DECaLS FITS: How to get metadata not in the Header?

I used this command to retrieve a very nice image:
curl -o ‘J000146+040120.fits’ ‘

But the FITS Header is devoid of information such as when the data were obtained, what the zero point for the pixel values is, what units the pixel values are given in, etc. How do I find such information?

Good question. The FITS cutouts from the viewer were not quite planned to be complete data-release-quality data products, so the headers are incomplete. For when the contributing images were taken, I think you would have to refer to the “singe-image cutouts” page. As for units, the images are in “nanomaggies”, ie, AB fluxes with zeropoints of 22.5 mag.



I guessed that the units (BUNIT is the standard FITS card I think) are nanomaggies, but did not know (and couldn’t guess) what the zero point is.

From “Overview of the DESI Legacy Imaging Surveys” (thank you very much for the reference!), I am a bit unclear if these images have been deconvolved or not. From the main text, it would seem that images - like this - are not; yet in Section 8 one reads “We construct initial image stacks using weighted sums of the optical (i.e., grz) images. Each individual image is first convolved with its PSF model and we then create image stacks for each individual band.” It seems that these image stacks are used as inputs to The Tractor, but are not what one can download using a command like the above. Have I got that right?

By definition, nanomaggies have a 22.5 zeropoint; a maggy is the flux from a mag-0 star, so 10^-9 of that means zeropoint 22.5.

These are coadded images; they have not been deconvolved in any way.

The inputs to the tractor are the individual exposures, not coadds.


Thanks! :slight_smile:

I read the definition of nanomaggies, some time ago, but clearly forgot about the zero point. :frowning: