The Astrometry.net Local Solver

Phoranso supports plate solving through a local installation of the well-known Astrometry.net solver, called ansvr, that runs entirely on your own Windows PC. Once installed, Phoranso can solve your science images directly on your machine, attaching a complete World Coordinate System (WCS) to each frame without ever contacting an online service.

What is the Astrometry.net local solver?

The Astrometry.net local solver (ansvr) performs the plate solving task using the same engine as the popular nova.astrometry.net online service, but as a self-contained server running locally on your computer.

Using a local solver in Phoranso brings several practical benefits:

• No internet connection required. All solving happens on your PC. This is invaluable at remote observatory sites with poor or no connectivity, and it keeps your image data entirely private.
• Blind solving. ansvr can identify a field with no prior knowledge of where the telescope was pointing, so you do not need to supply approximate coordinates. It recognises the field purely from the pattern of stars it contains.
• Full WCS with distortion correction. In addition to the basic plate solution, ansvr writes SIP (Simple Imaging Polynomial) distortion coefficients into the FITS header. These higher-order terms model the optical distortion of your system, so that the pixel-to-sky mapping remains accurate right out to the corners of the field — not just near the image centre. Accurate WCS and SIP information is the foundation for reliable identification of comparison stars and targets in Phoranso.
• Speed and reliability. Because there is no upload or queue, solving is fast and is never affected by the availability of an external server.
• Free and open. ansvr and the underlying Astrometry.net engine are free to use.

Credits. ansvr (the Astrometry.net Local Solver for Windows) is developed and maintained by Andy Galasso of adgsoftware, and packages the Astrometry.net plate-solving engine for convenient use on Windows. We gratefully acknowledge his work, without which this feature would not be possible. The installer, change log and original documentation are available at https://adgsoftware.com/ansvr/. The Astrometry.net engine itself is developed by the Astrometry.net team (astrometry.net).

Installing the Astrometry.net local solver

The Astrometry.net local solver ansvr is installed once. The procedure has two parts: installing the solver itself, and then downloading the astrometric index files that it uses to recognise star fields. The index file download is the most important — and most often misunderstood — step, so it is described in detail below.

Step 1 — Download and run the installer

1. Go to https://adgsoftware.com/ansvr/ and download the latest installer (named ansvr-setup-x.xx.exe; at the time of writing this is version 0.22).
2. Run the downloaded installer. 
3. The below window appears. Accept the default install location, or choose another folder, and click Next.
   
   
4. Accept the default Start Menu folder and click Next.
   
   
5. Accept the default port number (8080) and click Next. This port number setting doesn't affect Phoranso's usage of ansvr.
   
   
6. Click Install in the next window
   
   
7. Installation will begin
   
   

During installation the progress bar will appear to freeze around the halfway point for a considerable time. This is normal — the solver and its supporting environment are being extracted in the background. Be patient and let it finish.

7. When extraction completes, click Finish. You can leave both checkboxes ticked; this launches the index file downloader.
   
   

Step 2 — Download the index files

Index files are catalogues of star patterns that ansvr matches against your images in order to solve them. They are organised by the angular size of the field they are designed to recognise, and you only need the ones that match the field of view (FOV) of your own images. Choosing well here saves a great deal of disk space and download time, because the narrow-field index files are very large.

In the Index Manager:

1. Select the index file group(s) that bracket the field of view of your camera and telescope combination. 
   
   Not sure what your field of view is? Open one of your FITS images in Phoranso. Right click anywhere within the image and select Image statistics. This opens a window displaying your image dimensions in arcmin. For instance, 46 x 36.8 arcmin.
   
   We recommend ignoring the Narrowest field/Widest field size instructions provided by the Astrometry.net Index Manager window. Those are too generous in our opinion, and a narrower slice in practice solves as well. We suggest setting the Narrowest field to roughly 40 - 50% of your narrowest FOV, and the Widest field to 100% (as suggested by the Index Manager)
   
   In our example: Narrowest dimension is 36.8 arcmin, and 50% equals 18 arcmin. Widest dimension is 46 arcmin. Browsing through the Narrowest field sizes, select Index 4206, 16 to 22 arcmin. And for Widest field select Index 4208, 30 to 42 arcmin. 
   
    (Optional) Click the Folder button to place the index files on a drive with plenty of free space. The narrowest-field indexes can total many gigabytes.
2. Click Start. A separate window shows the download progress.
   
   

The download can take a long time. If you cancel it, simply re-run the index file downloader later — it resumes from where it left off rather than starting over.

Step 3 — Confirm that ansvr is running

1. Open Ansvr Settings from the Windows Start menu.
   
   
2. A page opens in your web browser showing the ansvr settings ansvr 0.22 config. If you see this page, ansvr is running correctly. Close the web page.
   
   

The installer places a shortcut in the Windows Startup folder, so ansvr starts automatically every time you log in and is always available in the background. There is no separate program window to keep open. If you ever wish to inspect what the solver is doing, choose Watch Ansvr Log from the Start menu; you can close that window again at any time without affecting solving.

Using ansvr in Phoranso

Once ansvr is installed and running, select it as your solver in Phoranso's solver settings. From then on, Phoranso will solve your images locally, attaching full WCS and SIP information to each frame.