Calibrating colors with background galaxies

[troypiggo]

Thankyou Vicent and Rogelio.  I wasn't criticising the methodology, although it may have read like that.  Just trying to understand.

[vicent_peris]

Well, it may say 7nm in the box, but I think it's actually bigger. Do you want a super 3nm filter with a superb transmission? 

Ok, I have it. I did photometry of 17 stars with SExtractor. The H-alpha image was acquired one hour after the R one; during this period of time atmospheric extinction was extremely stable, the only difference being the increase in altitude of the object from 45 (for R) to 52 deg (for Ha).

Assuming that the R filter has about 102 nm FWHM, the median brightness ratio of these 17 stars gives a width value of 8,3 nm for the H-alpha filter.


Regards,
Vicent.

[ManuelJ]

Well, it may say 7nm in the box, but I think it's actually bigger. Do you want a super 3nm filter with a superb transmission? 

Ok, I have it. I did photometry of 17 stars with SExtractor. The H-alpha image was acquired one hour after the R one; during this period of time atmospheric extinction was extremely stable, the only difference being the increase in altitude of the object from 45 (for R) to 52 deg (for Ha).

Assuming that the R filter has about 102 nm FWHM, the median brightness ratio of these 17 stars gives a width value of 8,3 nm for the H-alpha filter.


Regards,
Vicent.

And that's assuming that the Ha filter has the same transmission as the red filter. You are not counting also that the filter is centered in the Ha line.