Seeking advice on lowering the odds-to-solve value

Skip to first unread message

Prajwel Joseph

Nov 3, 2023, 3:36:47 AM11/3/23
to astrometry
Dear Dustin,

   I am running a local installation of software with custom Gaia index files. The intention is to solve astrometry for many far-ultraviolet images (with ~1'' FWHM). While we get successful WCS solutions for all our test cases using your fantastic software, I would like to make further improvements.

I went through the article. After reading the section "Verification of Hypotheses", I think modifying the odds-to-solve could be possible for the following reasons.
  1. We know the (RA, Dec) pointing information. Our FOV is half a degree in diameter. For the solve-field run, a 1-degree search radius centred on the known pointing information is used. 
  2. We also know the pixel scale very well.  
This being the case, how low can be a conservative value for odds-to-solve? I have presently lowered it to 1e6 from 1e9.

Thank you for any help.

With regards,

Dustin Lang

Nov 3, 2023, 10:16:13 AM11/3/23
to Prajwel Joseph, astrometry
Hi Prajwel,

You might find that the "6100-series" index files are perfect for your needs.  They are built from a GALEX catalog, and the scales (4-6) should be well matched to a half-degree field.

As for your question, one thing you can do is turn on the "--verbose" option to solve-field, and then it will print lines like

object 1 of 1000: 0 quads tried, 0 matched.
object 2 of 1000: 0 quads tried, 0 matched.
Got a new best match: logodds 2.99069.
object 3 of 1000: 5 quads tried, 37 matched.
object 4 of 1000: 16 quads tried, 149 matched.
object 5 of 1000: 28 quads tried, 189 matched.
Got a new best match: logodds 3.12527.
Got a new best match: logodds 3.42611.
object 6 of 1000: 64 quads tried, 441 matched.
Got a new best match: logodds 9.32062.
object 7 of 1000: 133 quads tried, 1155 matched.
object 8 of 1000: 201 quads tried, 1596 matched.
Got a new best match: logodds 65.8437.

the "object"s here are stars, so this is saying that, for example, after looking at the 8th star ("object 8"), solve-field formed 201 quadrangles of stars in the image, and found 1596 matches.  I would suggest looking at what those numbers are like for your example images.  If it matches 1,000,000 quadrangles, then a 10^6 odds ratio can easily give you a false positive result.  The other thing you can do is look at the lines saying "Got a new best match: logodds 9.32" etc... this is telling you the best matches found so far.  Usually these numbers will climb slowly as more-and-more convincing false positives are found.  It should give you an idea of what the best non-match is.


You received this message because you are subscribed to the Google Groups "astrometry" group.
To unsubscribe from this group and stop receiving emails from it, send an email to
To view this discussion on the web visit
Reply all
Reply to author
0 new messages