isoparix wrote: ↑Wed Jul 13, 2022 3:35 pm
These images are, I suppose, false-colour representations of infrared data. How much has the spectrum been shifted upwards - two octaves? ten?
Oh wow. You'll have to do with my very shaky infrared "expertise". (Yeah, using the word "expertise" here is a joke.)
Anyway...
JWST User Documentation wrote:
JWST NIRCam offers 29 bandpass filters in the short wavelength (0.6–2.3 μm) and long wavelength (2.4–5.0 μm) channels.
The JWST NIRCam is the camera detecting the shortest infrared wavelengths. There is another camera too, which I
think is called MIRI (Mid Infrared Instrument) (but you'll have to google, because I'm too lazy), which detects longer, mid infrared wavelengths.
So anyway. If I understand things correctly, and 0.6 μm is the same thing as 600 nm, then the shortest wavelength that JWST can detect corresponds to this visible color:
███
So this is the shortest wavelength that JWST can detect, and any emission at this wavelength will be mapped as blue
███. (The color sample I just showed you corresponds to a wavelength of 441 nm, which probably qualifies as the height of "blue-ness" that the human eye can detect.) The difference in wavelength between 600 nm and 441 nm is not so large, only some "160 steps on the wavelength meter" (or how do you put that in words?).
All right. But please note that 2.3 μm, 2300 nm,
also counts as a short wavelength to JWST, and this wavelength will, perhaps, be mapped as this visible color:
███ (This color sample corresponds to 550 nm, which is a wavelength that is sometimes used by Hubble as a green filter.)
Let's assume that 2300 nm is mapped as "green", or 550 nm, by JWST. The difference between 550 nm and 2300 nm corresponds to "1750 steps on the wavelength meter". That's a lot.
2300 nm is way, way off the optical scale of wavelengths that the human eye can detect. After all, when Hubble takes pictures in the infrared, it is usually no more infrared than - if I remember correctly - 814 nm. 2300 nm is almost three times longer than the longest infrared wavelength than Hubble can see.
To get an idea of wavelengths and color, go to
this page. Scroll right as far as you can get to see where the human ability to detect longer and longer wavelengths fizzles out. The scale ends at 780 nm, which corresponds to a dark brown color to the human eye. Beyond 780 nm, the human sensitivity will quickly drop to zero. 2300 nm, the longest wavelength that the JWST NIRCam classifies as "short", is almost exactly three times longer than the longest, darkest brown wavelength that is reasonably visible to the human eye.
Go to
this page to see a bit more about the JWST NIRCam filters and how their wavelengths are mapped.
Ann
