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PostPosted: Sun, 23-10-16, 17:38 GMT 
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Hi all,

thanks to Andrew's great Python scripting code the extraction of a 2 million sample (Gaia Data Release1 = DR1) of Gaia stars from the official data base is now available for celestia.Sci..

Unfortunately, by looking at the star distribution within celestia.Sci, I noted a nasty stripe pattern, which I thought should be also communicated to interested users.

Here is how it looks like:
[Click on image for obtaining a proper size]
Attachment:
2M_Gaia_MW_artefacts.jpg
2M_Gaia_MW_artefacts.jpg [ 226.52 KiB | Viewed 446 times ]


In case anyone has a sound recipe of how to eliminate the stripes, please let us know!! But clearly, scientists are usually not blind and thus these artefacts should be already discussed in the respective scientific literature.

Indeed....

The renowned journal Astronomy & Astrophysics (A & A) just released a special volume with 6 articles entirely devoted to Gaia data! Fortunately, 4 of the 6 papers can be accessed freely:
Here is the list of download links:

1) http://www.aanda.org/articles/aa/pdf/fo ... 990-16.pdf
2) http://www.aanda.org/articles/aa/pdf/fo ... 272-16.pdf
3) http://www.aanda.org/articles/aa/pdf/fo ... 512-16.pdf
4) http://www.aanda.org/articles/aa/pdf/fo ... 534-16.pdf

For any user with a desire to "dig deeper" I strongly recommend to display link 3)
and there to focus on chapter 6: Known limitations of Gaia DR1

No surprise, there is a corresponding image display (Fig. 11) with this figure caption:

Gaia DR1 source density distribution on the sky in the direction of the Milky Way bulge region. Note the prominent “striping” and the gaps in the source distribution.
Looking just like my "discovery" above.

Attachment:
gaia_stripes.jpg
gaia_stripes.jpg [ 60.75 KiB | Viewed 445 times ]

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In chapter 6.2 you will find a discussion about the origin of the phenomenon. I don't think I'll have to copy it for you here...
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We'll keep you informed about progress concerning this issue.

Fridger

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PostPosted: Sun, 23-10-16, 20:51 GMT 
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A common cause of visual striping in 3D datasets is to specify positions with limited precision, which can be reduced by adding random offsets.

That doesn't seem to be the case here, though.

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PostPosted: Sun, 23-10-16, 23:16 GMT 
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that looks like the Venus radar data but in a 3d point cloud

unfortunately the fix i used will not do for this

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PostPosted: Mon, 24-10-16, 13:23 GMT 
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Well, what I'd "pull first out of my pocket" concerns Fourier Transform (FT) methods. Further below, I will illustrate the particular power of using FT transform pairs for removing periodic structures with an instructive 2D example that I came across in the net. http://www.robots.ox.ac.uk/~az/lectures/ia/lect2.pdf

What's the trick? FT maps periodic signals into needle fine peak patterns, a dominating primary peak along with a number of secondary ones. Of course the primary peak position in Fourier space corresponds to the frequency of the periodiicity in real space.This fact guarantees that the modification of unwanted periodic stripe patterns (Gaia!) can be done with minimal amount of distortion of other non-periodic data!

Now look at the mentioned 2D example ("Forensic application") that should essentially be self-explaining:
[click on image by all means for an adequate size]
Attachment:
FT_example.jpg
FT_example.jpg [ 87.97 KiB | Viewed 413 times ]

Start with the top-left image that features (probably;-)) a woven table cloth with a doubly periodic weaving pattern along with a stain due to a finger print. After 2 dimensional FT, one obtains the red image on top right ( in Fourier space). Note the set of dominating very narrow peaks that now comprise ALL data associated with the original periodic background. The rest of the data is smeared out and thus entirely different in structure from the isolated peaks. What remains is just to remove the narrow peaks in Fourier space (see the red image on bottom right) and to transform the slightly modified data back into real space. The result looks pretty good (image on bottom left). The periodic background is entirely gone.

But for the Gaia DR1 stripe issue there is much more one will have to investigate, since there is always the aspect of positional star precision that will continuously compete with any such modification. Note also that we have a 3D problem with Gaia, not 2D as in the example.

So far I consider these FT activities more like a game one could play with. But I don't take them seriously for various reasons....

Anyway, let's see what Andrew's ideas look like...

Fridger

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