Please help me understand the star generator

[bretm]

I am working on a horsehead mosaic, and it seemed like a good time to try out the star generator.  I have generated an "image", and the StarAlignment process claims it finds 0 stars.  Figuring I had some bad setting, I reset all the Star Alignment settings with the little "Reset" button on the lower right, still with no success.

I have put a copy of the generated field here: https://www.dropbox.com/s/b8buj6gw24187fg/registration-reference.zip

It you unpack the image, and try to use Star Alignment to match it against itself, you should see the issue. I'm running V 1.8, RC7.

Bret

[bretm]

As an experiment I fed the image to astrometry.net, and it could solve it...

[pfile]

well, i guess you'll have to fiddle around with the star detection parameters in SA. i am not sure what the profile of the stars looks like coming out of StarGenerator but if they are too flat SA probably won't recognize them as stars.

rob

[Juan Conejero]

Hi Bret,

This is a limitation of the current star detection routine: it cannot work with images whose median values are zero or very close to zero, such as synthetic star fields where the background is pure black. This problem is now fixed and won't happen in  the final 1.8.0 version.

While we release 1.8.0 Ripley final (two-three days), this is the workaround:

- Duplicate your star field image.
- Set the duplicate as an inverted mask for the original.
- Open the NoiseGenerator tool (Noise Generation category in Process Explorer).
- In NoiseGenerator, uncheck the Preserve Median option.
- Apply to the star field.
- Remove the mask.

Now you have added some background noise, so the star field looks like a "natural" image. StarAlignment will work without problems with it. However, once this problem is fixed star detection will be much better, since the noise generated with the above procedure degrades the star field somewhat.

Sorry for the inconvenience.

[Andres.Pozo]

You can use also the script CatalogStarGenerator which has a larger set of parameters and catalogues, including the option of adding noise to the image.

[Juan Conejero]

In fact, I strongly recommend you use Andres' CatalogStarGenerator instead of StarGenerator, since the latter is less powerful and is now quite outdated.

[bretm]

Thanks everyone - CatalogStarGenerator worked as expected.

Bret