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Old 10-06-2009, 01:38 PM
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White Rabbit
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Lets talk about Noise.

Hi.

I have a few questions regarding noise. I find my 1000d to be quite noisy even when stacking multiple images.

Last night I took 4x5min subs of Cent A @ iso 800, and notice that when I stacked them the noise didnt really get any better. Now, on to my questions.

Is 4x5min the same as 5x4mins in terms of exposure it is but is it the same for noise reduction?

Would I have been better doing shorter and more subs?

I assume that the more subs you take the better what is the normall amount that people take?

I'll post a pic of what I'm talking about in a second.

Thanks
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Old 10-06-2009, 06:09 PM
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[1ponders] (Paul)
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The more subs you take the better within reason, but longer subs are better than shorter subs. With short subs there may not be much level difference between the signal and the noise, with longer subs the signal increases linearly (to a point) but the noise only increases with the sq rt of the signal (this is not an accurate statement and I'm sure someone will jump on it but you get the point i hope. i.e signal = 100 noise = 10, signal = 1000 noise = 33.3 ) The same rule applies with combining multiple images; 4 images = 4xsignal but 2xnoise, 10 images = 10 x signal but 3.3 times noise. Signal starts to swamp the noise.

Except in the dark areas of your image where there is not much difference between signal strength and noise.

Are you shooting in RAW mode and are you using in camera noise reduction? If not are you taking dark frames and subtracting them from the light frames?
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Old 10-06-2009, 07:01 PM
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Hi and thanks for the reply

Yes, I'm shooting in raw and yes I have the NR turned on I also have the high iso noise reduction turned on.

Here is a copy of what I'm talking about.

I know there are a lot of things wrong with this such as focus coma no flats were used either. this was a training run on my freshly formated computer and I wanted to test the auto guideing. Have a look at the noise and tell me if more data would clean it up, not that I'm intending on keeping it.

Thanks again for the help.

sandy
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Old 10-06-2009, 08:13 PM
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[1ponders] (Paul)
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What program are you using to combine your images? When you combine them are you saving them as 16bit images? What is you process for stretching them- are you using PS levels and curves?
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Old 11-06-2009, 08:30 AM
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HI

Sorry for not putting the relevant info in the first post. I used Deep Sky stacker to to stack and just use the standard settings register/stack. I did use photo shop cs3 where I applied some some levels and curves. As soon as you asked me what i used to process I though it might be noise created from pushing the curves to far, would I be correct?

thanks
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Old 12-06-2009, 11:52 AM
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thats what I'm thinking.
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Old 12-06-2009, 02:47 PM
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ide say its a combination of PS overkill + bad SNR + not a full reduction + Light pollution. When i take photos from down town perth towne if i don't do Flats Darks and Bias the final image is shocking. the light pollution shows up vingetting badly. and hence you cannot push levels and curves.

Moral to the story, take flats darks and bias when taking photos in light polluted areas, other wise you will be pushing 5hi7 up hill on a flat bed wheel barrow!
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Old 13-06-2009, 08:39 PM
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Yes thats right. Even in dark skies images should always be fully calibrated, that is, with darks, flats and bias frames. Any in camera noise reduction MUST be turned off also, including high ISO NR, otherwise it may interfere with the dark subtraction process. In other words we need raw data straight off the sensor with as little manipulation by the camera as possible, thus allowing the software to properly calibrate the images.

I find 10 minute subs is the sweet spot for me on averatge temperature nights. I once tried 3x30 minute subs on the Helix Nebula, but found thermal noise resulted in a noiser image than 9x10 mins. 5 minute subs also gives a slightly noisier total, however on very hot nights 5 mins is actually better
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