# Pairs in the same filter or different filters

**URL:** <https://www.rubin.community/t/pairs-in-the-same-filter-or-different-filters/4761>\
**Category:** Survey Strategy\
**Tags:** opsim, run-evaluation, sims-maf\
**Created:** [February 26, 2021, 2:02am UTC](https://www.rubin.community/t/pairs-in-the-same-filter-or-different-filters/4761 "2021-02-26T02:02:24Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![ljones](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/ljones/32/1804_2.png) [@ljones](https://www.rubin.community/u/ljones)\
**Post date:** [February 26, 2021, 2:02am UTC](https://www.rubin.community/t/pairs-in-the-same-filter-or-different-filters/4761/1 "2021-02-26T02:02:25Z")

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In section 6.2 of PSTN-051, we present a comparison of the impact of taking visit pairs in the same filter versus different filters. I feel like I’ve seen some confusion about this, though, so here’s a summary:

In the v1.5 simulations, we generally take visits in 1x30s exposures (i.e. single snaps), and visits are acquired in pairs – two visits to the same pointing in each night, separated by about 22 minutes. Then we have two simulations that vary the filters used for those pairs —  
In baseline\_v1.5\_10yrs.db: we take those pairs of visits in u+g, u+r, g+r, r+i, i+z, z+y, and y+y. (MAF output pages: [science](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=19), [SSO](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=20), [metadata](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=17), [glance](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=18))  
In baseline\_samefilt\_v1.5\_10yrs.db: we take each pair in the same filter (u+u, g+g, etc.).  
(MAF output pages: [science](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=22), [SSO](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=23), [metadata](http://astro-lsst-01.astro.washington.edu:8081/allMetricResults?runId=21)).

These are grouped into the ‘intranight’ set of simulations.

In baseline\_samefilt we have fewer filter changes, and thus can achieve more visits. We fit about 4% more visits into the entire survey, which is also translated into 4% more visits per pointing over the WFD area.

More visits is beneficial for science which wants more visits and does not need colors within a night. Solar system discoveries can benefit from this in particular, as visits in the same filter on the same night have a better chance of detecting the same object. Overall completeness for fainter solar system populations increases by about 3%, depending on population details. Likewise, static science metrics tracking coadded depth, such as the number of galaxies or coadded depth in i band, increase by about 2%.

However, science which needs photometry in multiple bands over a short amount of time, such as SN detection, suffers greatly when filters are not acquired in mixed pairs. For example, the number of TDEs with well-measured colors drops by 60% and the number of detected fast microlensing events drops by ~15%.

This is discussed in PSTN-051, section 6.2. An illustration of the response of metrics from a variety of science cases is available [here](https://github.com/lsst-pst/pstn-051/blob/master/plots/intranight.pdf) (Figure 59 from PSTN-051).

To create your own evaluation of the impact of mixed-filter vs same-filter pairs, please use the simulations mentioned at the top of this post and compare metric results between these two simulations.