SFD vs. true

[adrianlucy]

Hi Jo,

I had two questions about the scale on which E(B-V) is output: true (S&F 2011) vs. SFD (1998). I strongly suspect that I'm misunderstanding something; thanks in advance for your patience.

(1) What do I need to do to obtain E(B-V)_true from this code?

For the Marshall and Drimmel maps, my understanding is that sf10=True means that E(B-V)_true is rescaled in the code from E(B-V)_true back to E(B-V)_SFD, by dividing E(B-V)_true by 0.86, because the S&F 2011 Table 6 is supposed to be applied to the old E(B-V)_SFD values in order to get the new true filter extinctions. So if what I actually want to get out of the code is E(B-V)_true, I either need to set "filter=None, sf10=False", or set "filter=None, sf10=True" and multiply the results by 0.86. Meanwhile, for the Green 2015 map, the output of "filter=None" is E(B-V)_SFD no matter what I set sf10 to, so I'd always need to multiply the results by 0.86 to get E(B-V)_true.

Is that right? It's a little confusing to me, because the documentation says "Only use sf10=False when you have an extinction map in true E(B-V), not SFD E(B-V)." When in fact the purpose of sf10=True is to convert from E(B-V)_true to E(B-V)_SFD, not the other way around; the issue in Marshall and Drimmel isn't that the map is in SFD, but that S&F Table 6 requires SFD inputs.

(2) Is the Drimmel extinction being overestimated, since it was already normalized to SFD and the code is scaling it up to SFD again?

The Drimmel A_V map, in its initial construction by Drimmel et al. 2003, was already normalized to SFD. Doesn't that mean that A_V,Drimmel = 3.09 * E(B-V)_SFD
--> E(B-V)_SFD = A_V,Drimmel / 3.09,
which is already what we need, because S&F 2011 Table 6 takes SFD inputs. So why apply the 1/((1-self._sf10)+self._sf10*0.86) factor in Drimmel03.py? That gives the meaningless value of (1/0.86)*E(B-V)_SFD or (1/0.86)²*E(B-V)_true, and the extinction will be overestimated. Or did you somewhere scale A_V,Drimmel to E(B-V)_true and I didn't see it?

Anyway, I'm pretty sure I'm wrong, not the code, but I'd be very grateful for any help you can offer in explaining why I'm wrong.

Thanks again,
Adrian

[jobovy]

Hi Adrian,

Yes, the SFD scaling of the E(B-V) values is a recurring source of confusion. For your first question, it's important to note that sf10=True does not rescale E(B-V) from true to SFD-scale, because E(B-V)_true is not involved in any of the extinction maps (as the README states). sf10=True means that the code uses A_band/E(B-V) values from Schlafly & Finkbeiner (2010) to convert extinctions between different bands, because these are given by Schlafly & Finkbeiner (2010) to act upon E(B-V)_SFD, using them to go from A_band --> E(B-V) means that you end up with E(B-V)_SFD, not E(B-V)_true. So if you want to get E(B-V)_true, you should use sf10=True to get E(B-V)_SFD and then apply the re-scaling yourself. For most applications this subtlety doesn't matter, because if you use any regular filter, the E(B-V)_SFD factor drops out of the conversion for all maps that intrinsically return A_someband. Only in Drimmel is the scaling applied directly in the code, all the other maps are either (a) SFD or (b) give A_someband.

My understanding is that the Drimmel map was only rescaled to SFD at |b| > 30, so perhaps the 1/((1-self._sf10)+self._sf10*0.86) should only be applied for |b| < 30? That would be a easy change to make. Note that because extinction is quite small (and SFD is quite uncertain) at |b| > 30, the impact of that change is probably quite small.

[adrianlucy]

Thank you! About using regular filters: yes, the reason I need E(B-V) is in order to convert to SkyMapper filter extinctions. Anyway, for every map currently supported, I will use sf10=True and filter=None, and the output will in each case be E(B-V)_SFD (right?).

Ah, yes, skimming again through Drimmel 2003, I think you're right about where they scaled to SFD. Although it may be worth noting that even for |b| < 30, I think the Drimmel map is still in some way anchored to COBE/DIRBE emission data, upon which SFD is also based. So, maybe very complicated...

Thanks again! Let me know if I should close this issue, otherwise I'll keep it open if you want to ponder the Drimmel scaling.

[jobovy]

Yes, every map should be returning E(B-V)_SFD. One way to check this is by doing things like

cab= mwdust.Combined15(filter='Landolt B')
cav= mwdust.Combined15(filter='Landolt V')
cebv= mwdust.Combined15(filter=None)
(cab(2.,10.,4.)-cav(2.,10.,4.))/cebv(2.,10.,4.)
array([ 0.884])

(the fact that the ratio is not exactly 0.86 is because the Landolt filters aren't quite those that go into the standard E(B-V)).

Let's keep it open for now, maybe I will ping Drimmel to see what he recommends...