DM-6147

[r-owen]

No description provided.

[parejkoj]

These look good. The only thing I might add would be a clarification on what "suspect" means in the help description. Is that a term of art in LSST land, or is it supposed to be a general catch-all for "some undefined thing may be wrong here"?

[r-owen]

That is an excellent point. I will expand the explanation. For the record: it means a pixel whose value is high enough that the non-linearity correction may not work very well, and so very picky algorithms may wish to avoid using this pixel. I worry it's too granular, but HSC apparently finds it useful.

[parejkoj]

Ah, then can we call it something other than "suspect"? Because that sounds like a more generic thing. Maybe "non-linear suspect" or something?

[r-owen]

Ack!!!!!

[parejkoj]

I'm sorry about the naming complaint, but I was definitely confused by it when I saw the name, and I could see wanting to introduce a "suspect" mask in the future as a catch-all.

[r-owen]

I agree it's not a great name, but the mask plane is called SUSPECT so I think it's the best choice unless we rename the mask plane (and I don't want to do that unless it's really important)

[parejkoj]

Ok. Just document the name well.

[parejkoj]

The new description is definitely better. Thanks.

[RobertLuptonTheGood]

I like SUSPECT, as we may set it for other sorts of problems too.

[parejkoj]

If we're defining "suspect" as "pixels with values >= suspectLevel may be poorly linearized", then we shouldn't use "suspect" for other problems. If we want to use it for other problems, we should define it as "pixels whose post-ISR values are suspect for various reasons." And if we do that, the definition of suspectLevel will be meaningless, since it's not a value >= suspectLevel test anymore.

[RobertLuptonTheGood]

I think we're using suspect as "pixels that we don't trust". One type of untrusted pixel is those above a certain level, so suspectLevel is OK. I think I'm agreeing with John, but I'm not quite sure

[r-owen]

Robert: could you please provide a definition of suspect/suspect level?

[parejkoj]

That was what I initially suspected (see what I did there?) suspect meant, but in this context it appears that it has specific meaning. The documentation should be clear about this: possibly isrTask says what it currently says (because that's how suspect is used in that context) while afw says the more generic thing (suspect means we don't trust it).

[RobertLuptonTheGood]

Does this help?

Pixels whose values may be affected by unknown systematics are labelled SUSPECT. An example is that the NAOJ instrument scientist for SuprimeCam recommends not trusting the values of pixels above a certain level (this could happen if the non-linearity corrections above this level are unstable). We accordingly set the SUSPECT mask bit for all pixels above a level, suspectLevel -- which may be infinite for the LSST electronics. In the future we may identify other sources of suspect pixels.

[r-owen]

Thank you Robert. That is very helpful.

[r-owen]

Here is the description I came up with for the amp info catalog field (pushed to the ticket):

level above which pixels are considered suspicious, meaning they may be affected by unknown
systematics; for example if non-linearity corrections above a certain level are unstable
then that would be a useful value for suspectLevel; use `nan` if no such level applies

[parejkoj]

I'm not sure about the "level above which" part. Robert's description above suggests that there could be other reasons for a pixel to be suspicious (I think? @RobertLuptonTheGood ).

[r-owen]

In support of "level above": if pixels with low signal are hard to correct accurately then we are in deep trouble, since much of our signal is faint. I suggest we keep "level above" for now. It's probably sufficient, and if not, I suspect we will need to do much more work.