Magnitude color filter

[HealthyPear]

Is your feature request related to a problem? Please describe.
Apologies if this is already somewhat possible and I missed it.

Would it be possible to allow filtering of star magnitudes taking into account the color?
For example in gamma-ray astronomy usually we use photomultipliers which are sensible to a specific range of
wavelengths and for them stars with B-magnitude higher than 3 cause the the applied high voltage to be automatically reduced to protect the PMT from fast aging.

Describe the solution you'd like
An additional keyword that specifies to which band/color the value of magnitude refers.

[steveberardi]

Would a star's B-V index work? Because the star model does have that value, and you can filter on any model field when plotting stars.

For example:

p.stars(where=[_.bv > 1])

If you need B-magnitude though, and have a data source that cross matches with a star's HIP or TYC id, then you could create a list of HIP/TYC ids to filter by:

b_mag_star_hips = [...list of HIP ids you want]

p.stars(where=[_.hip.isin(b_mag_star_hips)])

As part of supporting custom star catalogs (planned for version 0.18 or later), I'd like to make things like the above easier to do as well. And, related to that, any chance you'd be interested in helping to write a function that determines the BV color index of a star from Gaia data? I've looked at their data docs, and see equations for doing the conversion, but don't understand the math/physics enough to make it work, and it sounds like you might have that knowledge 😄

[HealthyPear]

Would a star's B-V index work? Because the star model does have that value, and you can filter on any model field when plotting stars.

For example:

p.stars(where=[_.bv > 1])
If you need B-magnitude though, and have a data source that cross matches with a star's HIP or TYC id, then you could create a list of HIP/TYC ids to filter by:

b_mag_star_hips = [...list of HIP ids you want]

p.stars(where=[_.hip.isin(b_mag_star_hips)])

If I am not wrong, from the Big Sky catalog the magnitude of stars if the V magnitude - is this the same magnitude that one gets from Star.magnitude? Because in that case it should be just B = (B-V) + V - am I right?

As part of supporting custom star catalogs (planned for version 0.18 or later), I'd like to make things like the above easier to do as well. And, related to that, any chance you'd be interested in helping to write a function that determines the BV color index of a star from Gaia data? I've looked at their data docs, and see equations for doing the conversion, but don't understand the math/physics enough to make it work, and it sounds like you might have that knowledge 😄

I do not have first hand info on the Gaia data model, but accessing it should be relatively easy using astroquery

[steveberardi]

... from the Big Sky catalog the magnitude of stars if the V magnitude - is this the same magnitude that one gets from Star.magnitude?

Yes, the magnitude field on stars in Starplot is the Johnson V magnitude (see bigsky code here)

Because in that case it should be just B = (B-V) + V - am I right?

That sounds right to me. Hmmm, in that case, I wonder if something like this would work:

b = Star.find(where=[
    (_.bv + _.magnitude) > 3
])

Where (_.bv + _.magnitude) is the B-mag. I just tried running that and it returns 978,996 stars -- which sounds like too many? Not sure how Ibis handles a query like that, so maybe something's wrong there.

I do not have first hand info on the Gaia data model, but accessing it should be relatively easy using astroquery

oh, does astroquery also calculate the B-V? I'll look into that 👍

[HealthyPear]

oh, does astroquery also calculate the B-V? I'll look into that 👍

no but it should give you access to gaia data via astropy tables