Noise level comparisons please

[Padraic M]

Looking for input please on noise levels in stacked images...

 

The two samples below are centre crops from 2hrs 12.5 mins of Red (53x150s) at 100% zoom. Using an ASI1600mm Pro at -20c, gain 139, offset 21 and Baader RGB filters.

Processed in APP, calibrated, normalised and integrated with a 10% stretch. No light pollution removal or other non-linear processing applied.

 

The first sample is with flats, dark flats and dark calibration frames applied (20x each); the second sample is with flats and dark flats but without dark frames. The subs are dithered.

I have to say, I can't see any difference between them, with a slight preference for the one without darks.

 

Also, is this representative of what you would expect with this equipment, on twp clear moonless Bortle 8 nights? Obviously, the noise is carrying through into the rgb-combined image and while it may be down to poor skies, I'm wondering if I have a processing problem.

 

Blue, Green and Ha are similar. I have an obvious problem with the Ha darks, so I've definitely done something wrong there (walking noise patterns).

 

With darks...

 

Without darks....

[MarkAR]

Theres a good Adam Block YouTube video on how calibration works and how to use it in Pixinsight. The basics apply to any stacking software though.

The ASI1600 is CMOS I believe, I don't think you need darks. Having the wrong calibration frames can make things worse.

[AstronomyUkraine]

1 hour ago, MarkAR said:

Theres a good Adam Block YouTube video on how calibration works and how to use it in Pixinsight. The basics apply to any stacking software though.

The ASI1600 is CMOS I believe, I don't think you need darks. Having the wrong calibration frames can make things worse.

You do need darks with the 1600. You don't need bias frames, use flat frames instead.

[AstronomyUkraine]

1 hour ago, Padraic M said:

Looking for input please on noise levels in stacked images...

 

The two samples below are centre crops from 2hrs 12.5 mins of Red (53x150s) at 100% zoom. Using an ASI1600mm Pro at -20c, gain 139, offset 21 and Baader RGB filters.

Processed in APP, calibrated, normalised and integrated with a 10% stretch. No light pollution removal or other non-linear processing applied.

 

The first sample is with flats, dark flats and dark calibration frames applied (20x each); the second sample is with flats and dark flats but without dark frames. The subs are dithered.

I have to say, I can't see any difference between them, with a slight preference for the one without darks.

 

Also, is this representative of what you would expect with this equipment, on twp clear moonless Bortle 8 nights? Obviously, the noise is carrying through into the rgb-combined image and while it may be down to poor skies, I'm wondering if I have a processing problem.

 

Blue, Green and Ha are similar. I have an obvious problem with the Ha darks, so I've definitely done something wrong there (walking noise patterns).

 

With darks...

 

Without darks....

The noise is quite normal for RGB filters. Notice how the noise disappears where the signal is strong on the galaxy, most of the noise you see in the background will be reduced with more data. As for the walking noise, that's not a dark problem, but a dithering problem. With any NB filter, I would suggest dithering every frame. I would also increase your calibration frames from 20 to 30.

Edited April 14, 2021 by AstronomyUkraine

[Padraic M]

9 hours ago, MarkAR said:

Adam Block YouTube video

Thanks Mark, I'll look that one up.

My understanding after reading up on it was that darks are required but not biases, as the bias signal is already included in the other calibration frames. 

 

@AstronomyUkraine Interesting that you recommend dithering every frame for NB. I'll retake all calibrations at 30x and redo. See if it improves. It's somewhat reassuring that this is normal noise for the 1600. But, at 2h12m per filter already plus 1hr Ha, I'm not sure that I'll ever get more data on a target! Too much to see in the skies.

 

I'm not convinced yet that the Ha nose is related to dithering - it stacks fine without darks. When I look at the individual dark subs, some of them are surprisingly bright so I may have a light leak or a bad setting somewhere. I'll redo and retry. Next time I'm imaging I'll add more dither.

[AstronomyUkraine]

31 minutes ago, Padraic M said:

Thanks Mark, I'll look that one up.

My understanding after reading up on it was that darks are required but not biases, as the bias signal is already included in the other calibration frames. 

 

@AstronomyUkraine Interesting that you recommend dithering every frame for NB. I'll retake all calibrations at 30x and redo. See if it improves. It's somewhat reassuring that this is normal noise for the 1600. But, at 2h12m per filter already plus 1hr Ha, I'm not sure that I'll ever get more data on a target! Too much to see in the skies.

 

I'm not convinced yet that the Ha nose is related to dithering - it stacks fine without darks. When I look at the individual dark subs, some of them are surprisingly bright so I may have a light leak or a bad setting somewhere. I'll redo and retry. Next time I'm imaging I'll add more dither.

The darks should have a uniform brightness. The main thing with darks is taking them at the same temperature, exposure length and gain settings as the light frames. Flat frames can be taken at any temperature. I usually cover the end of my scope with a dark cloth or a coat just in case any light is leaking through the scope cover.

Edited April 14, 2021 by AstronomyUkraine

[MarkAR]

On 4/14/2021 at 1:07 AM, AstronomyUkraine said:

You do need darks with the 1600. You don't need bias frames, use flat frames instead.

Thanks for the correction, I alway seem to get that the wrong way round just going from memory.