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Computing visual Mag

thn's picture
thn
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Joined: 2013-08-17

Hello!

I'm a visual observer and I'm looking for visual magnitudes of objects I'm going to observe. Example: EW And. The VSX and the GSVS show: 10.8 - 11.8 p. SIMBAD has fluxes: 8.98 V,  12.14 B (=> B-V = 3.16).

Is there a formula that lets me transform the range from p-band to visual? Since the range is about 1 magnitude one guess would be 8.98 - 9.98 vis, assuming that the flux 8.98 V is at maximum. I also thougth that subtracting the B-V value from the p-values (assuming that p is in the blue spectrum) could give the answer, but that would mean a range 7.6 - 8.6 which looks wrong.

Since many ranges in VSX and GCVS are not in the visual range I would appreciate any help. Thanks in advance,

Thomas


P.S. I often look a light curves, they are a great help but in this case (EW And) there aren't any observations.

 

 

B-V for LPVs
FRF's picture
FRF
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Joined: 2010-08-09

Dear Thomas,

As a thumb rule you can calculate with an average B-V~1.5 mag for LPVs. So in this case the range of EW And in V will be around 9.3-10.3 which is in quite good agreement of the available Tycho-2 photometry of EW And: 9.459VT [1]

Although VSX gives Spec. type C7,3(Nb), so this star can be even redder, but probably not B-V=3.2

The I, R1, R2, B1 and B2 mags in the USNO B1.0 catalogue are: 8.74, 9.63, 9.56, 12.75 and 11.30 showing that the range in V might be probably much fainter than 7.6-8.6V


[1] http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-info=XML&-out.add=.&-source...

[2] http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-info=XML&-out.add=.&-source...

Clear skies,

Robert

EW And visual magnitude
Sebastian Otero's picture
Sebastian Otero
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Joined: 2010-09-19

Hi, Thomas,

The original discovery reports for several variables were based on photographic records and those photographic (p) magnitudes are usually more or less similar (but may not be equal) to the B magnitudes.
So it all depend on the star's color.
I have added a visual range for this star in VSX according to the available data (mostly Tycho). It is approximately 8.6 - 9.4.

Cheers,
Sebastian

-----------------------
Sebastian Otero
VSX Team
American Association of Variable Star Observers

Calculation?
thn's picture
thn
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Joined: 2013-08-17

Hi Sebastian -

could you give me the data and the calculation that lead you to this Vmag range? EW And was really more meant as an example. I want to do the calculation myself the next time. ;)

Thanks anyway,
Thomas

No calculation
Sebastian Otero's picture
Sebastian Otero
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Joined: 2010-09-19

Hi, Thomas,

There's no such calculation, I determined the V range from the available V data.
Since the p magnitudes are not standard magnitudes (different emulsions might be flagged as "p") and also the variable stars change their color during the variability cycle, there is no actual way to convert p to V.
As Robert said, for normal red giants a difference of more or less 1.5 mag. is the typical value and for carbon stars the difference is larger. You should just use this to have an idea of what you're going to find at the eyepiece.

Cheers,
Sebastian

OK
thn's picture
thn
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Joined: 2013-08-17

I'll take that as advice. (I'm learnig a lot here). Thanks Sebastian!

Best wishes,
Thomas

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484