display command changes pixel values?

stpjhj wrote on Feb 19, 2018

Hi!

When I use IRAF, I use ds9 to check images. I do this in following ways

cl> !ds9&
cl> display whatever.fits
 

But, ds9 shows the values of pixels on which mouse is hovering, and the value is different from original ones.

For instance, if I open a fits image of 16-bit integers by invoking display command, pixel values become floating points, which are close to the original value, but not identical.

Does anyone know how did this happen, and what can I do to see the image as is?

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Mike Fitzpatrick wrote on Feb 19, 2018

The DISPLAY command must scale the image from the original dynamic range of the pixels to the 200 levels used in the display. What you're seeing is an approximation of the pixel value based on that scaling.

When using DS9 you can load the image directly to see the actual pixel values from the file, e.g.

cl> !ds9 whatever.fits

Alternatively, you could use XImtool as the display, or the 'z' command in IMEXAMINE will print a 10x10 grid of pixel values around the cursor position.

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stpjhj wrote on Feb 23, 2018

I've never used ximtool, but I think I should give it a try.

Thank you.

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Last post on Feb 23, 2018