Unit mismatch between v24 (instrumental flux) and v29 (nJy) science images in image subtraction?

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#1

Hello,

We’re currently upgrading our AP pipeline from v24 to v29, and we’ve noticed a potential issue regarding flux units in image subtraction.

In v24, the science images (calexp) use instrumental flux units.
In v29, the calexp images are now stored in physical units (nJy).

However, for our current workflow, we plan to:

  • Use a template image built from v24 processing (instrumental units)
  • Subtract it from a new v29 science image (in nJy)

This means the image subtraction will be performed between two images with different flux units.

Our concern:
Will this unit mismatch cause incorrect pixel values in the difference image, leading to biased DIA Source flux measurements (e.g., psfFlux, apFlux)?

Thanks so much for your guidance!

#2

In this case, I think your current workflow will be fine. An overall scale factor is fit and applied to the template, so the final image difference will have the same units as the science image.

#3

Thanks for the clarification! That’s a huge relief — glad to hear the scale fit handles the unit difference automatically.