UPDATE: The measurements on this post have now been superseded but more reliable data from NGC6633. See blog entry for July 1st, 2012!
On 23/12/2011 Peter and Chris observed M41, an open cluster in the Milky Way plane at 06:46, -20:40. It was a very clear night and we got good images (although off center) in all bands. Image below is a centered B band frame.
Stars identified from the WEBDA database for this cluster are shown below:
Data for these stars are here:
The transformation to standard V and B are rather good. There is a small
colour dependence in the V filter (-0.08 mag/mag) and quite a large one in the B filter (+0.28 mag/mag). The scatter is quite low in the V transformation — 0.02 mag, and a little larger in B — 0.05 mag.
Instrumental magnitudes convert to true magnitudes as follows:
* compute instrumental magnitudes
V0 = -2.5*log10(ADU/sec)
B0 = -2.5*log10(ADU/sec)
* instrumental colour
(B-V)_0 = B0-V0
* transform instrumental colour to Johnson B-V
B-V = -1.056293 + 1.529783*(B-V)_0
* transformation to Johnson V (scatter is a very small 0.02 mag)
V = 15.15 + V0 – 0.08*(B-V)
* transformation to Johnson B (scatter is quite large at 0.05 mag)
B = 14.46 + B0 + 0.26*(B-V)
These are for a (rather small) 2.5 pixel aperture – designed to avoid crowding by nearby stars. The correction for the aperture size is ~0.1 mag brighter.
There are no standard magnitudes for VE1, VE2 or IRCUT. I have searched for
similarities with V, B or I, since these are the bands we have external magnitudes for.
The following definition of VE1 magnitude gives a very close 1:1 match between V and VE1:
VE1 = 16.34 – 2.5*log10(ADUs/sec) + 0.06*(B-V) (scatter 0.04 mag)
so VE1 is functionally quite similar to V.
On the other hand, VE2 is very similar to I. With this definition for the VE2 magnitude:
VE2 = 14.22 – 2.5*log10(ADUs/sec)
one gets very close to a 1:1 relation with I, with a scatter of only 0.03 mag. No colour term either — so VE2 this is an excellent match to I.
We didn’t get any good frames in the IRCUT filter. We’ll have to try that some other time!
Summary : Our V and B are much like their standard Johnson counterparts.
Our VE1 is functionally more or less the same as Johnson V, and VE2 is very close to Cousins I band. The latter seems a bit odd. I haven’t seen the filter curves, so they’ll be fascinating in this context. What is the IRCUT filter going to show? More to follow.
In retrospect – having looked at scatter in data and having various realizations about how the FW did not seem to work reliably – I would say that finding V and VE1 ‘very similar’ while IRCUT is quite different makes little sense … if V and VE1 are similar and VE1 and IRCUT have the same transmission curves, then we should have W=VE1=IRCUT.
Great work! IRCUT is a problem -we have one frame!