When to apply noise reduction

[maceemiller]

Hi all.

So I'm just lying here trying to get to sleep and I'm wondering when is the best time to apply any noise reduction in an image workflow?

I imagine it to be the very first thing to do in PP yet I dont actually know!

Any advice would be greatly appreciated.

Thankyou.....Carl

[yobbo89]

I would assume, usually the very last in the process, well that's how I do it. And if I have bad colour subs I will do noise reduction first and then do the lum channel /overall last

[bobharmony]

Carl, my tendency is to do noise reduction late in the process. As it removes some detail along with noise, I prefer to have the detail available throughout all post processing steps until the very end.

Bob

[UlteriorModem]

Actually for me it's one of the first things I do while the data is still in a linear state.

Typically I will do a crop (to remove any stacking artifacts), A background neutralization, then Multiscale linear transform 'MLT" (small scale noise reduction). MLT works best while the data is still in a linear, un-stretched state.

But I am using Pix Insight. It might differ for other programs.

[maceemiller]

Thanks for your replies..... I figured there would be 2 different times when people apply NR in their workflow!!

I can see benefits to do it at the start and also see benefits leaving it until the end.

I'm guessing a lot is down to personal preference/equipment.

[UlteriorModem]

One thing about Pixinsight's MLT process is that it is pretty good about knowing what is small scale noise and what is signal. But you have to be reasonable about it. If you go after it too hard and too may layers deep you can wipe out fine details. My recent process of the Cat's eye is a good example.

M94-1 by Tom Whit, on Flickr

See, just too 'soft' over did the noise reduction in the early stages.

Also Often I will use a range mask to protect objects while going after the background noise.

I may still do some additional steps after the image is stretched such as 'SCNR' which can eliminate small scale noise of a single channel (usually green).

[maceemiller]

Looks a great picture to me!

I use photoshop for post and a few days I noticed an article saying about using masks in NR.
Carried on browsing for what I was searching for though....

I'll have to try look it up as in your post a mask was mentioned and I love trying to learn!

[UlteriorModem]

Looks a great picture to me!

I use photoshop for post and a few days I noticed an article saying about using masks in NR.
Carried on browsing for what I was searching for though....

I'll have to try look it up as in your post a mask was mentioned and I love trying to learn!

Thanks, but I know there is a lot of fine detail in that image that got wiped out during noise reduction. I've been able to collect more data on this target over the last few nights and plan to re-visit this target again, probably this afternoon. This time I will go a little easier on the MLT.

Masks can be very useful, but again careful applications are needed as often times you can get really funky results with masks. Fortunatly PI allows to masks with 'soft' edges that helps tame down some of that funky results. Probably have something like that in PS.

The learning never stops... trust me.

[bobharmony]

Another thing to consider is what you are using for post processing. PixInsight has some NR routines that are meant to be used when the image is still in a linear state, so those would be applied earlier in the process, before stretching. StarTools is designed to apply NR near the end of the process. It claims to be able to track where in the image NR needs to be applied heavily and where a more sparing approach is warranted. My experience is that the claim is well justified.

I used to use PhotoShop for post, it seemed to like NR at the end. I also used the Noise Ninja plugin, which was very effective for NR.

Bob

[KathyNS]

There doesn't have to be a single answer. Noise reduction can be done at any time, and you may choose to do it more than once with different tools.

I typically do a noise reduction with ATrous Wavelet Transform (Multiscale Linear Transform would work as well) on the masters as the very first thing I do. And I usually use ACDNR to reduce remaining noise late in the processing, after colours have been combined. If the image is noisy, I might do some more noise reduction along the way.

[maceemiller]

Wow, some great replies here!

Definitely given me a better insight and hopefully others too.

The one thing I do know about image processing is it's not easy! This is why I say to my friends "astrophotography is like fishing.....you can have all the gear and catch nothing yet on a different day you catch a beauty with a string and hook"

Some understand, most do not!!