StarXTerminator Usage Notes

Keep in mind that star removal is an extremely challenging problem, and no star removal tool will ever perform 100% perfectly on all images. StarXTerminator was trained to work on images produced with a very wide range of instruments, from camera lenses to the James Webb Space telescope. It will produce excellent results on a majority of images, but occasionally there may be cases where stars are not completely removed, or some minor non-stellar structure is.

StarXTerminator was also trained to handle a limited range of optical aberrations such as minor focus errors, guiding errors, coma, field curvature, etc. It may not function well on images taken with instruments with serious optical deficiencies. Invest the time needed to get your optical setup functioning well, not only for better star removal, but also for the best detail and contrast in nebulas and galaxies achievable with your equipment.

If despite these efforts you find a case where StarXTerminator does not seem to perform well, review the usage notes below. If you are still getting poor results, feel free to contact support. Perhaps your instrument setup is unique and your data can be included in the training of the next version of the neural network. We continually train the neural network on new data, and RC Astro is always happy to receive suggestions for improvement.

Here are some usage notes and tips for the PixInsight and Photoshop versions of StarXTerminator:

PixInsight

• Use StarXTerminator as early in the processing flow as possible, ideally right after integration, with the data still in a linear state (i.e., prior to any stretching). This will generally produce the best results, and gives the added flexibility of being able to stretch the starless and stars images separately depending on the desired end result.
• StarXTerminator is trained on images stretched using a simple midtones transfer function (MTF). When processing linear images, StarXTerminator internally performs such a stretch automatically, then precisely reverses it after processing to return the image to a linear state.
• The MTF stretch is the same method used by PixInsight’s HistogramTransformation tool. Any processing that significantly alters star profiles relative to this method may reduce the effectiveness and/or quality of star removal. In particular, an arcsinh stretch and a generalized hyperbolic stretch (GHS) can create star profiles that are indistinguishable from small elliptical galaxies, and will result in StarXTerminator not removing, or only partially removing, the stars. Other processing operations such as masked stretch, high dynamic range processing, etc., may also alter star profiles enough that they will not be recognized as stars by the neural network.
• If generating a star image, don’t use STF Auto Stretch on the resulting stars image. StarXTerminator will translate the STF parameters of the original image to the stars image to make subsequent stretching easier. Auto-stretch will destroy this STF information and give a false impression of the significance of very faint background residual pixel values.
• If generating a star image from a linear image, don’t select the Unscreen option, which is for nonlinear (stretched) images. Simple subtraction is the best star extraction method to use with linear images, and will result in the best star color accuracy. For best results when recombining the (perhaps separately processed) starless and stars images, do it after stretching both, and use screen blending in PixelMath.

Photoshop

• Use StarXTerminator prior to any major processing of the image. The effectiveness and quality of the result from StarXTerminator will be adversely affected by any processing that significantly alters star profiles.
• To generate a separate starless and stars layer, follow this procedure:
  • Start with the image you want to separate on a layer
  • Duplicate this layer twice so you have three copies of the original image
  • Remove the stars from the top-most layer using StarXTerminator
  • Duplicate this starless layer so you have two copies of it
  • Drag one of the starless layers down so it is between the two remaining original layers. Name this layer “Starless”
  • Invert the top-most starless layer and set its blend mode to Divide
  • Invert the next layer down (a copy of the original)
  • Merge the top two layers into one. This will now be an inverted image of just the stars.
  • Invert that merged layer. This is now the stars layer. Name it “Stars”
  • Set the blend mode of this Stars layer to Screen
  • The combination of the Stars and Starless layer below should look exactly like the original image
  • You should end up with three layers in this order (top to bottom):
    • Stars (screen blending mode)
    • Starless
    • Original (in case you need it)
  • Process the Starless and Stars layers separately to taste

There is a Photoshop “action” to perform the above steps available here.