# Rubin Science Assembly, Thu Apr 3 2025: \[Science Preparation\] Galaxies Science **URL:** \ **Category:** Announcements\ **Created:** [April 1, 2025, 10:10pm UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951 "2025-04-01T22:10:16Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![DouglasLTucker](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/douglasltucker/32/1341_2.png) [@DouglasLTucker](https://www.rubin.community/u/DouglasLTucker)\ **Post date:** [April 1, 2025, 10:10pm UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951/1 "2025-04-01T22:10:17Z")
**Date:** Thursday April 3 2025, 9:00-10:00am PDT (2025-04-03T16:00:00Z) **Type:** Presentation and tutorial **Hosts:** Christina Williams and Douglas Tucker **Zoom:** [ls.st/dp0-events](http://ls.st/dp0-events) **LSST Science Preparation Seminar - Galaxies Science** **About the LSST science preparation seminar series:** Each seminar will start with a _very brief_ overview of the LSST science goals and planned data products that are relevant to each week’s topic, and then move into a hands-on demonstration of how to access and analyze simulated versions of those data products via the Rubin Science Platform (RSP). The focus of these seminars is on how to access Rubin data, rather than how to do the science. These seminars use the simulated Data Preview 0 datasets ([dp0.lsst.io](https://dp0.lsst.io/)). All are welcome, with students particularly encouraged to attend. There will be time for Q&A. Prior to attending it is recommended to get an account in the Rubin Science Platform: [rsp.lsst.io](https://rsp.lsst.io/). Slides and tutorials are available at [github.com/rubin-dp0/dp1-sci-prep-seminars](https://github.com/rubin-dp0/dp1-sci-prep-seminars). The recording will be posted in this topic thread. [Full description and schedule for Rubin Science Assemblies](https://rubinobservatory.org/for-scientists/events-deadlines/events-rsa). ---
**Author:** ![ChristinaWilliams](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/christinawilliams/32/2227_2.png) [@ChristinaWilliams](https://www.rubin.community/u/ChristinaWilliams)\ **Post date:** [April 4, 2025, 10:28pm UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951/2 "2025-04-04T22:28:04Z")
[https://youtu.be/iO1ZlzLWzk4](https://youtu.be/iO1ZlzLWzk4) ---
**Author:** ![babel](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/babel/32/2085_2.png) [@babel](https://www.rubin.community/u/babel)\ **Post date:** [April 11, 2025, 2:11am UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951/3 "2025-04-11T02:11:18Z")
Douglas and Christina, thank you both for this helpful talk (and for recording it)! Nicely presented and it addressed a lot of questions I’ve had. It sounds like the \_gaap??Flux type calculations will be deprecated, so some of these questions may not get resolved until DP1 is out, but I’m wondering if you have some thoughts at this point. I get that \_cModelFlux is best for individual fluxes and magnitudes of galaxies (whereas psf is best for stars), but should I be using the \_ap??Flux measurements for color calculations and, if so, what determines the best aperture size to use? I’m sure the various size options are calculated for specific reasons. I liked Christine’s comparison plots, but I’m not sure how to determine which aperture is best. Also, for a flux density g\_ap35Flux - is that 35 pixels? For the best (r - i) color, should I set m\_r - m\_i = -2.5 \* log(r\_ap??Flux / i\_ap??Flux)? When would I use the Kron flux? ---
**Author:** ![DouglasLTucker](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/douglasltucker/32/1341_2.png) [@DouglasLTucker](https://www.rubin.community/u/DouglasLTucker)\ **Post date:** [April 11, 2025, 8:34pm UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951/4 "2025-04-11T20:34:09Z")
Hi Bob! Good question! Although it is true that it appears that the `_gaapOptimalFlux` measurements will be deprecated, it is our understanding that the fixed aperture `_gaap??Flux` measurements are recommended for use for galaxy colors. Which `_gaap??Flux` to use depends a little bit on one’s science: smaller apertures for the central regions of galaxies, and larger apertures for colors that better approximate a total galaxy color. The number associated with an aperture flux (e.g., the “35” in `g_ap35Flux` is indeed the aperture size in pixels. Finally, it is our understanding that the Kron flux should basically never be used but has been included for completeness and for comparison with other surveys that utilize Kron fluxes. I hope this helps! Let us know if you have other questions! Thanks! ---
**Author:** ![babel](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/babel/32/2085_2.png) [@babel](https://www.rubin.community/u/babel)\ **Post date:** [April 11, 2025, 8:47pm UTC](https://www.rubin.community/t/rubin-science-assembly-thu-apr-3-2025-science-preparation-galaxies-science/9951/5 "2025-04-11T20:47:32Z")
That answers some questions I’ve had for quite a while - thanks for getting back to me Douglas : )