ASTAP, the Astrometric STAcking Program, astrometric solver and FITS viewer, is a free stacking and plate solver program for deep sky images. The plate solving capabilities can be accessed by CCDCIEL

ASTAP runs natively on Windows, Linux (AMD64), Raspberry PI (armhf) and MacOS. It will require about 500 MB disk space.

Installation instructions and information is available at:
http://www.hnsky.org/astap.htm
For plate solving you have to install both the program and the “G17 star database”.

Then configure the program path in CCDciel. For windows this is typical c:\Program Files\astap. For Linux this is typical /opt/astap

Two ASTAP settings are accessible in CCDciel:

• Radius search: Search radius in degrees. If there is no match, the program will move the search field around in a square spiral and increasing the distance from the initial position up to the radius specified. A radius of 30 degrees could be searched in a few minutes. Blind solving is possible by setting this option above 180 degrees. Scanning the whole sky can be achieved in typical 380 seconds but in most cases offset shall be much shorter and solving much quicker.
• Binning: For large images (>3000 pixels wide) it is beneficial to set binning to reduce image size and increase signal to noise ratio prior to plate solving. If your image is around 4000 pixels wide set this binning to 2. If your image is 5000 pixels wide set this option to 2 or 3.

In MS-Windows, the execution of the ASTAP solver will be shown by a small ASTAP tray icon on the right side of the status bar. If you move the mouse to the ASTAP tray icon, the hint will show the search radius reached.

If the search spiral has reached a distance more of then 2 degrees from the the start position then an ASTAP popup notifier will show the actual search distance and solver settings. Clarifications are given at the ASTAP webpage.

Tray icons are default off in the latest Win10 version. To set the ASTAP tray icon on, start a solve in CCDCIEL, go to Windows “Settings”, “Taskbar”, “Turn system icons on or off” and set the ASTAP tray icon permanent “on”

Images will require a minimum of about 30 focused stars up to more then 1000 stars. Exposure time as short as 5 or 10 seconds will be in most cases sufficient. Oval stars due to tracking errors or severe optical distortion will be ignored and solving could fail. If you have small amount of stars in the image, you could activate in ASTAP the option “small steps” for more reliable stacking. That is normally not required.

In case the plate solving fails a more detailed log is available in ASTAP for fault finding. Execute the ASTAP program manually and test the plate solving by loading an image in ASTAP. Open the “Stack” menu (ctrl+A), open the “Alignment” tab and have a look to section “ASTROMETRIC settings.”. You can set the maximum number of stars (500) and tolerance (0.005). Exit the program via menu file, exit will save these settings.

More information is available at ASTAP documentation