NIRISS team to review and agree with photom reference file updates

[stscijgbot]

Issue JP-711 was created by Alicia Canipe:

Please review the epic ticket for photom reference file updates and provide any comments or feedback. When you are done with your review, please close this ticket (or let Alicia know the task is completed).

[stscijgbot]

Comment by Kevin Volk: The ticket does not define exactly how the reference file needs to be changed.  Currently the NIRISS photometry reference file provides the scaling factors in (MJy/ster)/(ADU/second) for the imaging cases at a single reference wavelength per filter, and factors in (mJy/(ADU/second) for the spectroscopic modes at a reference wavelength along with a table of relative values as a function of wavelength.  Is the only change required that the reference wavelength values for spectroscopy be converted from the current units of mJy/(ADU/second) to (MJy/ster)/(ADU/second)?  Or does the wavelength-dependent table need to be put into (MJy/ster)/(ADU/second) units rather than being purely relative?

The NIRISS photometry reference file is providing multiplicative factors so I am a bit confused by the statement that the values will continue to be divided into the images in version 7.3.  I would hope that the NIRISS factors are not being used as divisor factors when they are mutliplicative values as was originally requested (I believe).

 

[stscijgbot]

Comment by Alicia Canipe: I believe the updates needed for the photom reference files will vary by instrument. Right now many of the current photom reference files have their correction factors in terms of sensitivity (e.g. (DN/s)/(flux)), so they get divided into the data (which means with the recent update to the photom step to multiply by what's in the current reference files, the results will be wrong). The updated pipeline step will assume that NIRISS reference files will be in units of MJy/steradian/(DN/sec), and it will apply this conversion as a multiplicative factor. Separately, SCSB will begin adding aperture corrections to the pipeline (Jira ticket here: https://jira.stsci.edu/browse/JP-706). So instrument teams will need to provide new reference files that include{color:#172b4d} aperture corrections that match those used to calculate the flux calibration factors.  {color}

[stscijgbot]

Comment by Kevin Volk: The aperture corrections used in generating the photometric calibration values may not be anything like the aperture corrections used in the source extraction step for general point sources.  In the NIRISS imaging case I am intending to use a much larger aperture for standard star observations to measure the response than the apertures discussed for general source photometry by the photometry working group, and I also expect that the aperture will be tailored to the individual photometric standard stars depending on what else is seen in the field.  In my view it would be a very bad idea to require that the apertures used in the standard star measurement be the same as the 20%/50%/80% encircled energy apertures currently discussed for the source tables, at least for NIRISS.  The main reason for this is that NIRISS imaging is generally badly undersampled and this creates significant uncertainties in the aperture corrections (i.e. much larger than 5%, in some cases).  The purpose of tailoring the aperture for the photometric measurements is to minimize the uncertainties on the aperture corrections, otherwise we will not be able to get anything like 2% photometric accuracy in a single measurement.  The same applies to the background aperture, which may be rather different in the photometric measurements than what is used in the source table case.

The aperture correction factors are specific for the source table generation, as that is where they are applied.  This is separate from measuring the photometric scaling factors from ADU/second to MJy/steradian.

 

[stscijgbot]

Comment by Alicia Canipe: [~jotaylor] can you add the keywords PIXAR_SR and PIXAR_A2 to the spectroscopic photom files? More information about the keywords is here: [https://jwst-pipeline.readthedocs.io/en/latest/jwst/photom/main.html?highlight=PIXAR_SR#pixel-area-data]

From Phil: {quote}        The photom step copies keywords for the pixel area from either the         photom or area reference file to the output file; however, this is only         done for imaging data.  As far as I can tell, none of the spectroscopic         area reference files include these keywords anyway.  The extract_1d step         needs the pixel area (solid angle) in order to convert the input surface         brightness data to flux density, and currently there's no good way to         get this information.  I would like to request that the new photom         reference files for spectroscopic modes include the keywords for pixel         area; the photom step can easily be modified to copy those keywords to         output.{quote}

[stscijgbot]

Comment by Alicia Canipe: Also, note that if you have two EXP_TYPEs that can use the same reference file (e.g., NRC_IMAGE and NRC_TSIMAGE), you can use the keyword P_EXP_TY (e.g., P_EXP_TY=NRC_IMAGE|NRC_TSIMAGE| ). 

[stscijgbot]

Comment by Alicia Canipe: Also, note that if you have two EXP_TYPEs that can use the same reference file, you can use the keyword P_EXP_TY. 

For example, assuming there is one imaging file that contains both NRC_IMAGE and NRC_TSIMAGE, the keyword structure is this:

EXP_TYPE does need to exist and is set to one of the two modes.

P_EXP_TY would then equal both modes as specified above. P_EXP_TY = NRC_IMAGE|NRC_TSIMAGE|

The pattern keywords are described here for any further clarification: [https://jwst-pipeline.readthedocs.io/en/latest/jwst/references_general/references_general.html#p-pattern-keywords]

[stscijgbot]

Comment by Alicia Canipe: Final format decisions are in https://jira.stsci.edu/browse/JP-1004 and https://jira.stsci.edu/browse/JP-1018. Closing this subtask since the files are in the process of delivery, so the review is complete.