Author Topic: Strange background noise  (Read 537 times)

Offline agengard

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Strange background noise
« on: 2019 May 08 00:57:47 »
Hi all!

I have a small problem and I can’t figure out the cause. Appreciate any help or suggestions.

During three consecutive nights in April, I shot M13, The Great Globular Cluster in Hercules, using my EdgeHD 8” f/7 and ASI294MC. I used a Baader UV/IR-cut filter, 60 seconds exposures at Unity Gain and -20 degrees. I used my usual Superbias, matching MasterDark and MasterFlat from the first night (using my LED-panel). I did not Dither.

I used Blink to sort out less good subs and subs with sat trails, and the Subframe Selector Process to weigh the better ones after FWHM, SNR and Eccentricity. I then used the best sub for registration and integration. It happened to be a sub from the second night.

April 21 I got 168x60sec exposures, April 22 - 240x60sec and April 23 - 219x60sec.

Using the best sub, mentioned above, as reference during stacking subs from any ONE of the nights, results in a photo with a reasonably smooth background and good looking stars. However, any combination of TWO or THREE nights results in a photo with lines in the background in a 45-degree angle. Still the stars look good.

The settings I use are mostly PixInsight Default, and they are also the settings I always use, most times with good result from a novice’s point of view.

In the jpg below one can see the difference between my integrations.

Here are links to XISF-versions. They are just integrated and cropped, no further development.

One night integrated:

Three nights integrated:

Is there an obvious reason to these lines? Is it a no-dither-multi-session-issue? I have stacked multi-sessions before without this phenomenon…

Below is a link to the same corner of my photo of M51 from the week before. Shot with the same equipment except for the filter. My development process was the same, however I used dithering, longer subs, LPS-filter and a few hours longer integration time. It consists of 233x180sec and 89x120sec.


Offline Greg Schwimer

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Re: Strange background noise
« Reply #1 on: 2019 May 08 07:49:34 »
This is likely due to not dithering or inadequate dithering. I think it is called walking noise. If you blink through the images in the sequence they were taken do you see evidence of drift in the direction of the noise?
- Greg
Scottsdale, Arizona, USA

Offline wvanreeven

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Re: Strange background noise
« Reply #2 on: 2019 May 08 07:52:13 »
If dithering was always done in the same direction then that could still account for the noise as well. In general dithering should be done in a general direction every time of course but perhaps the software used for guiding doesn't do that (possibly due to a bug).


Offline agengard

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Re: Strange background noise
« Reply #3 on: 2019 May 09 02:16:20 »
Greg and Wouter... Thanks a lot!

I did a web-search for “walking noise” and found several threads with images that show identical issues. My problem is clearly due to not dithering at all and a just little less than perfect PA.

When blinking the subs with the shortest time available, I notice the noise seems to creep in the direction of my lines. So I’m convinced this is the reason behind the problem.

Checking my one-night-integrations, I can see lines appear also there, but very faint. Of course, doubling or tripling the number of frames make the problem worse.

I never thought dithering was THAT important if one used enough Dark-corrected subs…

Thank you much for your help!