16-bit FITS - automatic conversion?

Posted by Andy Puckett on May 14, 2008; 7:44pm
URL: http://imagej.273.s1.nabble.com/16-bit-FITS-automatic-conversion-tp3696219.html

I have two questions, both related to opening a 16-bit (signed) FITS image:

1. There are no BSCALE or BZERO keywords in the header, but "Show Info..." reveals the calibration
function it claims to be using:  y = -32768.0 + 1.0x.  It also says 'Unit: "gray value"' although I
don't see this anywhere in the header (no BUNIT keyword).  Where is this coming from?  Does
ImageJ automatically convert between signed and unsigned values, despite the lack of a BZERO
keyword?  

2.  On this image, the status bar reveals two different "values" for every pixel.  For example:
          value = 235.00 (33003)
I see that they differ by 32768.  But which of these is the "raw" value that is stored in the FITS
image array, and which is the "calibrated" value?  The calibration function above suggests the
parenthetical value is the raw value, but my boss is telling me otherwise.  (I have opened this file
in SAOImage ds9 and IDL, and the values I get are the smaller ones, which aren't in parentheses
here.  So if those AREN'T the raw values, then all FITS readers must know to make this conversion,
right?)

3.  I'm using ImageProcessor.getPixelValue in my plugin to access these values, recently changed
from getPixel.  The documentation isn't clear on the difference between these two functions.  Does
one return the parenthetical value above, and the other return the other value?

Thanks,
Andy Puckett
University of Alaska Anchorage