Lens Module Use

Notes from users, documentation addendums.

Lens Module Use

Postby Guy » Wed Jan 25, 2017 11:13 am

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 chromatic aberration.

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:
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

Method:
This is a way of using the module which should give good results in most cases:
To correct Coma:
  1. Increase Curvature Linked setting until the stars in the corners are circular.
  2. Adjust Center X and Center Y to adjust 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 edges and at 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.
Description of Controls:
Auto Crop
  • Set if Lens module should automatically crop the image after correction
Curvature Linked
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 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 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 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 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.
Guy
 
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