How to convert nanomaggie to counts?

[HANJING SHI]

Hi all,

I hope this email finds you well. I am trying to use legacy cutouts to do MGE. And how can I convert nanomaggie into counts?  

Sincerely,

Hanjing

[Dustin Lang]

Hi,

If you are using cutout images from the Legacy Survey sky viewer, then (if you request cutouts at 0.262"/pixel) those are in nanomaggy units.  Those images are coadds from many input images, so there is not a single conversion back to counts -- it's a different conversion for each image that went into the coadd.

What are you trying to do?  What do you mean by MGE -- multi-gaussian expansion?  My guess is that what you actually want is an uncertainty estimate on each pixel's flux estimate.  If you add the "&invvar" argument to the cutout URL, you will get a multi-HDU FITS image with inverse-variances for each pixel, eg

https://www.legacysurvey.org/viewer/cutout.fits?ra=240.8505&dec=20.8906&layer=ls-dr9&pixscale=0.262&invvar

cheers,

dustin

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