Ensemble transformation

Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 08/11/2016 - 17:49

Looking at recent CH Cyg data, I see some observations that use an ensemble comparison and are also marked as transformed.  Exactly how is this done?  Transformation, as I understand it, is based on the delta(B-V) between the variable and comp.  Unless every star in the ensemble has the same B-V, I don't see how transformation can be applied.

In principle, if the field has enough stars with reliable magnitudes - and having a wide enough range of colors - one could fit transforms specifically to that field.  B and V for the variable could then be located on the BFSL for eV and eB.  Is this what is happening?

Tom

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Ensemble Transformation

Hi Tom:

As you noted, it is straight forward to transform a target magnitude using a single comp with known multi-filter magnitudes. It is equally possible to transform a target magnitude using another single comp of different color (B-V).

What VPhot does is iteratively calculate the transformed magnitude using many comps (ensemble) and then average these individual transformed magnitudes to calculate a final mean transformed target magnitude.

Sound reasonable?

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
Confused?

Brad:

VPhot has been able to do transformation (single comp or ensemble comps) forever. It just cannot do time series transformations. Thus the need for TA which can do time series transforms but only for single comps. It would be nice to combine the two possibilities! Maybe some day, but don't hold your breath. frown

Ken