Lose noise without losing stars?

[MilwaukeeLion]

Have qty 240 images - 30 sec - ISO 3200 - f3.5 - 16mm kit Sony lens with a6000  - forgot to shoot in raw, instead was set to 4.5M jpeg.  Question 1, Did some adjustments in lightroom, possbile to remove grain from upward light circled in red without losing like 70% of stars?  Below is 1. original, 2. adjustments with no noise removal 3. adjustments with noise removal.  Question 2, possible to get even lighting throughout sky, adjust dark in middle of image to match edges or vice a versa, if so, any tips appreciated.  Thank you ! ML

[cjdawson]

My understanding is that this is where Dark and Flat frames come into play.  The Dark frames will help to reduce noise, the flats will sort out the vignetting.

To take them, the darks should be the same exposure length as the main images, taking a series of 20 and stack them to make a master dark frame.

For the flats, the idea is that you leave the focus where it is, and put an even white source of light in front of the lens.  A cheap way is with a white t-shirt, place it over the optics so that the camera sensor is light with a diffuse white light, that is even across the whole area. Then take a photo and fill the camera pixels about 1/3 to 1/5.  This is easy done by setting the camera to Av mode and letting it set the exposure time.

Applying both the dark and flats, should help.

[ollypenrice]

Firstly noise reduction using Ps or similar: create a bottom layer and noise reduce that globally. Be quite aggressive  and go slightly too far.

Now work on the top layer. Use the colour select tool to pick up the background sky. Use the multiple sampler and the 'fuzziness' slider to ensure that all your noisy background is fully selected, but the stars not selected. Use a large eraser tool set to about 50% and wipe off the top layer. How does it look? If it needs more, give it a second pass with the eraser until you have what you want.

Secondly gradients. Flats will not remove these. There is a Ps plug in called Gradient Xterminator which works well. The best tool of all is DBE in Pixinsight.

To get a mottle-free background from most DSLRs you need a dither of about 12 pixels between subs. That will make a huge difference to your background sky noise, particularly colour noise.

Olly

[MilwaukeeLion]

Great, thank you CJ & Olly, I will try the PS tips and see how it looks.  I actually did take these in RAW 24M, the folder was hidden so I was working with jpeg sequence I had exported from lightroom the night I took those.  I'm re-organizing my computer to keep better track of my imaging sessions.  I need to pick up remote shutter release before using my APSC with my telescope, not sure the 1250 focal length of my 5SE is ideal for imaging with 1.5 crop APSC. As opposed to buying new full frame camera I ordered a JMI piggy back mount for the 5SE to attach my 150-600mm Tamron lens and APSC.  Once that arrives and I figure out how to polar align my mount (doesn't sound east with what I have) I will try a go at the darks, bias, flats.  Urgh, so much easier with my colour ccd, I just point at a planet, press record, run thorugh registax and poof finished product I like.

[Ruud]

In PS, sometimes Filter > Blur > Surface Blur will do the trick. 

[StargeezerTim]

I tried this yesterday. It seemed to work OK for me...

http://www.maclife.com/article/howtos/how_get_smarter_noise_reduction_photoshop#slide-0

[MartinFransson]

Firstly noise reduction using Ps or similar: create a bottom layer and noise reduce that globally. Be quite aggressive and go slightly too far.

Now work on the top layer. Use the colour select tool to pick up the background sky. Use the multiple sampler and the 'fuzziness' slider to ensure that all your noisy background is fully selected, but the stars not selected. Use a large eraser tool set to about 50% and wipe off the top layer. How does it look? If it needs more, give it a second pass with the eraser until you have what you want.

Secondly gradients. Flats will not remove these. There is a Ps plug in called Gradient Xterminator which works well. The best tool of all is DBE in Pixinsight.

To get a mottle-free background from most DSLRs you need a dither of about 12 pixels between subs. That will make a huge difference to your background sky noise, particularly colour noise.

Olly

I really need to start saving tricks like this in a word document. Excellent!

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