Is this walking noise?

[rohanpai]

Need some guidance here..

I got about 2 hours of data in total with Ha and Oiii filter. I had good polar alignment, guiding was going well and I was dithering after every image.

I stacked it all this morning with darks and flats and the image has walking noise.

If I am dithering, should it not eliminate the walking noise? Can this be due to dew?

 

[ollypenrice]

To my eye that looks electronic in origin rather than optical, so I'd put dew at the bottom of the list of suspects. Things to look into would include,

1) Do you see any trace of this type of noise in single subs?

2) Try stacking without darks, then without darks or flats, to see if they are introducing the noise. They may be.

3) Check your stacking parameters. Try different ones, ie Sigma, Median and Average.

4) Try a different stacking program. (You can download AstroArt for trial. It won't save but it's a great stacking-calibrating program.

There doesn't seem to be any very obvious object in the image. The stars are tight but what were you imaging? (My thought is that the image shown might have been stretched beyond its limit in search of an object which isn't there.)

Olly

[wimvb]

4 hours ago, rohanpai said:

If I am dithering, should it not eliminate the walking noise?

That depends. If you have dec backlash, then there may not be enough random movement to eliminate the drift pattern.

If you stack the subs without pixel rejection, you will see hot pixels following  the dither pattern.

In pixinsight you can blink the unaligned images to see the amount of dithering and drift.

Edited May 7, 2022 by wimvb

[david_taurus83]

What equipment did you use? Even with dithering it won't eliminate all noise. I notice it more in the blue channel if I use my DSLR. 2 hours isn't a lot of integration either for narrowband data.

[rohanpai]

5 hours ago, ollypenrice said:

To my eye that looks electronic in origin rather than optical, so I'd put dew at the bottom of the list of suspects. Things to look into would include,

1) Do you see any trace of this type of noise in single subs?

2) Try stacking without darks, then without darks or flats, to see if they are introducing the noise. They may be.

3) Check your stacking parameters. Try different ones, ie Sigma, Median and Average.

4) Try a different stacking program. (You can download AstroArt for trial. It won't save but it's a great stacking-calibrating program.

There doesn't seem to be any very obvious object in the image. The stars are tight but what were you imaging? (My thought is that the image shown might have been stretched beyond its limit in search of an object which isn't there.)

Olly

Hi Olly

1) Do you see any trace of this type of noise in single subs?
>> Yes. I can see some on a single sub as well. I have attached a single sub with Ha filter and Oiii filter.

2) Try stacking without darks, then without darks or flats, to see if they are introducing the noise. They may be.
>> I tried this and got the same result. The stacked image shows the same noise pattern

3) Check your stacking parameters. Try different ones, ie Sigma, Median and Average.
4) Try a different stacking program. (You can download AstroArt for trial. It won't save but it's a great stacking-calibrating program.
>> I tried Siril and DSS and PixInsight. All 3 gave similar results

I was imaging the Dragons of Ara (NCG 6188). I did a auto stretch in Siril and PixInsight after stacking and saw this noise. Even with manual stretch, I could see the noise start to show up quickly.

Oiii filter.fit ha filter.fit

[rohanpai]

5 hours ago, david_taurus83 said:

What equipment did you use? Even with dithering it won't eliminate all noise. I notice it more in the blue channel if I use my DSLR. 2 hours isn't a lot of integration either for narrowband data.

HI David

I will imaging using a WO ZS61 and ZWO 1600mm.

I gathered another 2 hours of data today. Stacked it all and see the same noise pattern on today's stack as well.

[rohanpai]

5 hours ago, wimvb said:

f you stack the subs without pixel rejection, you will see hot pixels following  the dither pattern.

Hey mate

Not sure how I can stack without pixel rejection. Can you help with what settings to use in DSS or Siril or PixInsight? I've got all 3 running at the moment ..

[michael8554]

How many pixels are you dithering the ASI1600MM ?

12 pixels is a common figure.

