Webb Office Hours Session 30: May 22, 2025
Q&A's:
Q1: Where are the Webb Office Hours procedures and guidelines?
A1: Webb Office Hours. Type your question into the WebEx chat. We will asynchronously copy questions from the chat to this main page and work through them as a group. If you have images to share please give WebEx permission to share your screen (you may need to log out and log back in again to enable this feature.)
Q2: I have NIRCam imaging data of a gravitationally lensed quasar. In the fits file there is an extension called ERR, how is this calculated? The final error seems to be very small compared to the science products (by 2 orders of magnitude). This seems too small to me, is this normal?
A2: Error propagation in the ramp_fitting step is implemented by carrying along the individual variances in the slope due to Poisson noise and read noise, in addition to the variance representing the flat field uncertainty, at all levels of calculations. The total error estimate at each pixel for the resampled products (i2d.fits) is computed as the square-root of the sum of these variance estimates. (see https://stcal.readthedocs.io/en/latest/stcal/ramp_fitting/description.html#ramp-error-propagation)
After looking at your data together (including how you completed your data reduction, using the default settings for the resample step), we don’t see evidence that your errors are too small, and there also seem to be no issues with how you ran the pipeline steps. The pixel values of the lensed quasar images are in the range of several hundred MJy/sr or more, and this would be consistent with the error values of tens of MJy/sr (given the exposure times of these images), which we validated here by comparing the science and error values in the final i2d files with the input single-exposure cal.fits files. In summary, it looks like your source is relatively bright (much brighter than the field galaxies, for example, even displaying diffraction spikes), so your high signal-to-noise ratio is likely expected.
Q3: We are also struggling to get an accurate PSF model with very few bright stars in our field. Can you help?
A3: Please see this JDox article: https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-performance/nircam-point-spread-functions
As we ran out of time during Office Hours, please submit your question to the JWST Help Desk (jwsthelp.stsci.edu) and we will answer you there.
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