12 replies to this topic

[Stylistic007]

I don't know what has caused this noise...

 

It's not the darks, bias, nor flats....I processed with alternates and still the same results.

 

Only difference is that I did not dither during my session.  Could it be this?

 

 

Images attached:

C9 Cave nebula

15 sec subs 

Stacked with background extraction

Auto stretch applied

 

a 1132.jpg = 1132 images stacked 

b 1-300.jpg = subs numbered 1-300 stacked of the 1132 original images

c........you get the point.

 

If you scroll through them, it looks like the noise is rotating clockwise....I just don't understand.

Attached Thumbnails

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Edited by Stylistic007, 10 May 2025 - 11:33 AM.

[bobzeq25]

Normal "fixed pattern noise" from the camera. It is a result of not dithering and an uncooled camera.

Autostretched with what? DSS does a pretty awful job, Pixinsight is pretty harsh if you transfer the ScreenTransferStretch to HistogramTransformation.

Edited by bobzeq25, 10 May 2025 - 11:21 AM.

[Stylistic007]

Here is the full view from the cropped selections above.

 

1132 15 sec subs stacked

Attached Thumbnails

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[Stylistic007]

Normal "fixed pattern noise" from the camera. It is a result of not dithering and an uncooled camera.

Autostretched with what? DSS does a pretty awful job, Pixinsight is pretty harsh if you transfer the ScreenTransferStretch to HistogramTransformation.

Thank you so much! Auto stretch from Siril. 

 

I only autostretched to emphasize the noise.

[Tapio]

Yes, it's 'dither or die' .

[Stylistic007]

On that topic....how often should I dither?

 

My default is 15 second lights and dither every 5 shots by 1pixel (on ASIAIR).

 

Should I view dithering in terms of exposure time, or frames taken?.....just curious as I did not realize how much dither has saved my other sessions.

[EPinNC]

I'm not doubting that it is walking noise, but this is the first time I've ever seen an example of a circular pattern.  I see that the mount used was a CEM40.  If the mount was an alt-az mount, could field rotation produce such a pattern?

[ngc7319_20]

I'm not doubting that it is walking noise, but this is the first time I've ever seen an example of a circular pattern.  I see that the mount used was a CEM40.  If the mount was an alt-az mount, could field rotation produce such a pattern?

 

Yes it is caused by field rotation.  Probably operating in Alt-Az mode.  Or rotation around guide star, if the polar align is off.

 

I would dither by more than 1 pixel, since hot pixels will sometimes affect neighboring pixels.  Dither on both axes.  

 

How often to dither will depend on how long it takes you to move vs. exposure time.  Don't want to spend all your time moving.  I guess I would move before the "fixed pattern noise" becomes visible above the other noise sources.

[Stylistic007]

I was imaging equatorally(?).  

 

My guiding was around 1" total.

 

This target was the closest to Northern the polar axis I have attempted.

 

The following images are cropped corners. Red outline is reference image.  As you can see there is rotation... but I don't know how to interpret these images:

(any comments welcomed)

 

Images are frames 1, 200, 400, 800, and the last sub 1132:

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Edited by Stylistic007, 10 May 2025 - 01:00 PM.

[bobzeq25]

It really doesn't matter. You need to dither. Getting a cooled camera would also be a good idea.

[Stylistic007]

Ya. I will dither from now on.  I didn't need cooling till about now.  I'm in Saskatchewan and my sensor temps were around ambient -30C to -6C.  

 

These images are around +6C.  I'm getting a "Phone Cooler" to attach to my ASI585MC for now (Nubia RedMagic 6 Pro Magnetic Cell Phone Cooler).

 

I'm still contemplating a mono set up.  

[flappah]

On that topic....how often should I dither?

My default is 15 second lights and dither every 5 shots by 1pixel (on ASIAIR).

Should I view dithering in terms of exposure time, or frames taken?.....just curious as I did not realize how much dither has saved my other sessions.

I’ve got my dithering set to at every three subs at a distance of 5 pixels. Used to do every 5 at 10 pixels but the 3 at 5 pixels gives me a slightly better result. I’m shooting with both the asi533mc-pro and asi585mc-pro camera at -10C and gain 100 (for the 533) and gain 252 (for the 585).

[sharkmelley]

Yes it is caused by field rotation.  Probably operating in Alt-Az mode.  Or rotation around guide star, if the polar align is off.

Agreed.  Field rotation around the guide star causes this.