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R and I APASS magnitudes in Seqplot

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R and I APASS magnitudes in Seqplot

Good morning,

I apologize in advance for asking a question that may have been already answered in the past.

I see that the Seqplot software can generate BVRI sequences extracted from the APASS catalog. As far as I know, the APASS catalog provides B and V Johnson magnitudes and g, r and i Sloan magnitudes. Therefore, I would like to know how the R and I APASS magnitudes in Seqplot are obtained from the original APASS database.

Thanks, Girolamo


CTX's picture
Sloan Filter Conversions


I can't comment at the moment as to which sloan filter conversion formula's  Arne has been using for Rc & Ic but I can point you to a good website that offers the majority of the sloan filter conversion formulas in use with various surveys:

I beleive Arne uses just the Sloan r & i filters to generate the Rc & Ic values.

As a sequence team member I use Jester to convert the Pan-STARRS sloan filters to BVRI when Rc-Ic < 1.15

On the other hand to convert the SDSS filters we use Lupton for the BVRI, I beleive this was the orignal choice of Arne.

While not a precise answer to your question the above should give you some idea of the process.

Tim Crawford




tcalderw's picture

Looking at the aforementioned SDSS transformation web page, it does not appear that Jester provides a way to recover Rc or Ic individually, only the Rc-Ic index.  There is a V-R formula, but since the 'c' designation appears elsewhere with R and I, I presume this V-R is Johnson.



HQA's picture
APASS Rc/Ic transformations

For seqcat/seqplot, we use the Jester et al. (2005) transformations for Rc&Ic, in conjunction with the already-available V.  Seqcat actually contains V and the astrophysical color indices (B-V), (V-Rc), (Rc-Ic) and (V-Ic).  Only when comp stars are selected does software convert the magnitude&colors into separate magnitudes by appropriate additions and subtractions.  Note that the color part of the Jester transformations is small, on the order of 0.09*index for (V-Rc) and essentially zero for (Rc-Ic), showing that the Sloan r'/i' filters are pretty close in bandpass to the Cousins equivalents.

Tom, Jester's equations are for Cousins Rc and Ic.  No professional that I know of uses Johnson Rj/Ij.


tcalderw's picture
Jester, etc

The SDSS transformation web page is inconsistent in its use of notation.  Jester's set of equations are listed as applying to UBVRcIc, and an equation is given for Rc-Ic.  No equation is given for V-Rc, it is specified as V-R.  Sloppy!

Anyway, I have another question.  If you go to the web page for u'g'r'i'z' -> ugriz conversion, you find the following:

"For SDSS calibration, the following color cuts are also imposed:

0.70 <= u'-g' <= 2.70 &&
0.15 <= g'-r' <= 1.20 &&
-0.10 <= r'-i' <= 0.60 &&
-0.20 <= i'-z' <= 0.40

The redward cuts are imposed because the u'g'r'i'z' standard star system is not well-determined for stars redder than about M0.  "

If you grind through the math,  r'-i' <= 0.60  translates to  Rc-Ic < 0.82  (approximately).  This is a more severe constraint than the  Rc-Ic <=1.15  Jester specifies.  It also seems a little hokey for Jester to put the color constraint on the output of the transformation rather than the input.  What is going on here?



APASS Rc/Ic transformations

I forgot to mention in my previous mail that I was actually referring to Rc and Ic magnitudes.

Thank you, Arne, for answering my question, and Tim for providing such useful information!


Girolamo Gallo


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