American Association of Variable Star Observers (AAVSO)
Sun, 03/31/2024 - 22:48

I have a question about the format for submissions through WerbObs.  If you look at the documentation here:

It indicate that CMAG and KMAG should be instrumental magnitudes.  I have been using Tycho to generate my report and it does put instrumental magnitudes there.

R LMi,2460398.59317,10.456,0.007,V,YES,STD,000-BBQ-749,-13.043,000-BBQ-737,-13.991,1.1040,na,X36328BKE,na
R LMi,2460398.59895,12.982,0.028,B,YES,STD,000-BBQ-749,-12.849,000-BBQ-737,-14.061,1.0910,na,X36328BKE,na

However, when looking at some other observations, I sometimes see that the true magnitude is used for CMAG and KMAG  rather than the instrumental mag.  This is even true for VPhot.  I would attach an image showing this but, of course, attachments aren't supported here.  Here is the data I see when clicking on someone else's observation:


ID# 1303371891

Magnitude Type STD

Name R LMI

JD 2460323.69269

UTC 2024/01/14 04:37:28

Magnitude 11.922

Uncertainty 0.029

Band V

Obstype CCD


CMag 0

Comp2/Check 111

KMag 11.044

Airmass 1.979

Charts X28725DQ

Software VPHOT 4.0.44

Transformed 1

Comments KMAGINS=-6.729 KMAGSTD=11.044 KREFMAG=11.055 TV_BV=-0.612 VMAGINS=-4.588


Note that KMAG is 11.044 which is not an instrumental magnitude.  In the Comments, the KMAGSTD is 11.044 which is the same a reported for KMag.  It seems like it should have the same value as KMAGINS which is -6.729.

Is the documentation wrong or is VPhot (and some other software I've seen) reporting this incorrectly?


American Association of Variable Star Observers (AAVSO)


Nothing is wrong. IF a comp ensemble is used, the check star magnitude is reported as a standard magnitude not instrumental magnitude. If a single comp is used, the CMAG and KMAG are instrumental mags.

The documentation mentions this.


American Association of Variable Star Observers (AAVSO)
Ah, okay.  I didn't notice…

Ah, okay.  I didn't notice the documentation says something different for ensembles.  That absolves VPhot.

However, there appears to be other software that is making this mistake (which is where I first noticed it).  I see the following software using standard magnitudes for non-ensembles.



DOPHOT 3.9.2

LESVEPHOTOMETRY V1.2.0.95 (not sure what it is using but CMAG and KMAG values are like +19.493 for mag 11 stars)

Maybe these tools don't create the report themselves and people are doing this by hand and entering the wrong magnitudes.  If this is important information, maybe there should be some check in WebObs to catch these.

American Association of Variable Star Observers (AAVSO)
AAVSO extended format (AEF)

I confess that I am the one who implemented this crazy format.  It was a way of including diagnostic information in the smallest amount of space.  Before we created the Extended Format, all observations were limited to an 80-character record so that the observations could be put on punch cards (remember those?).  CCD observations were handled very poorly.

For traditional variable-comp-check observing, having the comp and check as instrumental magnitudes enables the instrumental magnitude of the variable to be recovered (though non-trivial if the observation is transformed).  This helps, for example, in the case where the comp star is found to be variable and the researcher wants to use the check star instead.  For time series, instrumental comp and check measures help in monitoring extinction variations, and again, for any peculiar variation in either comp or check.  It also helps in understanding the true uncertainty of the time series observations.

For ensemble observing, I was always concerned about the quality of the catalog values for the individual comp stars, and by the lack of knowledge of what ensemble method was being used.  By having a check star, whose catalog value is known, measured by the same ensemble, the extended format tells you whether there is an offset in the calculation.  This is particularly true if new values for the ensemble and check stars are posted sometime in the future, since in the extended format definition, there is no way of knowing what comp stars and what magnitudes were used in the ensemble.  Because the check star is measured identically to the variable, any offset in the check star can later be applied to the variable.

This dual-purpose structure is confusing to many, and I bet that few researchers have ever gone back to improve the photometry.  Later extensions by George Silvis put comp star magnitudes for ensemble photometry into the comments/notes field if you are using VPHOT, and transform information if using TA, so there are extensions to the extended format.  Someday, someone might make an standard extended extended format!



American Association of Variable Star Observers (AAVSO)

Thanks for the…


Thanks for the history of the format.

My main concern is that a brief perusal of observations in the Light Curve Generator shows quite a few using the calculated magnitude for CMAG and KMAG rather than the instrumental magnitude.  This is true for non-ensemble observations.  If we care about this, it seems WebObs should have a check.  At the least it could see if the magnitude is negative, which most instrumental mags collected by amateurs should be.  Maybe a warning could be popped up noting the error.