Preprocessing artifacts

Image processing, astrometry, photometry, etc.
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Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Preprocessing artifacts

Post by Forum_2018 »

i've noticed repeatedly after preprocessing the resultant image looks like the pixel arrangement is always diagonal from bottom left to top right - see attachment
Is this a result of the stacking algorithm and should I be worried?

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

No one...not even Fabio? :(

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

Hello, I missed the message.
The cause seems to me an imperfect dark frame correction, which gets a diagonal shape cause the auto-alignment of several images. If you orient the camera with the equatorial axes the noise should get a horizontal shape.
Remember that the master dark frame must be at least the average of 4 frames (8 or more is better).
If you are using a bias as a dark frame, then again you should average several frames.
The image also seems extremely oversampled: you may work in binning 2x2 without losing any information.
Greetings,
Fabio.

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

Here is a photo from a book about astro photography by Ron Wodaski that shows similar artifacts due to stacking very short sub exposures!
Iver

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

Thanks Fabio. I was using multiple dark frames. What do you mean by over-sampled?

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

thanks Iver.

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

An image if oversampled if it contains "too many pixels" (or too high resolution) for the information that it really contains, see:

https://www.cloudynights.com/topic/3009 ... -an-image/

A test:

1) Open (in any graphic software) a good image.
2) Resize it to 50%.
3) Now resize it to 200%.

the result will have the same size, but it will be a blurred image, because step (2) decreased the information of the image.

However, if at the step (1) you start with an oversampled image (e.g. blurred from the beginning) then after those steps, the result is as good as the original.

This means that an oversampled image can be halved in size safely (e.g. work in binning 2x2) and processed at this size, without losing any information. Then, if needed it can be doubled in size as a last step.
Working on oversampled images is a waste of processing time and/or disk space.

This is the same thing of "megapixel myth" (Google it) that affected consumer cameras in the last years.

Forum_2018
Posts: 263
Joined: 17 Dec 2018, 14:45

Re: Preprocessing artifacts

Post by Forum_2018 »

It is commonly called "walking noise" Try dithering your exposures and make sure you are well polar aligned.

John
CCD-Freak

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