I mainly work with radio continuum data and recently I started working on some HST hydrogen recombination line datasets to compare with the radio data that I have, but I'm very confused about what is the correct way to compare them. For me there are two main issues:

  1. For integrated line flux measurements in radio, I know it's common to have Jy/beam km/s as units (i.e. moment 0 maps) which then can be converted to luminosity after aperture photometry. However, HST line observations were done with a filter, so the final observed product is in a way already a moment 0 map (?), but the data handbook only talked about how to convert DN/s to Jy, which is not the unit for integrated line flux, and will result in a spectral luminosity (erg/s/Hz) instead of integrated line luminosity (erg/s). I know I must missed something basic somewhere but I don't know what that is. Specifically, I'm looking at Paschen line images.

  2. Radio images have beam information associated and photometry should only be performed with aperture larger than the beam size, and pixels within a beam is correlated. However, HST data don't have such beam/PSF information associated with it (..?). And in papers that use HST data, they only talk about the pixel scale as the resolution unit. Does the PSF/beam not matter in optical/NIR observations? If I want to compare the radio and HST NIR data, how do I make sure I'm comparing apples to apples?

Sorry for the long wordy questions. I have not dealt closely with optical/NIR images, so I'm hoping anyone here with experience in optical data can enlighten me on this matter.

Thanks in advance!!

  • $\begingroup$ Certainly the PSF matters for optical/NIR observations (try googling for "HST PSF"). The precise PSF depends on the instrument, the filter, and (for broad filters) the spectrum of the source. $\endgroup$ Sep 25, 2021 at 16:03
  • $\begingroup$ Also, that's a really old version of the NICMOS Data Handbook -- maybe try the 2009 ("v8") version? stsci.edu/files/live/sites/www/files/home/hst/documentation/… $\endgroup$ Sep 25, 2021 at 16:25


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