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Using eXcalibrator w/ PixInsight Data


With version 4.2, eXcalibrator supports PixInsight (PI) 32-bit floating point and both 16-bit signed and unsigned FITS files.  This document assumes that the reader is familiar with eXcalibrator.

PixInsight offers multiple options for the FITS format. This introduces two problems for eXcalibrator, data scaling and the vertical flip. To keep things simple, eXcalibrator assumes that the files are created with the PixInsight default settings. EXcalibrator only checks the WCS File for the use of PI images.

Data Scaling

In the case of 32-bit floating-point data, PI scales the pixel values from 0.0 to 1.0. Internally, eXcalibrator equally rescales the three color files to a 16-bit (0.0 to 65535.0) scale.

The Vertical Flip

PixInsight allows the user to choose how FITS images are displayed. The default method is with no vertical flip. The dialog window, in Image1, shows the default settings. The Upper Left Corner selection displays a FITS image without a vertical flip. This agrees with most amateur astronomy software, including eXcalibrator.

With the Lower Left Corner setting, PI displays the image flipped vertically. Apparently, this method is preferred by the professional astronomers.

To display the PI FITS Format Preferences window click View | Explore Windows | Format Explorer and double-click FITS.



Now here is where it gets interesting

The PixInsight image solver always puts a vertical flip in the FITS Scale Matrix, relative to how the image is displayed. This gives a FITS image with no vertical flip in the data and a vertical flip indication in the Scale Matrix. This confuses many programs such as eXcalibrator and Aladin. Image2 is a partial Aladin screenshot showing this problem.



In the case for Image3, the FITS file was loaded with the Lower Left Corner option for the coordinates origin. This displays the image flipped vertically in PixInsight. Once again, the Image Solver flips the Scale Matrix vertically relative to the displayed image. This results in a saved image with the Scale Matrix in agreement with the image data. Now programs, other than PixInsight, can correctly analyze the image. See Image3 for the Aladin result.


eXcalibrator's Solution

When eXcalibrator loads the WCS File, the user is asked if the RGB data should be flipped vertically. If you are not sure, try it both ways. The correct choice will return more stars.

Here are some guidelines.

  • With PixInsight, plate solve one of the three color images and save all three as a 32-bit floating point FITS. In eXcalibrator, load the plate solved image into the WCS File text box. Finally, load all three files into their respective Red, Green and Blue text boxes.
    In this case, use the vertical flip function.

  • With other plate solving software...
    In PixInsight, save all three color images as 32-bit floating point FITS. With your favorite software, plate solve one of these three images and save it separately. In eXcalibrator, load the plate solved image into the WCS File text box and the other three files into their respective Red, Green and Blue text boxes.
    In this case, do not use the vertical flip function.