dr9 parameters

[Este Padilla Gonzalez]

Hello,

I am currently using stacked images for some image reduction. I am interested in finding the exposure time, the read out noise, saturate and gain of the image. I see that the header does not provide the info and the other file 'nexps' could help me calculate the exposure time by summing the pixels vales( I am assuming). However, I am unsure on how the get the info for the rest. I would really appreciate some guidance. 

Thanks,

Estefania

[Dustin Lang]

Hi,

If you are using the coadded images 

  (described here, https://www.legacysurvey.org/dr9/files/#image-stacks-region-coadd

  example "brick" directory here, https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr9/south/coadd/000/0001p000/ )

then those images are already the sum of many individual images with different exposure times and noise.

The images have a pixel-by-pixel uncertainty map, stored as inverse-variances, in the "invvar" files.

For example, each pixel in the file

    legacysurvey-0001p000-image-r.fits.fz

has an uncertainty sigma = 1/sqrt(invvar), where the invvars are in this file,

    legacysurvey-0001p000-invvar-r.fits.fz

So if you want to stack images, you can do inverse-variance weighted stacks.

cheers,

dustin

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[Jim Acosta]

Sorry to re-open this issue but I think it is important to ask again for the gain value. In DECam characteristics, it says that its gain is about 4e-/ADUs. The problem here is that the data units in the coadded images are in nanomaggies. Is there a gain value or a way to convert coadded image data units in electrons? In my particular case I want data units in electrons to make simulations of DESI galaxies.

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[Dustin Lang]

Hi,

Unless you're starting from *very* low-level simulations, you really probably don't need the gains.  And if you do, then using the coadds is probably the wrong thing to do.  For example, a coadd may include one image with good seeing and clear skies with a short exposure time, and another image with worse seeing, light clouds, and a longer exposure time.  The coadd would be a weighted sum of those images.

Anyway, if you really want to, you can find the average zeropoint of the individual exposures in the "survey-ccds" files, eg

  https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr9/survey-ccds-decam-dr9.fits.gz

and if you assume that all the images in the coadd are photometric and have that zeropoint, then I guess you can convert the coadds in nanomaggies (zeropoint = 22.5) back into count-like units, and assume the typical DECam gain.

cheers,

dustin