Decon Module Use (v1.5)

[Guy]

Here are some notes relating to using this module. It is not the only way to use the module and experimentation is encouraged.
Please let me know if anyone sees any errors or has any additional advice they think helpful.
I will update this post as needed.
To see a full alphabetical list of module topics click here

Decon Module

Purpose:

• The Decon module tries to reverse the effects that atmospheric turbulence has on the data. It allows recovery of detail in seeing-limited data sets that were affected by atmospheric turbulence.

Description:
For a general overview see Deconvolution: Detail Recovery from Seeing-Limited Data
The Decon module is noise-aware and is able to generate its own de-ringing mask. De-ringing will still try to coalesce singularities.
In some implementations deconvolution is used to correct bad collimation or other optical defects. However, this implementation is only designed to correct atmospheric seeing-related issues. Other modules deal with the optical effects.
The Decon module incorporates a regularization algorithm that automatically finds the optimum balance between noise and detail and allows you to control this trade-off.

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.
The processing tutorial video M8 in Color with modest data describes the Decon module between 4m45s and 5m49s.
The Hangout discussion of StarTools with Ivo discusses the Decon module between about 0h39m and 0h54m.

When to use:

• The Decon module should be used just after the final global stretch (Develop or AutoDev).
• Other modules, such as Sharp and HDR should be used after it as they can make use of the detail the Decon module reveals.
• The mask that the Decon module creates can be re-used in the Sharp module to stop it causing stars to bloat further.
• If there is no oversampling, or if there is a lot of noise, the benefit of this module will be limited.
• Use only once.

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:

1. Select 'Generate mask automatically' to create an inverse star mask - or use AutoMask preset to create one - this masks out overexposed stars which cause ringing.
2. Select Image Type - Deep Space or Lunar/Planetary.
3. Select a preview area rectangle in the image to see the effect of changes - this speeds up anaysis. Choose a bright, detailed and noise-free area.
4. Zoom in so you can see the detail.
5. Adjust the Radius setting - until just before the smaller stars start showing signs of ringing.
6. Change the Iterations setting to see if increasing it gets better results.
7. Toggle top "Pre Tweak/Post Tweak" button to see effect of last adjustment if needed (StarTools v1.5+).
8. Normally other default values work well - e.g. for Regularisation - but you can experiment if you want.
9. When done, select 'All' to apply this to the whole image - this may take some time.
10. Press 'Keep'.

What result to look for:

• Elements should appear more focussed as the blurring effect of atmospheric turbulence is compensated for.
• Edges should look more distinct and without the exaggerated coalescing caused by increasing the radius a lot.
• If there are ringing artefacts around the stars it indicates the Radius parameter is too high.
• An increase in noise blotches in the background in the 'After' image indicates the Regularisation setting is too low.
• An increase in blurring in the foreground in the 'After' image indicates the Regularisation setting is too high.

Ways of getting better results:

• The Decon module works best when there is little noise - Bin your oversampled data to improve the SNR if needed - but consider leaving some degree of oversampling to allow deconvolution to bring out finer detail. See the Bin module notes for a discussion of the issues relating to Bin vs. deconvolution.
• Data that is not oversampled is not a candidate for deconvolution if the aim is to reverse seeing-related issues.

After Use:

• Use the Sharp module (if needed) as we can use the same star mask as used here.
• Modules like Sharp and HDR make use of the results of the Decon module.

Description of Controls:

Mask:
For general instructions on using masks see Mask

• In this case the mask is used to mask out the bright stars so that they can be treated separately to avoid ringing effects.
• If no mask is set you are asked if you want to create star mask - 'Generate mask automatically', 'Generate mask manually' or 'Don't create mask now (Lunar or Planetary)'
• Select 'Create mask automatically' if the subject is a DSO.
• If not prompted because a Mask is already set - select the AutoMask preset to create a mask automatically:
  • Shrink-Grow to get rid of single pixels
  • Include star halos in the mask
  • 'Keep'

Radius:
The size of the blur that Decon will try and remove.

• Default is 1.5 pixels. Range is 0.0 to 20.0 pixels.
• Related to the seeing - Seeing-induced blur is normally 3-4.5 arc-seconds. The camera/lens combination gives a resolution between 1 and 5 arcsec/pixel depending on the equipment combination.
• Adjust the Radius until just before the smaller stars start showing signs of ringing.

Regularization:
Sets the balance between detail, noise and smoothness.

• Range is 0.00 to 5.00
• Default is 1.00 (optimal noise and detail).
• Adjust as needed. reduce below 1.00 to get extra detail at the expense of noise, increase above 1.00 to reduce noise at the expense of detail, up to a maximum of 5.00

Iterations:
Sets the number of iterations the deconvolution algorithm goes through.

• Default is 6, Range is 1 to 51.
• Increase this value incrementally if further improvement can be seen - there will be a point beyond which you will not get a better result.

Image Type:
Different Deconvolution modes for Deep Space and Lunar/Planetary targets.

• Deep Space - Deconvolution makes no special provision for extra dynamic range.
• Lunar/Planetary - This mode frees up dynamic range for any deconvolved highlights.
• Default is 'Deep Space'.

Mask Behaviour
The use of the mask can have different effects in different modes.

• 'Normal' - Retains the original pixels where no mask is set (non-green).
• 'De-ring Mask Gaps, Show Results' - Deconvolves the whole image but assumes the mask is a star mask and applies de-ringing to them.
• 'De-ring Mask Gaps, Hide Results' - Deconvolves the whole image but assumes the mask is a star mask and applies de-ringing to them. In this case the de-ringed parts don't bleed into the masked parts. The deringing is hidden by copying the original non-deconvolved image in the non-masked parts (stars) and then gradually show the deringed, convolved image in the masked part - with the transition being controlled by the 'Mask Fuzz' setting.
• Default, if the mask is used, is 'De-ring Mask Gaps, Hide Results'

Mask Fuzz:
If a mask is used, Mask Fuzz controls the blending of the transition between masked and non-masked parts of the image.

• Only has effect when Mask is active (DSO subjects) - see Mask Behaviour above.
• Smooths the transition around the bright stars.
• Default 8.0 pixels. Range is 1.0 to 40.0 pixels.
• Experiment to find most natural look.

Background Notes

Deconvolution and Oversampling
Deconvolution is used to sharpen up an image, however this can amplify noise and introduce artefacts including ringing. It works best on data which is oversampled and has a high signal to noise ratio.
However, data that is on the cusp of being oversampled, where faint stars are spread over 3 pixels, may still benefit from a small amount of deconvolution. Every optical system, no matter how expensive, spreads a point light over multiple pixels to a degree (see Airy disk). Decon can reverse this spreading as well - just take it easy - it is very easy to overdo this.
For further details regarding oversampling, binning and deconvolution see the Bin module background notes.

Deconvolution and singularities
Singularities in the data are those areas where there is a discontinuity in the valid data - where the valid data is missing - such as in the saturated white cores of stars. These areas normally cause bad ringing artefacts.
StarTools uses a novel de-ringing algorithm which ensures stars are protected from the Gibbs phenomenon (also known as 'panda eye effect'), while actually being able to still coalesce singularities, such as over-exposed white cores of stars, into point lights.