# [AstroPy] Help with pysynphot and vegamag.

Thomas Vaughan tevaughan at gmail.com
Wed Jul 20 11:35:28 EDT 2011

```My original post didn't mention that I'm trying to use pysynphot.

The short of it is that I'm wondering whether my original use of
vegamag is correct below.

On Tue, Jul 19, 2011 at 14:45, Thomas Vaughan <tevaughan at gmail.com> wrote:
> I'm trying to do some calibration work for a star tracker that uses
> Iota Persei as the standard.
>
> ---
> ---
>
> 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)
>
> ---
> --- 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

```