The dither needs to be in imaging cam pixels.

With some imaging software it might be in guidecam pixels, requiring a calculation to give the correct dither for the imaging cam.

Michael

[alacant]

56 minutes ago, rohanpai said:

Not sure how I can stack without pixel rejection. Can you help with what settings to use in DSS or Siril

You need to stack with pixel rejection. Siril has many options:

Here are your aligned frames showing the walking pixels:

Edited May 7, 2022 by alacant

[wimvb]

1 hour ago, rohanpai said:

PixInsight? I've got all 3 running at the moment ..

When you open the image integration process, under "Pixel Rejection 1", "no pixel rejection" should be the default setting. If not, just select it

[wimvb]

52 minutes ago, alacant said:

You need to stack with pixel rejection

Normally, I would agree. But in this case the OP needs to diagnose possible problems with dithering. Following hot pixels across the image is a way to do that. For that, he needs to stack without rejection.

[wimvb]

If dithering works, you should see a hot pixel distribution similar to this. If the hot pixels are along a line, you can expect walking noise.

[wimvb]

 

To decrease the effect of walking noise /outliner pixels, apply cosmetic correction on the calibrated images prior to integration in PI.

Cosmetic Correction is under the "Image callibration" section of processes. Tick the check box "Use Auto Detect" and "Hot Sigma". Lower the Hot pixel rejection slider to 2.

Save an instance of CC to the workspace and use it during the BatchPreprocesseing script. I don't know if it is available in WBPP.

2 hours ago, rohanpai said:

>> Yes. I can see some on a single sub as well. I have attached a single sub with Ha filter and Oiii filter.

Sorry, I don't see it in a single sub

Ha

OIII

Edited May 7, 2022 by wimvb

[rohanpai]

On 08/05/2022 at 01:36, wimvb said:

If dithering works, you should see a hot pixel distribution similar to this. If the hot pixels are along a line, you can expect walking noise.

I used sum stacking and I don't see any hot pixels along a straight line

I was dithering using a asi120mc-s with OAG. I can try dithering to 12pixels and see if tht makes any difference.

 

 

 

[rohanpai]

On 08/05/2022 at 00:16, michael8554 said:

12 pixels is a common figure.

I have this as my settings in PHD2. I am not sure if PHD2 will do the minimum configured move and then go ahead with the next capture. Should I try changing this?

My imaging camera is a asi1600mm with 3.8um pixel size and guide camera is a asi120mc-s which is 3.75um pixel size.

[wimvb]

1 hour ago, rohanpai said:

I have this as my settings in PHD2. I am not sure if PHD2 will do the minimum configured move and then go ahead with the next capture. Should I try changing this?

My imaging camera is a asi1600mm with 3.8um pixel size and guide camera is a asi120mc-s which is 3.75um pixel size.

I believe that dither settings are under the "Guiding" tab. The Minimum Move setting here is only for guiding, not for dithering. 

Edited May 9, 2022 by wimvb

[alacant]

6 hours ago, rohanpai said:

don't see any hot pixels along a straight line

?

There are hundreds thousands of them. Here are a few of the brighter ones:

[michael8554]

 

PHD2 responds to Dither commands from external software.

The external dither setting may already be in imaging camera pixels.

Or it may be the amount the guidecam is being dithered, requiring an adjustment to the PHD2 multiplication settings to translate into image camera pixels.

Believing the Algorithm settings have anything to do with Dithering suggests you need to step back and read the PHD2 Instructions.

Michael

[rohanpai]

15 hours ago, alacant said:

There are hundreds thousands of them. Here are a few of the brighter ones:

Thanks. I was expecting them to be moving much more

I'll try different dither settings next time on the same target.

Weird enough, I used the same setup and setting to image the Carina nebula and this noise pattern didn't occur. The only difference was the sensor temperature (i wasn't cooling the camera for both sessions - so Carina was about 15C and Dragons of Ara was about 11C) and I did not have my dew heater strapped around the OTA for Carina.