Hello, I am reviewing the AAVSO Extended File format:
For the "MTYPE" section, it states:
MTYPE: Magnitude type. STD if standardized (Click here for definition of standardized) or DIF if differential (very rare). If you are currently using ABS for 'absolute' we will still accept it. Differential requires the use of CNAME.
Reading further in the document it states:
"Differential photometry example (not recommended)"
Am I missing something? Why would differential photometry not be recommended? From the AAVSO CCD Photometry Guide, it even says to use differential approach:
"Only differential photometry is covered in this Guide because it is far easier, normally yields excellent results, and is more forgiving when observing conditions are not idea"
Perhaps the MTYPE section should instead refer to it as "Instrumental" mag rather than "Differential" mag? Since one is supplying instrumental magnitudes (for CMAG and KMAG) rather than calculated magnitudes?
The description of MTYPE does need a better explanation.
DIF refers to reporting the difference of target and comp star magnitudes. So its range is probably -2 to 2, depending on the comp chosen. You can see how this would be incompatible with the data reported in the Light Curve Generator (LCG) as it is dependent upon the observers choice of comp.
STD goes the extra step of adding the comp star's reference magnitude from that difference. This is the important step that makes your data comparable to that of other observers in the LCG.
When we speak of "differential" observations we are talking about comparing data of target and comp from the same image, the same FOV. This is as opposed to "all sky" observations where target and comp are a slew apart and would need extinction corrections to be made comparable.
This topic would probably be better located in the Photometry forum.
I think the problem is that there isn't a consistent distinction in the notes on the AAVSO Extended File Format web page between differential photometry and differential magnitude.
Differential photometry contrasts with all-sky photometry, as George wrote. The magnitudes determined by differential photometry may be standardized or differential. Standardized magnitudes are true magnitudes, which may be transformed or not, as defined in the relevant field of the AAVSO Extended File Format. A differential magnitude is of course simply the difference between the instrumental mags of a variable and a comp star, or a check star and a comp star.
The label MTYPE (magnitude type) in the AAVSO Extended File Format is correctly defined.
It is perhaps unfortunate that the example given further down the AAVSO Extended File Format web page is titled "Differential photometry example (not recommended)". It may have been better titled "Differential magnitude example (not recommended)".
Roy and George:
Unfortunately, differential photometry has two different (although related) usages/meanings in the literature and the web! One is related to all-sky photometry BUT there is a similar definition used in single image photometry.
It is confusing BUT so engrained in the web that IMHO it is not worth trying to clarify it?
I agree that differential photometry can mean two different things (single image photometry yielding standardized magnitudes, and single image photometry yielding differential magnitudes calculated as instrumental mag of variable minus instrumental mag of comp).
However the practical point is that the definitions and examples on the AAVSO Extended File Format web page should when possible use terms that are internally consistent, as per the last sentence in my previous post.
>This topic would probably be better located in the Photometry forum.
The motivation in posting to the software forum is that it pertains to a file export format. Specifically, I am developing software that can export to this file format. So it seemed logical to me to post in the software forum.
>It may have been better titled "Differential magnitude example (not recommended)".
I agree, that could be a good fix and would make it a bit more clear.