[Aavso-photometry] Differential vs. absolute magnitudes Aavso-photometry Digest, Vol 52, Issue 10

[Brad Walter]

 Gianluca, The terminology in the new WebObs means only this:
Differential magnitude is target mag - comp mag.  You must report the comp
star information so AAVSO can add back the correct comp star magnitude. 
"absolute" magnitude is target - comp (both instrumental magnitudes) + comp
magnitude that has been transformed to a standard system, as you get from
the photometric sequences in VSP.

For example, say my target is GJ436 (HIP 57087) and I am going to use the
106 Comp (000-BCT-043)from the VSP photometry sequence as my comparison
star. In a particular image I get the following instrumental magnitudes and
net counts from my aperture photometry:
		Star		vmag		net counts
Target	GJ36	 	-8.605	144,403
Comp  	106		-8.713	159,415

The differential magnitude, in this context is just vmag GJ436 - vmag
comp106 = 0.108

The vmag listed in the VSP sequence for the 106 comparison star
(000-BCT-043) is 10.624. Therefore, the untransformed "absolute" magnitude
is 0.108+10.624 = 10.732. You could also transform your measurements of the
comp and target star magnitudes and calculate T-C+C = T (capitals indicate
magnitudes transformed to a standard system). Now you have a transformed
"absolute" magnitude, keeping in mind that "absolute" magnitude in this
context is always an apparent magnitude, not the Absolute Magnitude as used
in  HR and CM diagrams, which denotes a magnitude measured at a distance of
10 parsecs. Also absolute magnitude in this context most frequently applies
to magnitudes obtained using differential photometry. The term "absolute" as
used in WebObs really is confusing nomenclature. 

If you are using Maxim DL for photometry, I don't think you can get the raw
instrumental magnitudes shown above = -2.5*Log(net counts*Gain/EXPTIME)
which for the Comp 106 star = -2.5*Log(159415*2.3/120)for the example 120
second image and my camera with gain of 2.3. If I recall correctly, Maxim
forces you to always enter a magnitude for the comp star and it's output is
always t-c+C for the target and k-c+C for the check star. 


> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org 
> [mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of 
> aavso-photometry-request at mira.aavso.org
> Sent: Monday, March 10, 2008 11:00 AM
> To: aavso-photometry at mira.aavso.org
> Subject: Aavso-photometry Digest, Vol 52, Issue 10
> ------------------------------
> 
> Message: 2
> Date: Sun, 9 Mar 2008 21:53:18 +0100
> From: gianlucaros at gmail.com
> Subject: Re: [Aavso-photometry] Differential vs. absolute magnitudes
> To: aavso-photometry at mira.aavso.org
> Message-ID:
> 	<13b407510803091353s7c6886b5q706e60dc72503582 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> I still have some confusion about differential vs. absolute 
> magnitudes. If I have understood properly transformation 
> coefficients improves the accuracy of the measurements and 
> zero point is necessary only for all sky photometry. 
> Concerning the new AAVSO format that asks to specify 
> differential vs. absolute magnitudes, does this request mean 
> to specify that magnitudes are obtained with either 
> differential or absolute photometry? Is the latter possible 
> only if you have the standardized magnitude of your comp 
> star? Can I find information about standardized magnitudes 
> for comp stars in the photometric table of the VSP or shall I 
> calculate myself? I apolologize for asking something that 
> perhaps has been already addressed by some of you.
> Gianluca