I’ve seen the light. Well, I’ve seen how we will control and tame the light on the future ;).
Last two days I’ve been playing with a nightly build of SiriL. I was looking for the new “unlink channels” when you perform autostrech. I’m using a OSC and if after the stack I select auto stretch to see what I’ve captured, I get the famous green swampy screen with some brighter blobs on it. Now, with the link channels (really, unlink) you can get a pretty colour balance view and you can start to crop, remove background and all this preparatory task before stretching or touching the colour of your stack.
And talking about background extraction: definitively you should learn to use the new RBF method. It’s a great addition to the powerful arsenal of tools that SiriL put in our hands.
Then I’ve discovered the new feature that moves me to create this post: Generalised Hyperbolic Stretching. We never will give enough thanks to @ajeb78 for bringing the method developed by David Payne to SiriL. It’s a stretching algorithm that allows you to finely control where and how the stretching is applied. Read the original post from David Payne here Linker error (trying to add additional feature)
How it looks? Well, the best way is to compare with other methods. So, we start with a deep image of M101. It’s a 106x180" stack from an ASI533MC OSC camera on a 150 f4 newtonian telescope from a decent sky (Bortel 2).
After removing the remaining background inconsistencies and applying photometric color balance, the image, generated with the standard MTF stretch and some saturation enhance is something like this
It’s very difficult to get all the detail out of the core with this simple stretching algorithm. Also, when you pump up the color saturation, the chromatic color aberration introduced by my comma corrector shows ups as a nasty cyan ring around bright stars. You can see the most prominent one just below the galaxy’s center.
Without leaving SiriL we can use asinh. This method bring to us the ability to apply a different stretching to the bight and dark region of the image. Also, the colour are best managed and after the stretching you can enhance saturation with less problems
Now, you can see more detail on the central region of the galaxy while maintaining the visibility of the outer regions. There are still some blue fringing, but at least is not as invasive as it was with an MTF stretch + color saturation
The last algorithm and by far the one that gives you more control is the Generalised Hyperbolic Transform. With it I was able to generate this image
The differences are more subtle that between the first two, but this difference is what makes a good image, great. You have a lot of control over the contrast near the bright region of the nucleus, so you can tame better the light. Also, the colour stretching is performing after the bright one. So, you use GHT to stretch the “luminance” and the use colour saturation tool to increase the colour. This allows you a level of control that is well over asinh.
I’m very exciting about the future versions of SiriL, we will be able to almost finish the image completely from stacking to stretching inside SiriL.
The only thing that I’m performing outside SiriL is the sharpening / deconvolution / HDR-ization? and noise reduction. For this task I’m currently using StarTools, a comercial product.
I’m sorry for that, because until this point all my acquisition / processing toolchain is open source: OnStep (open source hardware and software) to control the telescope, KStars/Ekos and INDI drivers to perform the session control, guiding, platesolving and image capture. And for all the data processing I’m using SiriL. Only for the last step, I’m using not OpenSource software.
Apart of this digression surely you will want to see the final results ;). Well, this is the “Almost SiriL” produced image. Here I’ve used StartTools HDR module to bring up the detail near the galaxy core and the noise reduction to try to reduce the noise, obviously
Sorry for the long post, but I think that we come to this forums always to complain about things that are bothering us on this exact point in time, but we forgot what this team of people are offering to us. His continuous work on SiriL is amazing.
Thank you very much for allow us to produce this kind of images.
