About an image of Legacy Surveys

[Daviti Kobaidze]

Dear who it may concern,

 

I’m currently analyzing an image of an astrophysical object from the DESI Legacy Imaging Surveys catalog. This image is available in the (g), (r), (i), and (z) bands. Additionally, there’s a color image that combines all these bands, referred to as the “DESI Legacy Surveys color (g, r, i, z)”. While the wavelength ranges for each individual band are provided, I’m confused about the colored image, which encompass all the wavelengths from the (g), (r), (i), and (z) bands. Are these wavelengths overlaid within the range of the colored image? Could you explain how this colored image is constructed and how its wavelength range is determined? 

Thank you in advance.

[Dustin Lang]

Hi,

This page describes the filters on the Dark Energy Camera, https://noirlab.edu/science/programs/ctio/filters/Dark-Energy-Camera , and you'll see that the g,r,i,z filters together are wider than the human visible spectrum, so we have to map them into R,G,B.

What we do is, the z image goes into the Red channel of the color image, the i image is split 1/3 into Red and 2/3 into the Green channel.  The r image goes 2/3 into Green and 1/3 into Blue, and g goes into Blue.

Or, alternatively,

R = 0.75 z + 0.25 i

G = 0.5 i + 0.5 r

B = 0.25 r + 0.75 g

but there is also some non-linear stretching to allow the large dynamic range in astronomical images to be expressed; that part is basically the same as the grz images.

The code is here,

https://github.com/legacysurvey/legacypipe/blob/main/py/legacypipe/survey.py#L567

cheers,

dustin

--
You received this message because you are subscribed to the Google Groups "DECam Legacy Survey" group.
To unsubscribe from this group and stop receiving emails from it, send an email to decam-legacy-su...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/decam-legacy-survey/06f47434-1394-412d-aee0-017f54e76bc0n%40googlegroups.com.