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WEbObs page check magnitudes

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giufrust's picture
WEbObs page check magnitudes

A simple and basic question about WebObs page: the variable mag. to be put into the 3rd window must be corrected by  the difference between catalogue and observed check magnitude, or not? Sorry, but I could not find the answer in the helps. I'm using ensembles and not-transformed magnitudes. Thanks

HQA's picture
check magnitude

Hi Giuseppe,

The variable's magnitude is NOT corrected by the catalog/observed check magnitude.  Your ensemble solution should already produce a magnitude that has been adjusted by the catalog magnitudes of the stars in your ensemble, so that is sufficient.  The check star is a separate star, reported using the same ensemble that you used for the variable, so that it may be possible to adjust your data in the future if better photometry becomes available for the ensemble stars.


giufrust's picture
Check magnitude

Thank you, Arne. That's the way I used to do, but I wished a confirmation.

WBY's picture
Related and Perhaps Uninformed Questions

An ignorant but related question

For years WebObs files I have submitted contain the raw instrumental magnitudes of the comp (for single comp observations) and the raw instrumental magnitude of the check. My stupid question is whether I should be reporting the (un-transformed) standardized magnitude of the check star. It seems to me this is a matter of convention rather than substance since if you identify the comp and provide its raw instrumental magnitude then if you report the the check star AUID and raw instrumental magnitude  you can directly calculate its standardized magnitude, and if you report the standardized magnitude, instead, you can directly calculate the the raw instrumental magnitude. So it doesn't seem to matter which you report, except is one  from is used by convention and perhaps making it easier for a data user comprehend when the check and comp stars' magnitudes are both raw instrumental magnitudes.

Is there a convention? does AAVSO care if the check magnitude is a raw instrumental magnitude or an un-transformed standardized magnitude?

As a second issue: if you are doing ensemble photometry and want to provide users with the same ability to analyze your results as with single comp observations, don't you need to identify all of the comps, their individual instrumental magnitudes and the individual weights assigned to each comp (or state they are all equal)  in the Notes field of each image? If that is correct and we want to make our ensemble photometry as useful as possible to data users, then don't we, as Ricky Ricardo might say, "got a lot of writin' to do" in the notes, to an extent that becomes impractical if you are using more than a few comps in your ensemble?   Any suggestions how to do this better?

Brad Walter, WBY

SGEO's picture
check star mags and ensembles


The documentation for the Extended Format Record that is submitted to AID via WebObs ( here at ) states:

  • CMAG: Instrumental magnitude of the comparison star. If not present, use "na". Limit: 8 characters.
  • KMAG: Instrumental magnitude of the check star. If not present, use "na". Limit: 8 characters.

So, instrumental mags they should be.


Now, as for ensembles, you have pointed out their weakness in AID: they can't be adjusted for changes in photometry. Or this can be seen as a weakness of the Extended Format record: it doesn't allow for the description of the ensemble. What ever. I would believe this is enough of a problem that ensembles should be deprecated. When we pushed the use of transforms a few years back Arne stated a clear preference for transforms over ensembles ( ).

Is there a way to extend the AEF record to hold ensemble definitions? Sure, there is a way if we are prepared to do a whole lot of writing. The Notes field can be extended with subfields. We could document a convention something like this: number the comps and create subfields   |C01NAME=    |C01INSTMAG=     |C01ERR=       and so on for all the stars in the ensemble. But still, there are many ways to do an ensemble with many kinds of weighting schemes. These would all have to be defined and then noted in a field (eg  |EMETHOD= ESIMPAVG). I think this path would quickly breakdown. Too much is being put on the researcher to verify the observation.

A simpler fix for ensembles might be to put enough information into the Notes subfields so that a researcher can ignore the ensemble. If you provided the |VMAGINS=   |CMAG=     |CNAME=  (the target's instrument mag, one comp star's instrumenal mag and name) then the researcher could quickly get a  verifiable single-comp measurement.

TransformApplier provides this kind of information in the Notes field for exactly this reason: to let the researcher access the raw data and skip over any post processing.


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