<p>I opened a FITs image data from <a href="https://www.tifr.res.in/%7Edaa/tirspec/" rel="nofollow noreferrer">TIRSPEC</a> (NIR imaging) (an HgCdTe infrared imaging sensor) on DS9 and I saw that some of the bright stars have black patches at the center (negative values).</p>
<p>Can you please explain what this means?</p>

CC BY-SA 4.0
2022-10-29T11:54:49.640Z
I've got an answer prepared, but before I post, I'd like some more details.  What is the source of the sky image?  What telescope? What imaging technology (cmos, CCD, something made by NASA?)  Also why do you tag "star-formation"?  This has nothing to do with DS9 (that just displays the image) And everything to do with the telescope and imaging technology, so please tell us more about that.
James K
CC BY-SA 4.0
2022-10-31T09:11:05.267Z
It's the data from TIRSPEC (NIR imaging). CCDs have been used to image.
Rian
CC BY-SA 4.0
2022-10-31T12:46:45.237Z
Assuming it's this [TIRSPEC](https://www.tifr.res.in/~daa/tirspec/), it's a Hawaii1K HgCdTe detector, not a CCD.
Peter Erwin
CC BY-SA 4.0
2022-10-31T12:47:47.903Z
My guess would be that the centers of the stars saturated the detector, and something in the post-processing translated that to negative counts.
Peter Erwin
CC BY-SA 4.0
2022-10-31T17:49:22.227Z
Yes you get the same thing with JWST images, it seems to be a feature of HgCdTe technology that it returns zero or negative values when oversaturated.
James K
<photography><image-processing><infrared><fits>
What do negative values on bright sources in TIRSPEC mean?
Rian
# What do negative values on bright sources in TIRSPEC mean?

(Asked by: username_0 on 2022-10-29)

I opened a FITs image data from [TIRSPEC](https://www.tifr.res.in/%7Edaa/tirspec/) (NIR imaging) (an HgCdTe infrared imaging sensor) on DS9 and I saw that some of the bright stars have black patches at the center (negative values).

Can you please explain what this means?

**Tags:** photography, image-processing, infrared, fits
> **Comments:**
> **username_1**: I've got an answer prepared, but before I post, I'd like some more details.  What is the source of the sky image?  What telescope? What imaging technology (cmos, CCD, something made by NASA?)  Also why do you tag "star-formation"?  This has nothing to do with DS9 (that just displays the image) And everything to do with the telescope and imaging technology, so please tell us more about that.  
> **username_0**: It's the data from TIRSPEC (NIR imaging). CCDs have been used to image.  
> **username_2**: Assuming it's this [TIRSPEC](https://www.tifr.res.in/~daa/tirspec/), it's a Hawaii1K HgCdTe detector, not a CCD.  
> **username_2**: My guess would be that the centers of the stars saturated the detector, and something in the post-processing translated that to negative counts.  
> **username_1**: Yes you get the same thing with JWST images, it seems to be a feature of HgCdTe technology that it returns zero or negative values when oversaturated.  

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## 0 answers

