veil nebula new svdecon

[alacant]

Hi everyone
IMHO, the 1.8 alpha decon is excellent. It reverses blur without adding noise.

A lot depends upon which stars you choose. This is with around 10 stars of all sizes. Maybe a little overdone...?
But then that's comparing with the vast majority of other amateur astro-photos which seem to try to imitate the conceived correctness of but a few operators.
The new decon is going places. My handling of it however leaves a lot to be desired!

Does anyone have any other examples or guidelines about star choice?
Cheers and clear skies,
Steve

veil

[admin]

Hi Steve,

Finding good samples hopefully shouldn;t be too hard. Just make sure they fit the critera, and ideally you will want to have samples across your image (so that the spatially variant distortion can be modeled the most accurate). From the docs;

Recommended workflow

^ The SVDecon module is ideally operated by selecting samples of good quality. Top left; original image. Top right; the resulting deconvolved image by selecting the samples indicated. Bottom left; "Sample Quality" view without samples selected. Bottom right; "Sample Quality" view with three good quality samples selected. The blue bounding boxes should ideally fit the entire green "blobs" (signifiying the apodization mask for each sample).

Selecting samples for Spatially Variant deconvolution

In the 'Sampling' view, A convenient rendering of the image is shown, in which;

• Candidate stars are delineated by an outline.
• Red pixels show low quality areas
• Yellow pixels show borderline usable areas.
• Green pixels show high quality areas.

Ideally, you should endeavour to find stars samples that have a green inner core without any red pixels at their centre. If you cannot find such stars and you need samples in a specific area you may choose samples that have a yellow core instead. As a rule of thumb, providing samples in all areas of the image takes precedence over the quality of the samples.

^ Detail should snap into focus, stars should coalesce into point lights and halos around over-exposing stars should be diminished.



You should avoid;

• Stars that sit on top of nebulosity or other detail.
• Objects that are not stars (for example distant galaxies)
• Stars that are close to other stars
• Stars that appear markedly different in shape compared to other stars nearby
• Stars whose outline appear non-oval or concave or markedly different to the outlines of other stars nearby

Star samples can be made visible on the regular view (e.g. the view with the before/after deconvolved result) by holding the left mouse button. Star samples will also be visible outside any preview area, this also doubles as a reminder that any selected PSF Resampling algorithm will not resample those stars (see 'PSF resampling mode'). You may also quickly de-select stars via the regular before/after view by clicking on a star that has a sample over it that you wish to remove.

The Sampled Area

The immediate area of a sampled star is indicated by a blue square ('bounding box'). This area is the 'Sampled Area'. A sampled area should contain one star sample only; you should avoid parts of other stars in the blue square surrounding a sample. The size of the blue square is determined by the 'Sampled Area' parameter. The 'Sampled Area' parameter should be set in such a way that all samples' green pixels fall well within the blue area's confines and are not 'cut-off' by the blue square's boundaries.

Star sample outlines and apodization mask

The star sample outlines are constructed using the apodization mask that is generated. You may touch up this mask to avoid low-quality stars being included in the blue square 'Sampled Area', if that helps to better sample a high quality star.
Number of samples and location of samples

Ideally samples are specified in all areas of the image in equal numbers. The module will work with anywhere from one to hundreds of samples. The amount of samples you should provide is largely dependent on how severe the distortions are in the image, and the quality of the samples ( see the aforementioned 'You should avoid' items).

When clicking a sample, the indicated centre of a sample will not necessarily be the brightest pixel. Instead, the indicated centre is the "luminance centroid". It is the weighted (by brightness) mean of all pixels in the sample. This is so that, for example, samples of stars that are deformed or heavily defocused (where their centre is less bright than their surroundings) are still captured correctly.
Heavily distorted PSFs

For images with heavily distorted PSFs that are highly variant (for example due to field rotation, tracking error, field curvature, coma, camera mounting issue, or some other acquisition issue that has severely deformed stars in an anisotropic way), the 'Spatial Error' parameter may need to be increased, with the 'Sampled Iterations' increased in tandem. The 'Spatial Error' parameter relaxes locality constraints on the recovered detail, and increasing this parameter, allows the algorithm to reconstruct point lights from pixels that are much less co-located than would normally be the case. Deconvolution is not a 100% cure for such issues, and its corrective effect is limited by what the data can bear without artifacts (due to noise) becoming a limiting factor.

Under such challenging conditions, improvement should be regarded in the context of improved detail, rather than perfectly point or circle-like stellar profiles. While stars may definitely become more pin-point and/or 'rounder', particularly areas that are (or are close to) over-exposing, such as very bright stars, may not contain enough data for reconstruction due to clipping or non-linearity issues. Binning the resulting image slightly may somewhat help hide issues in the stellar profiles. Alternatively, the Repair module may help correcting these stars.

Hope that helps!

[alacant]

Thanks Ivo
Yes, that guide is perfect.
Very much a case of RTFM, of which I hold my hand up in the guilt of knowing there was minimal reading on my part.
Sorry for the noise.