Output Files and Stellar Aberration

[David Wyrick]

Hola,

I was successful compiling astrometry.net on cygwin! Hurrah! Now, I have a question about the output files.  

<base>.wcs a FITS WCS header for the solution.

<base>-indx.xyls a FITS BINTABLE with the pixel locations of stars from the index.

<base>.rdls a FITS BINTABLE with the RA,Dec of sources we extracted from the image.

<base>.axy a FITS BINTABLE of the sources we extracted, plus headers that describe the job (how the image is going to be solved).

<base>.match a FITS BINTABLE describing the quad match that solved the image.

One thing I was noticing when plotting/comparing the pixel locations of the stars identified in the index (given in <base>-indx.xyls) with the pixel locations of the quad match (given in <base>.match) was that there was an offset between them. Are the pixel locations of the index not corrected for stellar aberration and earth wobble effects? Additionally, I used wcs-rd2xy to convert the RA/DEC locations of the sources (given by <base>.rdls) to xy pixel locations and it is exactly the same as <base>indx.xyls. The file <base>.axy contains all of the sources extracted, including the stars described in <base>.match. So, a few questions.

1) Does Astrometry.Net correct for stellar aberrations when calculating the xy pixel location of the index locations and/or quad match pixel locations?

2) Does wcs-rd2xy consider stellar aberrations/earth wobble effect by looking at the wcs header you give it? 

3) Is there a detailed description of what the contents are in each of these FITS files? When I read <base>.match into matlab, it's read in as a 1x35 cell array with seemingly random numbers/format. The other ones are more straight forward. 

Thanks for anyone that can help!

-David

[David Wyrick]

Furthermore...

in the <base>.corr file, which I assume from the name correspondence, relates the RA/DEC and XY pixel location values of the index star positions to that of the stars detected in the image. The thing that is confusing me is, if indeed a transformation between RA/DEC of the index stars onto the FPA was calculated, is why the stars detected in the image have the same RA/DEC values as the index stars, but NOT the same XY positions... Shouldn't there be a difference in RA/DEC values as well due to stellar aberration and earth wobble effects? I calculated the differences in RA/DEC of the index stars after applying corrections for stellar aberration and EOP. I found differences of 45 microradians in RA and 27 microradians in DEC, yet the PIXEL offset of the index stars to the stars detected in the image are not that large. (taking the pixel distance * IFOV ~ 3microrad)

So another few questions,

4) Any ideas for this discrepancy?

5) Does solve-field need a header with time/date values if you are solving .fits files? I'm taking binary data and simply writing that to a fits file with no header from matlab to run Astrometry.net. I do have time/date information though.

Thanks again!

[Dustin Lang]

Hi,

All our astrometric results are in J2000, ie, the coordinates of the reference catalog (2MASS/Tycho2 probably).

Does that explain what you're seeing?

cheers,
--dustin

[David Wyrick]

Dustin,

I figured that out. What is still confusing is why the stars detected in the image have different RA/DEC values than the those given for the index stars? By only 3 microrad or so. What causes this difference? 

So, just to confirm, you don't consider any stellar aberration effects?

Cheers,

David

[Dustin Lang]

In the corr files, you will find that the pixel and RA,Dec positions of the image and reference stars aren't exactly the same -- there isn't an *exact* mapping from image pixels to RA,Dec that applies for every star in the image; it's a least-squares problem.

Also, we don't take proper motions of stars into account; these are J2000 positions.

Correct, we don't do anything about stellar aberation.

cheers,
--dustin

[David Wyrick]

Dustin,

Alright, thank you for confirming my suspicions. And thank you for creating Astrometry.Net. Absolutely wonderful work. 

Cheers,

David