Date: Fri, 21 Nov 2003 09:52:17 -0800 (PST) From: Alex Conley To: nsuntzeff@noao.edu Subject: Bessel filters Dr. Suntzeff, I have a question for you reguarding the Bessel (1990) passbands. In your sn1998bu paper you have a footnote pointing out that the Bessel passbands given in Bessel (1990) include, essentially, a factor of lambda already. Unfortunately, I have been using the passbands as given when doing K-corrections, so am natrually a little concerned about extra factors of lambda that I was unaware of. So, my question for you is: Where does this information come from? I looked at Bessel's original paper, and he never seems to define what he means by passband. Some of his later papers explicitly do give lambda*R(lambda), but I have been unable to verify that this was the case in the 1990 paper. K-corrections give me a headache. Alex Conley Date: Fri, 21 Nov 2003 15:32:33 -0300 (CDT) From: nick suntzeff x284 To: aconley@panisse.lbl.gov Subject: Re: Bessel filters Dear Alex, I agree about K-corrections. Where I figured out that the Bessell passbands were lambda*R(lambda) is lost in the cobwebs of my memory. But I know I asked Mike about it and he told me that the functions are to be integrated on energy flux, not photon flux. I am sure in one of his 1990 papers he has a footnote stating that you have to correct the 1990 curves by lambda if you want to integrate on photon flux. In addition, I have taken spectrophotometry and synthesized BV photometry both with and without the lambda factor to verify which one gives zero color term as a sanity check. The best way if you want a final verification is just to email Mike. He is a really nice guy, and will answer you immediately. In our high-z group, we have had many a discussion on what the proper form of a "sensitivity" function is. For me, I always assume that a sensitivity function is to be integrated on photon flux, not energy flux, since a CCD is a photon counter. That way, if I measure a filter curve and a QE for a detector, I can calculate a sensitivity function by merely mulitplying the curves together. The final curve is a sensitivity function appropriate to photon flux. Here is Mike's email: bessell@mso.anu.edu.au cheers nick Date: Fri, 21 Nov 2003 10:52:38 -0800 (PST) From: Alex Conley To: bessell@mso.anu.edu.au Subject: The definition of passband Dr. Bessel, I have a question for you reguarding the definition of passband used in your 1990 UBVRI paper. Are the passbands given in table 2 in terms of photons, or have they had a factor of lambda multiplied in? In some of your later papers you explicitly tabulate lambda * R(lambda) so that magnitudes can be calculated from energy spectra (i.e., mag = const - 2.5 * log10( \int f_{\lambda} * \lambda * R d\lambda ) ), but I haven't been able to verify if this is true of the passbands in Bessel (1990). The question arises because I'm trying to calculate K-corrections based on your filter passbands, and noticed that Nick Suntzeff in his SN1998bu paper has a footnote to the effect that your passbands already include lambda. I contacted Nick, and he told me that the best way to make sure was to write you directly. This wouldn't have a particularly huge effect on my calculations, but it's certainly worth trying to do right. Alex Conley Supernova Cosmology Project Date: Mon, 24 Nov 2003 08:01:55 +1100 From: Mike Bessell To: Alex Conley Subject: Re: The definition of passband Hi Alex In the 1990 paper the passbands are in energy as they involved photomultipier tubes that were essentially current integration devices. In my later papers where CCDs were used or where photon counting was used I specifically discussed the issue and included a lambda mutiplication factor and noted it. But not for the original UBVRI system paper. Best regards Mike * Mike Bessell, RSAA, IAS, ANU, Mount Stromlo, Cotter Road, Weston, ACT 2611, Australia 61 2 6125 0268 (w), 61 2 6230 7662 (h), 61 2 6125 0260 (fax), bessell@mso.anu.edu.au or Michael.Bessell@anu.edu.au http://www.mso.anu.edu.au/~bessell * Unfortunately I seem to have misplaced my response to Bessell. Basically, I asked for clarification since I wasn't sure if 'But not for the original UBVRI system paper.' meant that he hadn't used the lambda factor or had included it but simply not noted it. So I asked him specifically if (given that R is the numbers he tabulated in Bessel 1990) if I should use mag = const - 2.5 log ( Int lambda * R * f_lambda dlambda ) or mag = const - 2.5 log ( Int R * f_lambda dlambda ) Date: Mon, 24 Nov 2003 11:00:14 +1100 From: Mike Bessell To: Alex Conley Subject: Re: The definition of passband > >If I am using a CCD, should I multiply the Bessell passbands by lambda >when I integrate against f_lambda, or is that already included? > It is not whether you use a CCD but rather when you use those passbands to compute synthetic mags and colors you should use mag = const - 2.5 * log10( Int f_lambda * R dlambda ) Mike * Mike Bessell, RSAA, IAS, ANU, Mount Stromlo, Cotter Road, Weston, ACT 2611, Australia 61 2 6125 0268 (w), 61 2 6230 7662 (h), 61 2 6125 0260 (fax), bessell@mso.anu.edu.au or Michael.Bessell@anu.edu.au http://www.mso.anu.edu.au/~bessell *