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I'm downloading some images from the legacy survey and the units are a bit challenging to understand. I've already figured out how to get my fits and header, but I cannot go from nanomaggies (the units it comes in) to AB mag using:

AB mag = 22.5 - 2.5*log10(nanomaggy)

Because some of the counts in the downloaded fits are negative. I'm assuming there was some sort of background subtraction done, but I do not see any zeropoint in the header or anything that can help me make the entire data non-negative, such that I can convert the units from nanomaggies to magnitude if the physics units are negative.
asked Jan 22 by anonymous
reopened Jan 24 by datalab | 36 views

1 Answer

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Could you please give a specific example?  In particular, are these negative values in pixels or in a catalog value?

If this is in pixels, are you trying to convert individual pixels to magnitudes, or are you integrating over an aperture, & the total flux is still negative?
answered Jan 22 by baweaver (220 points)
This is in pixels and I am trying to convert individual pixels to another unit. I'm assuming I am missing some piece of logic to go from nanomaggies to magnitude
What is the specific use case for converting pixels to magnitude units?  That conversion is not particularly meaningful for individual pixels.  If you are trying to measure the flux of objects in an aperture, you should sum the pixels in flux units over the aperture first, then convert to magnitude units.

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