how to stretch a noisy image

[blackdragon72]

What is the steps/processes to stretch a noisy image, eg, the image without enough exposure? If I use HT to bright out the detail then background is bright and noisy (even noise reduction is applied prior to HT).

[Phil Leigh]

At the risk of stating the obvious, if the data is bad then it cannot be magically corrected in post-processing. Stretching does not add noise, it merely makes noise more easily visible. You can try noise reduction before stretching (TGVDenoise with local support) or after stretching. You can try Atrous Wavelets noise reduction (with a strong mask to expose the background) to remove larger scale noise.

[Geoff]

You could also apply a mask to the background before the final stretch.
Geoff

[NGC7789]

Another method is downsampling. I don't know what camera you have but if you have the pixels you may choose to sacrifice some to improve the signal to noise ratio. Good down sampling may preserve enough detail for a satisfying image, especially for screen display where you are ultimately throwing away the pixels anyway.

[blackdragon72]

Thanks for all the suggestions.

How does downsample work? Is it same as binning?

[NGC7789]

In PixInsight it's the Resample tool. It's one of the tools that has documentation so you can read up. Go over the different resample algorithms and then experiment to see what gives you results you like.

[blackdragon72]

And does masked stretch help in this case?

[NGC7789]

And does masked stretch help in this case?

I would say when it comes to noise you attack with all the tools you can.

But it shouldn't be forgotten what was mentioned earlier in the thread. There is no substitute for good data. If you cannot remove the noise to your satisfaction you have to shoot the target again using techniques or equipment that gather less noise. We are fortunately in astrophotography that most of our targets are still there to shoot again (and again and again).