Just a question about the .fits files.

[Bill Gaylord]

Sorry If I sound like a complete idiot but how exactly do I make use of the .fits files. I know some types of .fits files are basically images while others hold data. 

I was just asking because I recently found Astrometry while trying to figure out how to tell what stars were in the pictures of the night sky I took. (Using a pretty old (7 years) Canon Camera).

 

[Dustin Lang]

Hi,

Right, some are FITS images, some are FITS tables, and some are just FITS headers.  On the nova.astrometry.net site, if you look at a solved image, for example, this recent one,
http://nova.astrometry.net/user_images/1770829

the "new-image.fits" link is a FITS image with a WCS header.  If you open that up in the SAO ds9 program (http://ds9.si.edu/site/Home.html), you'll get RA,Dec coordinates along with x,y coordinates as you move around the image.

The "wcs.fits" file is a FITS header -- 80-character lines of text describing the WCS (World Coordinate System), or mapping between x,y and RA,Dec.

The "rdls.fits" and "axy.fits" files are FITS tables, listing RA,Dec coordinates of reference-catalog stars, and X,Y coordinates of stars detected in your image, resp.  They can be viewed using the "tablist" program (included in the Astrometry.net source code), or read in using your favorite programming language.  The "corr.fits" is a table listing stars that we think match between your image and the reference catalog.

Hope that helps!
--dustin

[Bill Gaylord]

Thanks for the quick reply.  btw astrometry.net is an awesome site.  :D Very usefully for figuring stuff out when your just pointing the camera up and taking pictures.  

And okay. I found astropy for python seems to be a nice utility for reading them. 

[Bill Gaylord]

Tho I have another question are the ordering of the stars / objects the same between the tables?   

[Dustin Lang]

Nope -- rdls.fits are all objects from the reference catalog that would be within your image.  axy.fits are sources we detected in your image.  These two files are not matched up or ordered in any specific way (as far as I can remember!).  The corr.fits file ("correspondences") lists stars that we think match.

cheers,
--dustin

[Bill Gaylord]

So I am better off using the anotations part of the api to get the locations of all stars that the system labeled? (As the axy and corr only show the stars with the green circles when looking at the red-green view.)

[Dustin Lang]

axy are the red stars (detected in your image)
rdls are the green stars (reference catalog)
corrs are red + green matched stars (pair we think matched)

cheers,
--dustin

[Bill Gaylord]

Oh. Okay that makes more sense now, Thank you.