[AstroPy] Help with vegamag.

[Thomas Vaughan]

I'm trying to do some calibration work for a star tracker that uses
Iota Persei as the standard.

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--- What I Start With
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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
(Maiz-Apellaniz, 2005)
And I have v_vega = 0.026, which is the Johnson V magnitude of Vega
(Bohlin & Gilliland, 2004)

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--- What Doesn't Seem To Work
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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.

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--- What Seems To Work But Doesn't Make Sense
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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
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Am I missing something obvious?

-- 
Thomas E. Vaughan