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Help Photometry with Sloan Filters

JAC's picture
Joined: 2013-12-10

While trying to prepare the report of the observations for HD355501 Filter r , i stumbled upon the photmetric data of Vizier and some doubt struck on me

1- the flux for the comparison star HD355499 seems to be equal for the sloan g and r filter that would render a magnitude for g and r filter the same. since i dont have the zero points that might not be true.

can anyone shed some light on the subject

the source data is 

and the photometry table for this object, regarding the chart 13548BUG

while strugling to calculate the magnitude to use for diferential photometry i calculated the magnitude of 9.28 for sloan g filter from the expression 

assuming that g.r = 0.04 from measurements i made

V for hd355499 is 9.153

V +0.587 . 0,04 + 0.11 = g

right know i assume it is well calculated but, i'm not sure

taken from arXiv:0710.5801v1 by taylor S.Clonis, C. Martin GAskell



HD 355499
HQA's picture
Joined: 2010-05-10

This is not a heavily-studied star, and has no history of variability.  Simbad gives its spectral type as A5, which matches the (B-V) value from Tycho2 of approximately 0.43.

The Vizier tool is pretty neat, giving you a spectral energy distribution of the star, using the available photometry along with appropriate conversion factors that take magnitudes and convert them into flux.  Again in the case of HD 355499, it shows the flux peak between B and V, which is about correct for this stellar temperature.

Your transformation process to derive a g' magnitude looks ok.  Remember that this is only an approximate value.  The transformation coefficients from Taylor are for a generic star, and any individual star may differ.  It may be a giant instead of a dwarf; it may have stronger emission/absorption than others; the Tycho2 magnitudes may have larger error; the transformation itself has uncertainty.

Part of the APASS project is a bright-star extension, giving photometric calibration with Sloan filters for stars as bright as V=7, so this star will eventually have true Sloan calibration in about a year.


thank you arne. what would
JAC's picture
Joined: 2013-12-10

thank you arne.

what would the best process to have the R magnitude of this star since it doen'st appear in the photometry table?

since I have the measures  for sloan r' and need the comparison star value for R?




Arne can you help me here? I
JAC's picture
Joined: 2013-12-10

Arne can you help me here?

I need to delete the submission for Sloan G due to an incorrect value for the magnitude used in the standartization of the magnitude values.

is there an expedite way of delete a batch or make a search only by star and filter used in the WEBObs

Meanwhile with the Help of OJJ i could find the value for sloan r' filter. submission of data made to WebObs

Thank's in Advance


Your observations have been deleted
BSJ's picture
Joined: 2010-03-12


I just removed all of your Sloan g measurements of HD 355501 (ASAS J202135+1525.6) from the database. I hope that is what you wanted.

Normally, the best thing to do if you need some observations deleted and it is too time-consuming to do it through WebObs Search as in this case (because it was just for one filter), the best thing to do is to write to me about it directly with all the details. 

If you have further questions please send me an email (sara [at]

Clear skies,

Thank you Sara
JAC's picture
Joined: 2013-12-10

Thank you, and my apologies for the submission.

it was my intetion to remove them and resubmit with the correct mag values.

it realy was time consuming and prone to error through webObs,

ill retain the directive for future problems, that i hope won't happen.

Kind regards


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