What is the 'wht' map produced by Astrodrizzle using IVM and ERR weighting?

mead11mead11 Member Posts: 1
After running Astrodrizzle to combine several FLT files into one image, a '_drz_sci.fits' and '_drz_wht.fits' file are produced. Using the 'IVM' and 'ERR' weighting schemes gives hugely (1x10^11) different values in the 'wht' map, and I am confused about what this actually is. The '_drz_wht.fits' file produced when setting final_wht_type = 'IVM' has values close to what I'm expecting for the rms, but then why is it called a 'wht' map?

In the end, I am looking for an RMS map for use with Source Extractor and GALFIT, which does not include the contribution from the objects themselves. Can you advise on how to produce what I need? I am aware from a previous discussion that the handbook Section 4.2.12A is incorrect on this matter.


  • avilaavila Member, Institute_Staff Posts: 10
    edited November 2015
    The difference you are seeing might be because the 'ERR' scheme uses the noise from the objects themselves (as well as other sources of noise) while 'IVM' does not.

    For Source Extractor you want to use IVM weighting when you drizzle. You then have to take the inverse of that weight map to create the RMS map you require. When using IVM, Astrodrizzle uses the sky and flat to make the weight map where:

    ivm = (flat)**2/(darkimg+(skyimg*flat)+RN**2)
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