Fri, 01/20/2017 - 05:54
Hi,
I've noticed that the new light curve generator (LCGv2) now permits display of data derived from Sloan filters. As an example for KIC8462852 there are two observers (BMAH and CWJA) who've submitted Sloan filter observations. Currently I do not see that the AAVSO web site can generate photometry comp star tables with Sloan filters listed. Am I incorrect? Does anyone know what source these observers are getting thier comp stars from?
--OJJ
AAVSO Photometric All Sky Survey (APASS) DR9 (Henden+, 2016) - stars up to 14-th mag
The SDSS Photometric Catalog, Release 9 (Adelman-McCarthy+, 2012) for stars faint than 14-th mag
Use AAVSO access form for APASS: https://www.aavso.org/download-apass-data
or for both:
VizieR online access: http://vizier.cfa.harvard.edu/index.gml
Velimir
Hi ,
At one time Arne suggested I could use a Sloan i' filter, submitting it as Johnson I then putting in the comments field that the filter used was actually Sloan i'. His thinking was that the band passes of Johnson I and Sloan i' were sufficiently close enough that I could get away with this as long as I provided the comment. At that time, I'd assumed that he meant I could use AAVSO sequences for the comp and check stars which I did and my results come very close (with slightly higher-brighter bias above the mean) than everyone else's Johnson I measurements so I felt reasonably confident I was on the right path. Reference the the plot in the attached PDF
But recently seeing some of the SI submittals in the new light curve generator, I see that Sloan i' and Johnson I the Sloan i' appears to be about 0.48 mag lower than the Johnson I data. Again, see the the plot in the attached PDF.
So rather than use the AAVSO sequences I tried the APASS sequences for the same comp and check star (At least I think these are the same based on RA and DEC). The result was that my Sloan i' data now comes close to those seven SI data points shown the the plot in the attached PDF. So if Sloan i' and Johnson I have characteristics very close to each other why the big differences seen in the plot below and my APASS experiment mentioned above? In short; what sequences should I use for Sloan i' ?
Thanks in advance,
John Ott (OJJ)
While the Sloan i' (or SI) bandpass is very similar to the Cousins I (or Ic), what differs is the standard stars that define each system and how they are measured. Cousins is "Vega-based", which means it is defined such that Vega (or at least comparable A0 stars) have the same magnitude in each filter. You know this is not true flux, as you can look at a typical spectrum and see that an A0 star peaks in the blue and gets fainter in the red. Meanwhile Sloan is "AB-based", which means it is based on the actual flux of the star. In the Sloan system, an A0 star does NOT have the same magnitude in each filter, but instead fades in the red.
The difference in these two filter systems results in an offset, depending on which filter (wavelength) is used. That is why if you take an image in SI, you will get two different results depending on whether your comp stars are Cousins Ic, or Sloan i' (SI). So if you use an SI filter, be sure to add a comment to your observation. If you use the magnitudes from the AAVSO photometry table, which is currently Ic, you would indicate "I" for the filter and mention in the comments that the actual filter was SI. If you used APASS or SDSS i' magnitudes for the comparison star, then you would indicate "SI" for the filter and then put in the comment field the actual magnitude you used for your comparison (since that value is not found in the VSP photometry table).
Either way works.
Arne
Thanks for that pevious answer Arne.
With regard to do with the previously mentioned slightly higher (brighter) bias in my Sloan i' data; if you take a look at the .PDF I attached you will notice that in Sloan i' only, after transforms I'm running about 0.04 magnitudes brighter than the mean. My first guess would be that this is simply the slight difference in Johnson I and Sloan i'.
I wouldn't think that this is a difference between other folk's non-transformed data and my transformed results as transforms have if anything, made the my submitted measurments < 0.02 magnitudes dimmer.
I suppose this might be attributed to the fact that I'm at 8,300ft and the less atmospheric extinction, less water vapor at this altidude might account for the slightly brighter values. However, I am using the recommended altitude extinction values in the Transform Applier.
As an aside; my transformed V-band measurements come out right on the V-band mean for the same star at the same time.
-- John Ott