Bright Star Saturation

[jgizis (John Gizis)]

Hi, I’ve been working on bright stars… the LSST Science book estimates saturation for 15 second exposure in 0.7 arcsec seeing as u, g, r, i, z, y = 14.7, 15.7, 15.8, 15.8, 15.3 and 13.9. Does anyone know if this estimate has changed a lot?

I understand DM will correct for non-linearity as best they can, but how close do you expect to report high quality (say 1% results). I see there was a discussion some years ago that didn’t reach a simple conclusion at:

CCD Nonlinearity Near Saturation

We are attempting to define the behavior of LSST photometry around the saturation and nonlinearity limits in different bands. Currently, two properties we are trying to find out are the expected full well depth of the LSST pixels and the linearity behavior - essentially how the gain changes with star brightness, and the resulting shape and size of the resulting bleed trails. I realize that information might not be known for sure, but I imagine there are basic specs for that. I have reviewed t…

My goal is simply to understand what stars will have high quality photometry on the LSST system in the case of 15s, 30s exposures, and in the case of additional short exposures.

[pepper (Joshua Pepper)]

I haven’t heard anything new about estimates for the precision at the bright end. The noise model from about 7 years ago is still the most current one that I am aware of.

There has been a lot of work done in the interim on doing precision photometry of saturated stars with TESS and K2. I’m not sure how much of that will translate to ground-based photometry, but some of it should be applicable. Someone should test that with DecCam if they haven’t already.

[jgizis (John Gizis)]

A related question is there a nominal bright star limit number in the Observing Strategy simulations that we can look at with MAF?

[yoachim (Peter Yoachim)]

Right now we have stellar density models available in MAF, but I think they all start around 15th mag. If you want to make something on the brighter end we can probably do that (I imagine we could bin up gaia DR3 or something).

For the saturation limits, that seems like the kind of thing that one could calculate from the syseng_throughputs repo. This notebook might have the numbers you need to check if the bright end has changed much: https://github.com/lsst-pst/syseng_throughputs/blob/master/notebooks/Overview%20Paper.ipynb