Hi,
let's get you started with the Blind Solver. In this article, I want to introduce you to the software that I use behind the scenes of the PI script (and that I did
not write, I just use it in my script).
The Blind Solver is a product of a university project (of which I was
not part), documented on
http://astrometry.net/. If you are interested in how this software works, an article
http://arxiv.org/PS_cache/arxiv/pdf/0910/0910.2233v1.pdf published by the project is probably the best introduction. Dustin Lang's thesis
http://www.astro.princeton.edu/~dstn/ is an extended version of that. In essence, the Blind Solver works by creating a large database containing relative positions of groups of 4 stars (asterisms). When given an astronomical image, the Blind Solver identifies bright stars, and queries the database for the asterisms of those stars. Using the asterisms as fingerprints, it reduces possible positions/scales/rotations in the sky to a few candidates that can be validated using the other stars in the image. It can do so without knowing scale or rotations, because asterisms are invariant to these factors.
The result of the solver is center position, rotation, scale of the solved image, plus some more obscure information about the WCS coordinate system
http://www.atnf.csiro.au/people/mcalabre/WCS/. This information can ultimately be used to label a star image, as I do in my application.
As far as I know, this capability is quite unique. Most of the astrometric tools I found on the web are only for "refining" already known star positions. Searching for more general tools, I found the PinPoint Astrometric engine
http://pinpoint.dc3.com/ which is a commercial product. And I found the Elbrus Star Locator
http://www.astrosurf.com/pulgar/elbrus/elbrusin.htm, which is open source. Both appear to require approximate information about image scale and rotation. When I tried Elbrus Star Locator, I could not convince it to solve my images. (I would be interested to hear if there are other programs solving this problem...)
Compiling the Blind Solver and installing the database requires some work. For those of us who want to give the Blind Solver a quick try, there are two options:
1. Register for the alpha version of the web service, as explained on
http://astrometry.net/use.html, or
2. Upload your image to the astrometry group on Flickr
http://www.flickr.com/groups/astrometry/ . The photostream of this group is screened by the astrometry.net Blind Solver web service, and annotations are added to your images automatically (sooner or later).
This, of course, is no option if you want to solve large amounts of images as I did for my Milky Way film
http://pixinsight.com/forum/index.php?topic=1853.0, or if you want to use the Blind Solver to find the right position in the sky with your telescope. For that, you need the locally installed version of the Blind Solver, which usually solves an image within seconds. In the next forum article (maybe tommorow), I will give you some hints on how to install the Blind Solver software on Fedora 11 (which may also be helpful for other UNIXes).
Cheers,
Georg
