StarXTerminator (SXT)
StarXTerminator (SXT) removes the stars from an image and, if requested, also delivers an image containing only the stars. Nebula and stars can then be processed with the tools that suit each and recombined afterwards.
Purpose
| Task | Result |
|---|---|
| process the nebula without stars (contrast, saturation, local contrast) | starless image |
| process the stars separately (colour, size) | star image |
| recombine both at the end | screen blend via PixelMath |
Whyseparate the stars?
Every contrast, sharpening and saturation step on the nebula also affects the stars: they bloat, get dark halos or colour fringes and, in dense fields, cover the nebula. Separated, each image gets the right tools, and recombining stays under control.
Principle
- SXT is trained on non-linear data and works most reliably there. In a stretched image halos and faint stars are also fully visible, so the separation captures all of them.
- The image is processed in tiles. A larger tile overlap avoids visible tile boundaries and captures large stars with extended halos better.
- The star image can be computed in two ways. This determines how it must be recombined later:
| without Unscreen stars | with Unscreen stars | |
|---|---|---|
| Star image | difference: original − starless | computed so that a screen blend reproduces the original exactly |
| Recombination | addition: starless + stars | screen: ~((~starless) * (~stars)) |
| over bright nebula | values can exceed 1.0 → clipped stars, hard edges, dark rims | mathematically never above 1.0 → stars blend in softly |
The option affects only the star image; the starless image is identical with and without Unscreen.
Whyscreen rather than addition?
Screen is defined as 1 − (1 − a)·(1 − b). For small values it behaves like an addition, for large ones it approaches 1 – the result can never overshoot, like two overlapping light sources. When starless and star image are changed separately, they no longer match exactly; an addition then produces values above 1 and hard edges over bright nebula, a screen blend does not.
Parameters
| Option | Effect | Typical choice |
|---|---|---|
| Generate star image | also create a star image | enabled for the image the stars will later come from; otherwise off |
| Unscreen stars | compute the star image for screen recombination | enabled |
| Large overlap | larger tile overlap | enable for dense star fields or large halos |
Operation
- Apply to the stretched image.
- Result: the image itself becomes starless; with Generate star image a star image is created as well (e.g.
…_stars). - Process both images separately; the star image contains nothing but stars on a black background – processes can be applied there without a star mask.
- Recombine with PixelMath at the end (see below).
Typical applications
Stars from the colour image
When a colour image and a separate luminance are processed separately, the star image comes from the colour image – only it contains star colours; a star image from the luminance would be grey. The luminance is made starless without Generate star image.
Recombining the stars
PixelMath, destination Replace target image, on a duplicate of the finished starless image:
~((~Starless) * (~(Stars * s)))
Symbols: s = 1.0~xis the inversion1 − x; the expression computes the screen blend.- Replace the image names with the actual IDs.
- Replace target image keeps FITS keywords and the astrometric solution; Create new image discards both. The duplicate protects the starless image as a fallback.
s | Effect |
|---|---|
| 1.0 | stars at full intensity |
| 0.7–0.9 | additional dimming |
After a Makes stars smaller instead of merely dimming them – faint stars recede, bright ones remain as accents. Easiest in a separated star image. Open fundamentals →, s = 1.0 is usually right: the stars are already smaller; dimming them further only takes away brilliance, and the image looks flatter.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Obvious star remnants, tile boundaries | overlap too small | recompute with Large overlap |
| Clipped stars, hard edges over bright nebula | addition instead of screen, or Unscreen not enabled | Unscreen and the screen formula |
| Stars too bright or too dim after recombination | formula does not match how the star image was computed | check consistency |
| Dark halos around bright stars | background under the star estimated too dark | check at 1:1 after recombination; usually barely visible over nebula |
| Astrometric solution missing in the final image | PixelMath with Create new image | recombine with Replace target image on a duplicate |
| Grey stars in the final image | star image from the luminance | star image from the colour image |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| processing nebula and stars separately | only shrinking stars without processing the nebula → Makes stars smaller instead of merely dimming them – faint stars recede, bright ones remain as accents. Easiest in a separated star image. Open fundamentals → with a star mask |
| dense star fields where stars cover the nebula | linear data → stretch first, e.g. Stretches linear data so that contrast is created around a chosen brightness value – in several small passes, alternating with a black point. Open fundamentals → |
HaLRGB workflow · MorphologicalTransformation · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · PixelMath · RangeSelection (Range Mask)