Why does StarXTerminator fail to remove stars?

These are the most common reasons for poor star removal performance:

  1. Optical aberrations (coma, field curvature, etc.) are too severe
  2. The image is excessively oversampled
  3. The image has been stretched using alternative stretching method(s) such as generalized hyperbolic stretch (GHS) or arcsinh

StarXTerminator is trained to recognized stars with a limited amount of optical aberrations such as coma. There is a limit, however. If you find that stars near the center are removed but stars near the corners are not, this is most likely the cause. You may be able to achieve better removal if the image is first processed with BlurXTerminator in the “correct only” mode, but even here there is a limit. The correct solution is to resolve the optical aberrations through better tuning of the optical system and/or better optics. Some systems may simply require cropping away the strongly aberrated portions.

For best results, run StarXTerminator (and BlurXTerminator) on images with stellar FWHM values less than 8 pixels, and use it early in the processing flow, before extensive stretching operations.

Stellar FWHM values between 3-4 pixels are adequately sampled, meaning that there is no resolution gain at higher values. Any image with FWHM values over 6 pixels can be safely down-sampled by a factor of two, using either in-camera pixel binning or a simple integer resize, without losing information. There are benefits to down-sampling oversampled images: image files will be 4x smaller, all processing will run ~4x faster, and the signal-to-noise ratio (SNR) of the image will be improved.

To provide a factor of two of margin, StarXTerminator and other RC Astro tools are designed to work well up to 8 pixels FWHM. Above this, performance will suffer.

As mentioned in the usage notes, StarXTerminator is best used early in processing, prior to extensive stretching, especially if using stretch methods other than a simple mid-tones transfer function (MTF).