Notes

The Astrometric Reduction Tool (ART) is based on the Astrometry.net source code with modifications. All of the HEALPix (index) files are available on this server. My code modifications are intended to speed up the plate reduction process to deliver results quickly. ART is the web front-end for testing the code. This server rejects any file that is not in the FITS image format. While it is designed for the purposes of our Observatory, it should work well for any FITS image with a few caveats.

ART performs best when the image is properly reduced: bias, dark subtracted and flat fielded. Likewise, the FITS header should contain the essential information about the object and imaging system using these keywords:

NAXIS, NAXIS1, NAXIS2, FOCALLEN, APTDIA, XPIXSZ, YPIXSZ, XBINNING, YBINNING, OBJCTRA, OBJCTDEC

Without these keywords, you should submit your images to the queue at nova.astrometry.net instead of using this resource since there won't be any real advantage. ART is only allowed to search for a plate solution for a maximum of 60 seconds; although, the gateway will likely close in a shorter time span.

 

 

Usage

Create New File If checked, a new FITS file will be created with the WCS (World Coordinate System) written in the header. If unchecked, the WCS file created by the plate solver will be returned. Note that the WCS file that is returned i.e.(somefilename.wcs), is actually a FITS header, which can be opened and manipulated in IRAF or whatever you use for managing headers or copying WCS solutions.

Include SIP If checked, adds the Simple Imaging Polynomial (SIP) to the WCS. SIP is a convention for representing non-linear geometric distortion of the coordinates. If your FITS image viewer does not support SIP, you should either download a copy of SAOImage DS9 or uncheck this option.

Include Solution Write the solution parameters and information to the header. This can be helpful for better understanding the imaging system. The keywords used in the header to store this information take the form "INFO_XXX" where "XXX" is an integer value. The comment section of each keycard identifies what the value represents.

Search Depth Search only the brightest N stars in the field. This is a slight modification of the original source code. Unlike the Astrometry.net sort depth command-line argument, this requires a single integer value. A plate scale of more than an arc-second per pixel will probably solve quickly with a sort depth of 10 or 20. I usually set the sort depth to 50 for a plate scale of 0.5 arc-sec/pixel or less. Enter a '0' to ignore this option.

Plate Scale If you truly know the plate scale of your imaging system, enter it here and uncheck the "Calculate Scale" option. A known plate scale will solve more quickly. The scale tolerance allowed by my code is ±0.05 arc-seconds/pixel. If you're not certain of the plate scale of the imaging system, leave this set to 0.0 and check the "Calculate Scale" option.

Tolerance This adds a cushion to the plate scale. The default tolerance is 0.05, which means if the plate scale is 1, ART will search scales between 0.95 and 1.05 arc-sec/pixel for a solution.

Calculate Scale If you're not sure of the plate scale for the image and the appropriate keywords are in the header, checking this will greatly decrease the time it takes to find a solution. Checking this ignores any value entered for the "Plate Scale" option.

Search Center The keywords (NAXIS, NAXIS1, NAXIS2, FOCALLEN, XPIXSZ, YPIXSZ, XBINNING, YBINNING) are used to estimate the initial plate scale and set the scale boundaries and determine the field size in degrees. The keywords (OBJCTRA, OBJCTDEC) are used to determine an initial center from which to perform the search. This forces the solver to look through the index files at only the points that are within a 2 field radius of the center.

Downsample Bin the image before running source extraction and solve. The WCS will be scaled to the original image before being returned. Use this if you have an unbinned image from a big detector that's not being solved. If this is the case, try downsampling by 2 and set the "Search Depth" to 50.