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Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Colour filters

Optical design Posted on Dec 27, 2011 02:52PM

The filter transmission as measured by Rodrigo for the B, V, VE1 and VE2 filter up to 800 nm and the IRCUT filter for a much wider range as given by the manufacturer.



M41 and photometric calibration of filters

Post-Obs scattered-light rem. Posted on Dec 27, 2011 03:50AM

UPDATE: The measurements on this post have now been superseded but more reliable data from NGC6633.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:

http://www.univie.ac.at/webda/cgi-bin/frame_list.cgi?ngc2287

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.

Transformations

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.