma:

• Stars at the edges and at the centre are round - any elongation has been compensated for.
• Diffraction spikes will, unfortunately, now be curved.

When correcting chromatic aberration:

• Star red and blue fringes are reduced or eliminated.

Ways of getting better results:
To remove coma:

• Use a coma corrector/field flattener.
• Do this sort of adjustment to the subs before stacking if you can - as each sub will be slightly rotated.

To remove chromatic aberration:

• Use an Apochromatic refractor.
• Use a reflector.
• For any axial chromatic aberration - use the Decon module or the Filter module - which should remove any remaining blue fringes.

After Use:

• Do Crop if required - and continue with workflow.

Special Techniques:
Easier Colour Alignment:
Sometimes it is difficult to adjust for R,G,B channel misalignments because you can't see the colours early in in the processing.
To help - you can do some processing to bring out the colours, apply the lens module, then use Restore to go back and process as normal.
Restore will keep the changes made by the Lens module.

• AutoDev module - to see what you're doing
• Wipe module - to see the color changes
• Color module - to bring out the colours
• Lens module - to correct for the errors between the colour channels.
• Press Restore button - select 'Original' option - this reverts to the original Linear, Binned and Cropped image - but keeping the changes made by the Lens module.
• Process your corrected dataset using the normal workflow - from Wipe onward.

Description of Controls:
Auto Crop

• Set if the Lens module should automatically crop the image after correction.

Curvature Linked
This parameter defines if the field curvature of the red and the blue channel should be adjusted the same.

• For coma correction/field flattening both red and blue curvature should be the same.
• Changing this setting changes the Curvature Red and Curvature Blue to the same values.
• Default is 100.00%. Range is -100.00% to +300.00%.
• Change this value until the coma in the corners has been compensated for. Values of 150% are common.

Curvature Red
Sets the field curvature of the red channel independently of the blue channel.

• Default is 100.00%. Range is -100.00% to +300.00%

Curvature Blue
Sets the field curvature of the blue channel independently of the red channel.

• Default is 100.00%. Range is -100.00% to +300.00%

Center X
Sets the X-axis position of the centre of the distortion.

• Default is 0.0 pixels.
• Change this value if the image has been cropped unevenly or if the optical axis is offset.

Center Y
Sets the Y-axis position of the centre of the distortion

• Default is 0.0 pixels
• Change this value if the image has been cropped unevenly or if the optical axis is offset.

Red Shift X
Shifts the red channel along the X axis.

• Default is 0.0 pixels. Range -50.0 pixels to +50.0 pixels.
• Adjust this parameter in conjunction with Red Shift Y until the red fringe around the stars has been removed by moving it towards the centre of the star.

Red Shift Y
Shifts the red channel along the Y axis.

• Default is 0.0 pixels. Range -50.0 pixels to +50.0 pixels.
• Adjust this parameter in conjunction with Red Shift X until the red fringe around the stars has been removed by moving it towards the centre of the star.

Blue Shift X
Shifts the blue channel along the X axis.

• Default is 0.0 pixels. Range -50.0 pixels to +50.0 pixels.
• Adjust this parameter in conjunction with Blue Shift Y until the blue fringe around the stars has been removed by moving it towards the centre of the star.

Blue Shift Y
Shifts the blue channel along the Y axis.

• Default is 0.0 pixels.
• Adjust this parameter in conjunction with Blue Shift X until the blue fringe around the stars has been removed by moving it towards the centre of the star.

Background Notes
Optical Distortion

• The Lens module in StarTools uses Brown's distortion model to correct for "simple" radial distortion, which can alleviate some of the worst effects of coma. Ref: Wikipedia article on distortion in optics.
• Image distortion may be off-centre for a number of reasons, such as the camera not sitting perfectly flush/perpendicular to the imaging plane, or movement between sub-frames affecting the stacking process.

Chromatic aberration
For a general introduction to the subject see the article: Chromatic aberration.
This can be split into two types:

• Transverse or Lateral chromatic aberration - colorshifts due to the different channels not quite lining up - this is the type that the Lens module addresses. With transverse chromatic aberration the colour fringes are on one axis.
• Axial or Longitudinal chromatic aberration - caused by different wavelengths/colours coming into focus at slightly different distances - this can be mitigated using the Decon or Filter modules. With axial chromatic aberration the colour fringes are all around the object.
• For further information see this article on Chromatic Aberration.