[AstroPy] Help with vegamag.
tevaughan at gmail.com
Tue Jul 19 16:45:55 EDT 2011
I'm trying to do some calibration work for a star tracker that uses
Iota Persei as the standard.
--- What I Start With
I have vt_iper = 4.107, which is the Tycho-2 V_T magnitude of Iota Persei.
And I have vt_vega = 0.058, which is the Tycho-2 V_T magnitude of Vega
And I have v_vega = 0.026, which is the Johnson V magnitude of Vega
(Bohlin & Gilliland, 2004)
--- What Doesn't Seem To Work
The definition of vegamag suggests that I do something like this:
vm_vt_iper = vt_iper - vt_vega # V_T vegamag of Iota Persei
new_spec = spec.renorm( vm_vt_iper, 'vegamag', bp_vt ) # Tycho V_T bandpass
Then, for a sanity check, I do
v_obs = psyn.Observation( new_spec, bp_jv ) # Johnson V bandpass
v = v_obs.effstim( 'vegamag' ) + v_vega
When I do this, though, the V magnitude comes out really off, like
around 4.0 instead of 4.05.
--- What Seems To Work But Doesn't Make Sense
To get an answer that looks right, I have to do reverse the signs as follows:
vm_vt_iper = vt_iper + vt_vega
Then, at the end,
v = v_obs.effstim( 'vegamag' ) - v_vega
This gives an answer around the published value of 4.05, but the
reversal doesn't make sense to me by the definition of vegamag.
--- The Question
Am I missing something obvious?
Thomas E. Vaughan
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