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Hello,

I have a student using IRAF to compute photometry in various bands from the spectrum of an object. She has an optical spectrum and is trying to compute the color (B-V) from the spectrum using the Johnson filter response. She has a file with the filter response (wavelength and transmission efficiency at that wavelength) for B and V. She is using the sbands task to try to compute the magnitude in B and V. The "bands" field in sbands has 4 columns with a name for the band, central wavelength, width, and filter file. If we are providing the filter file, it is unclear how the central wavelength and width are being used or why they are even needed at all. Could you explain to us or point us to someone else who could explain how these number are used to compute the output magnitude? It would seem that you would only need to convolve the filter response file with the spectrum to get the magnitude, but it appears that the central wavelength and width values are also being used in this calculation.

Thanks very much for any help you can provide.

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The filter function is used to weight the pixels covered by filter in the spectrum. Fluxes/magnitudes are based on that. The center and width are used to isolate and check the spectrum covers the desired region. It is also needed in setting things like continuum and normalization for equivalent widths. As far as I can tell from refreshing myself on this, the magnitudes should not depend on these. When you say that it appears that it affects the calculation in what way? If you change these a little you get different measured values? If you can provide me with an example I could check more carefully.

Regards,
Frank

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Yes, I noticed that the magnitudes would change if I changed the central wavelength or width but kept the filter function the same. Here is an example of one of the spectra we are using and the filter response files. Thank you for looking into this! We appreciate your help!

[Files attached]

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The center and width along with the filter function are used like kind of like an intersection. As long as the center and width cover the bandpass function and don't extend beyond the range of the spectrum then the bandpass function determines the flux. If the center/width cover a smaller part of the filter function then you will get reduced flux. For the spectrum you sent I saw that the whole spectrum could be roughly described as centered at 5900A with a width of 4900A.

But what I think you are seeing is that your bpass.dat doesn't go to zero at one end; i.e. the last value is "5500 0.009". The program extrapolates the first and last values as being the same. So your bpass.dat is like having a "red leak". In that case, depending on the region described by center+width you will get more or less light. If I add a point "5550 0.0" to bpass.dat, I called it bpassfixed.dat, then it is independent of center+width as long as that is wider than the band pass but within the spectrum.

\$this->_split2($m[0])
onedspec\$this->_split2($m[0]) sbands spec48 STDOUT sbands.in

# SBANDS: NOAO/IRAF V2.16 valdes@tiamat.tuc.noao.edu Thu 11:46:11 05-Nov-2015
# bands = sbands.in, norm = no, mag = no
# band filter wavelength width
# V vpass.dat 5900. 4900.
# V vpass.dat 5900. 4800.
# V vpass.dat 5900. 4700.
# B bpass.dat 5900. 4900.
# B bpass.dat 5900. 4800.
# B bpass.dat 5900. 4700.
# B bpassfixed.dat 5900. 4900.
# B bpassfixed.dat 5900. 4800.
# B bpassfixed.dat 5900. 4700.
#
# spectrum band flux
spec48(1) V 7218.05
spec48(1) V 7218.05
spec48(1) V 7218.05
spec48(1) B 14607.1 <- variation due to red-leak
spec48(1) B 14604. <- variation due to red-leak
spec48(1) B 14601.1 <- variation due to red-leak
spec48(1) B 14443.9
spec48(1) B 14443.9
spec48(1) B 14443.9
[/code]


Yours,
Frank