PixInsight: Avoiding Star Bloat when Stretching
I have found that my images are very susceptible to star bloat, I think is mainly due to me having very fast (f/4) optics but also potentially due to a mis-matched coma corrector. Over the last few months I have been trying to find a method that would let me keep the bloat to a minimum. There are numerous tools that can be used in the workflow both pre and post stretching. Whilst these tools are quite effective I felt there should be some merit in trying to minimise the bloat as early in the workflow as possible. Some examples of how to reduce stars are:
- Deconvolution (While image is linear)
- Masked Stretch
- Morphological Transformation: Erosion
- Morphological Transformation: Morphological Selection
- Star Reduction Script
I do use Deconvolution as part of my processing and can confirm it is very effective a tightening the stars but I need a little more than it can offer by itself. My new stretching process is relatively simple & not time consuming, it follows the following process:
- Extract Lightness from ‘original’ image, or clone if it is a mono image, rename it as ‘lightness’.
- Use Histogram Transformation to apply an Auto-STF stretch to the ‘lightness’ image.
- Create a good starmask from the ‘lightness’ image, rename it as ‘starmask’.
- Apply ‘starmask’ to ‘original’ image, then invert the mask to protect everything except the stars.
- Apply a small Histogram Transformation stretch to ‘original’ image.
- Remove ‘starmask’ frim ‘original’ image.
- Use Histogram Transformation to stretch image to desired result.
I have found that this works very well, as soon in the preview image below.
The following will be a worked example showing each step.I am starting with a Luminance image for Messier 51 – The Whirlpool Galaxy, it is a stack of almost 7 hours of 3min subs. It has been calibrated and stacked before being linear processed with:
- Dynamic Background Extraction – Gradient Removal.
- Deconvolution – Detail Recovery & Star Reduction.
- Multiscale Linear Transform – Noise Reduction.
Clone Image. With image selected, ‘Edit’ Menu > ‘Duplicate’. Rename ‘lightness’.
Apply Auto-STF to ‘lightness’ using Histogram Transformation.
Create a starmask that fits well, it needs to suit all stars in your original image. This can be challenging but I find the following settings generally work well for me.
Apply starmask to original image.
The image below shows the inverted starmask applied and the original image having a temporary Auto-STF stretch to show the fit around the stars.
Hide starmask, either ‘Ctrl & K’ or ‘Mask’ menu > ‘Show Mask’
Apply a stretch to the image using Histogram Transformation, just stretch enough that the mid-brightness stars or galaxy(s) are starting to show a bit. Like below, I have included the stretch settings (see middle slider position).
Please note if you push this stretch two far you will end up with dark rings around your stars.
Remove starmask and continue to stretch the image as normal, using Histogram Transformation with real-time preview active.
Two additional iterations of different stretch amounts gave me an end result as shown below:
And that is basically it done, I would now move onto the non-linear portion of my workflow.