Changes in the .fits header...

[wvinton]

Karen: First of all, I'm eternally grateful for your creation of this program, and your immediate responses on the forum!

Now, here's a question I can't find an answer to in the documentation.... It appears that when I use the data processor to calibrate images with biases, darks and flats, that some values in the .fits header get changed. I have not done an extensive study to see which ones - but the most obvious is the AIRMASS.  For example, in one of my original science images, the AIRMASS was 1.00611; after calibration the value was 16.803.  Scanning through some of my other image headers (which all started out with AIRMASS values around 1.006) I see values that go up as high as 386, and some end up as -1.00.

So, I'm puzzled by this. Is this an unavoidable part of calibration? When I calibrate should I uncheck the "General" under FITS header updates?

Also, if I do something like a mean combine of a stack of images to enhance SNR, the FITS header is completely zapped, ("No valid FITS header").

 Is there any way to keep the header categories intact during something like a mean combine of a stack?

Thanks again for everything!

Bill V

[karenacollins]

Hi Bill,

When images are combined in AIJ, the header is not currently automatically copied forward as you have found (because it is an underlying ImageJ code that is doing the work and it is FITS header unaware). I hope to have time to fix that in the near future. For now though, you can manually copy a FITS header from one image to another (i.e. after you median combine), by using AIJ_Toolbar->Plugins->Astronomy->'Copy FITS Header'. If the header has WCS (i.e. plate solve information), you will need to click the auto-contrast icon () above the new image so that the new WCS solution is recognized.

For more info about FITS header updates, see Appendix A.7 in the AIJ paper expanded version ( https://arxiv.org/abs/1701.04817 ), but I'll give an overview here:

If you don't want to change the fits header, disable both 'General' and 'Plate Solve' options in the 'FITS Header Updates' sub-panel. If you do want to update the headers, enable "General" and then click this icon in the same sub-panel to define what you want to update. If you are updating the FITS headers, you need to define your observatory location and target RA and DEC. That is done by pressing the DP Coordinate Converter icon and entering that information on the DP Coordinate Converter panel as shown below. The time for each observation is extracted from the FITS headers.

Alternatively, DP can automatically extract the RA, DEC, Lat, and Long info from the image headers, if available, by selecting the 'Target Coordinate Source' and 'Observatory Location Source' in the "FITS Header Updates" panel as shown here:

For this mode to work, you also need to specify the proper FITS header keywords that appear in your image headers in the "General FITS Header Settings" as shown below. That is the same panel as mentioned above which can be accessed by clicking the icon.



Hope that helps clarify things a bit,
Karen

[wvinton]

Karen: Thank you so much for your wonderfully clear and timely explanation. I'll explore these issues along the lines you suggest.

Bill V