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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 16-bit data.

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.

 


Image1

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.



Image2

 

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.


Image3

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.

  • When using PixInsight for the plate solve, make a copy of one of the RGB files, plate solve it, and load this into the eXcalibrator WCS/Sum File textbox. Do not plate solve the other three RGB files. In this case it will be necessary to use the vertical flip function.

  • Another PI solution is to plate solve all three color files and use one of them as the WCS/Sum File. In this case, the vertical flip is not required.

  • With other software, plate solve one of the color files and use it as the eXcalibrator WCS file.  The option also does not need the vertical flip.