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This page summarizes a discussion between members of the DM Science team and the DESC on the uniformity of coadds in a Data Release. 

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Logistics

Meeting Date: 2022-12-09

Present: DM: Leanne G, Robert L, Jim B, DESC: Katrin H, Eric G, Humna A

Background

The DESC are concerned about the uniformity of the coadds released as part of the annual DRs. The discussion was in the context of the SCOC recommendations for rolling cadence, and how rolling cadence will contribute to non-uniformity of coadds. DM were asked if there is a software/DM solution to this problem. This informal chat was set up to better understand concerns from the DESC and to brainstorm possible solutions.

Discussion

DM: We would like to understand what the DESC defines as non-uniform, what definition of non-uniformity matters? Coadds are going to be non-uniform because conditions, weather, air-mass, are non-uniform. DESC code is going to have to handle that in any case. What is the push to make coadds more uniform when DESC code will have to handle non-uniformity in any case?

DESC: The DESC has both static and time-domain science goals and so any recommendations about the survey cadence need to balance those goals. Rolling has a lot of benefits for SN1a discovery and we have supported it. We know that rolling cadence makes things less uniform in the interim before a return to uniformity. The initial implementation of rolling cadence took 1 year of data non-rolling and then rolled over 2 parts of the sky alternating over 2 years resulting in uniformity again after three years.  This strategy was acceptable; the DESC do not need every DR to be uniform. The new seasonal rolling cadence strategy is highly optimized for the time domain but will not result in a return uniformity every few years and there will be no uniform data releases between DR1 and DR9. This is a big problem for cosmology. DESC will have a pipeline to handle non-uniformity, what they are worried about is a significant increase to the amount of non-uniformity that would be introduced by the new seasonal rolling cadence strategy. 

The DESC  estimates (roughly) that after 10 years,  the depth variation as a function of everything will have a 10-12% variability. They want to avoid a situation where a DR that would have had 15% variation now has 30% depth variations (estimate from simulations with the seasonal rolling cadence strategy). This is less of an issue after year 5.


Open Questions:

  • How non-uniform was Kids/DES at the intermediate data releases?

Possible Solutions discussed

  1. Modify the observing strategy to ensure a return to uniformity every few years. The new seasonal rolling cadence could be maintained in the interim data releases. The DESC does not require every DR to be uniform, one uniform DR every few years would suffice.
  2. Remove data from a coadd to force uniformity. No one was very enthusiastic about this option as it involves removing good data.
  3. Add noise to the data
  4. Shift the Data Release schedule(or the cutoff date for data to enter a DRP)  to match when the survey naturally returns to uniformity in the rolling cadence scenario. 

Publish a small number (2-3)  special “uniform” data release at key intervals during the 10 year survey alongside the regular DR that are sufficiently uniform to meet the DESC’s needs. 

Actions

  1. DM will feed the details of this meeting back to the DM Science team, PST and SCOC.
  2. DESC will provide a statement about how much uniformity will be difficult to deal with and why. 
  3. DM (via PST) will talk to other SCs to understand if they have similar concerns and if any of the possible solutions proposed here would meet their needs.
  4. DM will put some (but not too much)  initial thought into the feasibility of the possible solutions, but will wait for a response from the DESC on 2). 

Supporting Material

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