Notes a discussion about jointcal photometry
These are notes I took during a conversation between @KSK, @RHL, @jbosch, and @parejkoj about how to move forward on how to output jointcal’s fitted results, what we need to do to get it to match meas_mosaic’s output, and what we’d like to do further in the future.
Any errors in the below are mine: please correct me if I misheard something.
Jim: there’s a “little known” way to save the calibrated catalogs, but it may have bit-rotted.
- save something like meas_mosaic’s output as a math.boundedField
- redo calib to force people to specify their corrections.
- add a boundedField thing to calib.
- have a calib that is just a magnitude zeropoint.
Spatial vs. SED dimensions
What to do with the spatial-varying component?
- save the jacobian, or not?
- have to include the wcs with the calib?
- spatial structure in the filters: may be color-dependent.
- Are all of these things multiplicative factors that are commutative?
- If they don’t all commute, then we have to think harder about the order and may have to make the interface less general.
What about the SED dimension?
- Do we just have a 3d polynomial (spatial+SED)?
- Polynomial is probably not the most appropriate choice.
- All our refcats are afw.tables, so how do we represent the colors.
- Our refcats have an entry for each object in each flux.
- We need to generate obs_*-appropriate fluxes and colors (which each obs package should be able to do).
- We should add placeholders for colors to jointcal, so we’re ready in the future.
- Try reducing the catalog to just one star, to see the spatial structure: even just fitting a single zeropoint should be an improvement over single-frame procesing.
- Save the “single zero-point per chip” that jointcal can currently fit as a new Calib (probably as part of the Exposure that is persisted in the “wcs” dataRef).
- Figure out how the Jacobian behaves before figuring out how to put it into Calib.
- Currently assuming uniform illumination of the flat-field-screen, if that’s not true the Jacobian can matter a lot.
- match meas_mosaic: spatially-varying polynomial over each chip, and the Jacobian. Put all that information into the calib.
- There may be discrete jumps at the edges of the chips because of coatings on the chips: cannot assume a smooth function across the whole field.
- Eventually fit all the parts separately like we plan to do with the WCS.
- We should do some experiments to decide how we want to represent the spectral and spatial dimensions.
- How do we cover the “known spatial structure in the filters”?
- We need star SED info to go into jointcal.
- Can we extract the known spatial structure in the HSC filters from the photometry?
- Can we look at time-varying water vapor as a factor of position?
- Can we say that there is one function per CCD?
- Look at the change in y-band between exposures (and between CCDs) as a function of i-z.
- Represent transmission as a function of up to three separable things: spatial, time, wavelength, with each function only dependent on one of those.